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Urgent!!!! P0300 Misfire, Rough Idle, Fuel Trim Error

12-12-2005, 04:33 AM
Hey guys,

I've got a good challenge for you if you're up for it and see if you can help me figure out the root case of my problem without causing me to drain my bank account. If you do have the answer, I would be forever grateful and will show my appreciation accordingly.

I have been sitting here looking through another forum and this forum and am no closer to finding my answer. I decided to stop trying to second guess and chase down a problem in the dark and draining my bank account.

I've noticed a lot of people had similar problems to mine but no true solution.

I have a '96 4x4 4-doors BLAZER with CSFI and Eng Code W, of course 4.3L Vortec, what else would it be???

I've decided to stop wasting time pouring through the forums with no answers in sight. I probably could spend a whole week and not find any answer. I mean, I figured that's what this forum is for, to seek help. Plus I figured that since it's a 1996 truck and now this is Year 2005, I would think that in those 9 years that has passed, SOMEBODY would have the answer which is why I'm here. I'd really appreciate the answer because I want to get my truck fixed ASAP so I can get through this winter season.

Here's the symptoms I'm currently experiencing:

1) hard to start if I turned the key and hold it but if I turned the keyquickly in like three short bursts then it starts up fine.
2) random erratic idle
3) sometimes don't accelerate smoothly from stop and had to mess with the gas pedal to get it going.
4) sometimes misfire at 55mph
5) service engine lights on sometimes it goes away
6) got the misfire P0300 code (multi-cylinders)
7) Cyl 2 and 3 are misfiring very high
8) excess gas smell (possibly running rich due to #2 and #3 misfiring and possibly running lean, causing other four cylinders to run rich)
9) also got Fuel Trim codes Bank 1 and Bank 2 which I think are P0171 and P0172.

Here's the list of parts that I've replaced but no success in solving the problem. In fact, they are the common parts being replaced by other Blazer owners while trying to track down the root cause without success. I'm trying to go through process of elimination to further narrow down to the root cause.

Coolant Temperature Sensor
Throttle Position Sensor
Ignition Coil
Spark Plugs
Spark Plug Wires
Camshaft Sensor
Fuel Pressure Regulator (my old one was faulty cuz the diagphram ring around was ripped, like a blown speaker).

My problem is exact the same as DANNK's problem but the postings has stopped back in Feb 2005 and I was hoping that he found a solution so I'm not sure what's up with that.

He replaced the following items which didn't solve his problem:

Replaced fuel filter (6 months ago) - seperate issue
Replaced fuel pump (5.5 months ago) - seperate issue
replaced plugs and wires (3 months ago)
replaced distributor cap and rotor (2.5 months ago)
replaced ignition coil pack (2 months ago)
replaced ENTIRE fuel injection system (2 weeks ago) ($350!!!) (not easy)

My fuel pressure is within spec.
Key on, 60 psi pressure held steady
Engine running approx 56 psi. pressure held steady when idling.
Fuel pressure held steady after shutdown.
Therefore, that rules out fuel filter.

Seafoam cleaner does nothing for me. It was recommended by my mechanic but he's full of sh*t.

Any suggestions? It got to be ONE SINGLE PART that is causing all of this. I'm at a dead-end. Thought I would replace fuel injectors but after reading the forums, it doesn't seem that it's the problem. And plus I haven't see any solutions anywhere. Nobody seems to really have any solutions.

I've seen that EGR replacement has fixed it but I've also seen that it was only temporary and problem came back.

I've seen Crankshaft Sensor replacement has solved the problem but not always.

Gas stations is not relevant since it has nothing to do with gas in my case. I've been using 87 octane which is what the manual recommended anyway.

What about the Catalytic Converter being bad? Any way I can tell if it's bad or not?

It could be MAP sensor, it could be MAF sensor, it could be Idle Air Valve assembly, it could be Top Dead Center sensor located on tranny's bellhousing. Hell there's too many possibilities and I don't want to waste money on those if it turns out NOT to be the culprit.

My hunch is telling me that it's probably more likely a combination of three things, maybe four........ Crankshaft Sensor, EGR valve, Catalytic Converter (due to bad fuel regulator which is replaced), and MAP sensor. Hell if I know.

That's why I need your help. I'm hoping that there is actually someone out there WITH THE SAME PROBLEM as mine and have FIGURED OUT THE ANSWER to the problem would respond to me so I can get over with my ordeal.

I don't want any second guessing or whatsoever, if you KNOW FOR SURE that you got the answer, please do share with me. If you are not SURE then I'm willing to listen to your suggestion because I'm at my wit's end.

Thank you very much.

12-12-2005, 04:51 AM
I had the same problem a litle over a year and a half ago, cleaned the Egr valve, prblem went away, problem came back, cleanedthe egr valve again, problem went away, problem came back, said muck it, replaced the egr valve, problem still gone.
I have a 95 Blazer 4 door LS, this is what I had success with, this is my two cents, still a newbie with fixign Blazers, BlazerLT and a few others on this forum kick butt on what hey know. For sure you'll get your answer here.


12-12-2005, 09:55 AM
My instincts say you are both probably right - EGR + Cat.

12-12-2005, 10:40 AM
I Have a feeling that its your EGR valve. Try taking it off and check for chunks of carbon. Clean it out and make sure that the plunger moves freely and closes all the way. Like XPC2004 had said. I too had the same trouble. I needed to remove and clean 4 times before it stoped.

12-12-2005, 11:13 AM
There are alot of things not really cover. Po171 P0172 are the same bank 1 lean 1 rich. So I question that. Your ign switch deal doesn't add up either. Cycling key fast will not let fuel pump prime. Egr most of time don't cause random rough idles, every time I have came across egr problem it idles bad always. Replaceing all those sensor without any diagnostic test is a shot in the dark. The map sensor in this case is only used my the computor to see if egr valve worked as it should. Because this engine has MAF sensor. Sounds like people forgot the basics. How is compression, Engine vacuum, timing, What are the fuel doing? LTFTRIM , STFTRIM, O2 sensor readings,vaccum leaks?.

12-12-2005, 11:26 AM
Let me ask you this......... have you had those problems like I'm having right now?

As for the key, when I first bought it back in May 2005, it only took one turn of the key to get it started and it started right away, fast. Now, if you turn the key all the way on to start position and it will start but won't fire. So I shut off, then do a rapid succession of key start and sure enough, it fired up.

As for EGR, can it be affected by outside temperature? Does EGR moves slower in very cold weather thus making the problems go away?

If it was the compression issue, I would have the problem everyday but why is it that one day, there isn't anything wrong with my truck and it runs beautiful? Why is that another day, my truck decides to crap out on me. Same goes for vacuum. Timing.

The bottom line here........ there has to be a part that decides to cause my truck to act up on me and when it doesn't. Because it was running fine from May until September until that one day, that's when everything went wrong then it got a mind of its own on when it decide it'll act up and when not to act up. I could be driving one day all day and it'll run perfect, got all the powers, no hestitation. Another day, it would bog down and misfires.

Exhaust was coming out clean.

Something is causing my engine to flood out. Refer to my original post carefully as I've described everything.

12-12-2005, 06:12 PM
Common problem with all Blazers... EGR. Think EGR... EGR... Exhaust Gas Recirculator... one tiny bit of carbon and it acts up.. please read the following link,


When my Egr got gummed up it did Exactly what you are describing... exactly. Cleaned it numerous times, but eventually ou need to replace it.
I also have read a few other posts about mechanics installing a mesh to prevent the larger bits of carbon from gumming it up... don't kow about that one...
Slight carbon build up will do what you describe, throttle it, and if th ecarbon loosens and flys on down then it's all good, my check engine light would go off too, I get he codes read and it had egr problems.
The link explains exactly what an eg valve does...

All info is good info, just trying to do my part.
Egr is a comon problem, I think I mentioned that..

12-12-2005, 07:43 PM
Okay let me step back. As for the first line, read my profile. I own a shop. As for second thing, the key thing don't add up but let's think about it. You say no sprak until rapid succession of key. Well that seems like a place to start. As for you third, since egr in electronic on this model I wouldn't think temp. And yes seen many egr problems on this model. Just the least little bit of carbon to break off and get trapped under seat of the valve. But then I don't see misfire on 2 cylinders. I will see lost of vacuum. I will see multiple misfires. Well I see you don't know the basics, so many times that is always the problems, Basics. And since I read your first note carefully as you put it in number 8 you said, 8) excess gas smell (possibly running rich due to #2 and #3 misfiring and possibly running lean, causing other four cylinders to run rich). I wonder where you got,( Exhaust was coming out clean). But yea your right, you keep on changing parts one day you might get it right. I only ask you some questions and gave you something to think about. But heck my truck runs just fine. Good luck with your quest. I won't waste my time with this one for sure.

12-12-2005, 09:25 PM
Here's my latest update on my '96 Blazer problem.

Folks.....please forgive me if this is rather long message but I want this to be informative and share my experience as well as my perspective and opinions. However, I really appreciate all of your comments that you have provided to me. Thank you for taking the time to do that.

I felt it would be unfair to those who may join this forum or continue to experience same problem as mine by not sharing my experience with everyone.

I'm not saying that the way I fixed my problem will work for everyone. But at least I I think I would probably be safe to say at least try this approach first as the best starting point instead of wasting your money on those other unnecessary parts like everyone and I did when you might want to try the ones I've done first before trying the rest of the parts.

I just want to spare everyone the financial and emotional heartaches that I had to suffer and others before me as well and to spare those from getting ripped off by dishonest mechanics. There are such honest mechanics out there and a big salute go to them for conducting honest business.

If someone disagree with me with anything I said below or wish to add additional information, I welcome your reply. I'd like this to be educational for other unfortunate vehicle owners with same situation since the Big Three are so incredibly unreliable in help their customers out. I'd like for this to be the first stop before blowing all kind of money on various parts that I mentioned in my first post.

One last thing..... for those who are new to this and are experiencing same problem.....please read my first post under "Urgent!!!! P0300 Misfire, Rough Idle, Fuel Trim Error" thread which contains my description of my current situation in details and the parts that I've replaced thus far up to this point without success.

Here we go...........

================================================== =

RKSNC, XPC2004,Gabe25, & Cailen
I appreciate and thank you all of your comments that you've posted. I took EGR into consideration. However, I have something interesting to share with you with my experience that I've had today.

I started with EGR as this is what I kept hearing from other gentlemen that have posted their replies to my issue.

I checked EGR which is PCM controlled, not vacuum controlled, and I pressed on the plunger and it moved very freely, not even a hint of stickiness. I looked inside and it doesn't seem like a bad carbon buildup at all. I looked on the block and it didn't even look that bad. NOTE: my truck's computer did not generate P0400, P0401, or P0402 codes which refer to EGR problem which is why I ruled EGR out in my case.

This was done at Noon today and I decided to put it back on and tried a different avenue.

Internet Research and MAF & MAP sensors
I came back in the house and decided to do a search in Yahoo on three keywords: P0300 Fuel Trim

And guess what.........there were so many different cars, makes and models that all reported same problem. The common theme I've found in my research all pointed to MAF and MAP sensors.

I decided to run to AutoZone to buy MAF ($100) sensor and O'Reilly to get the MAP ($50) sensors and replaced both this evening. I know some of you EGR guys might think "What the hell is Ralph doing????? It is the EGR!!!!!!!".

But hear me out. I come from B.S. in Computer Science (programming) background and the logical structure of any computer, whether if it be a PC, engine PCM, watch, cell phone are all similar and is similar to the engine itself even if it's mechanical. The point is that it relies on proper working function of various components.

If one part/component goes bad, it causes erratic and unfunctional behavior, whether it be a computer or an engine. However, since the auto engineers decided to incorporate computer into automobiles to control the engine, the same logical structure still applies. It's all about information being received by the engine's computer and how the computer adjust the fuel/air mixture based on sensors information. This is how I approached the engine's computer.

Below information are for those new folks who are experiencing exact same problem and are not familiar where the MAF and MAP sensors are located and what they do.

MAF sensor
MAF on my engine is located on the air hose path between the filter and the throttle body (butterfly). It's a huge square thing with plug-in and inside is a round thing with elements in it to measure air in order to determine how much fuel to mix. Bad air info would cause it to either run rich or lean. In my case, it was more of a rich because it kept flooding and I could smell gas.

Here's the defintion I found for MAF......
A device used in many fuel injected engines to measure the amount of air entering the engine so the computer can control the air/fuel mixture. Located ahead of the throttle body, the MAF sensor uses a heated wire or filament to measure airflow.

MAP sensor
MAP is located on the upper manifold on the passenger side, near the fuel injection assembly. The definition I found for this is as follows:

The MAP sensor is connected to the intake plenum with a hose and constantly measures the intemal collector pressure. The measured amount of manifold air pressure is converted into an electrical signal and sent to the control unit. The control unit uses this signal in conjunction with a signal from the CMP sensor, to calculate the amount of intake air.

Here's another definition of MAP.........
Refers to a manifold absolute pressure sensor, a variable resistor used to monitor the difference in pressure between the intake manifold at outside atmosphere. This information is used by the engine computer to monitor engine load (vacuum drops when the engine is under load or at wide open throttle). When the engine is under load, the computer may alter spark timing and the fuel mixture to improve performance and emissions.

My Opinion on MAP and MAF sensors and fuel pressure
In my opinion, both MAP and MAF sensors basically measure air and send the information to the computer to determine the proper air/fuel mixture and spark timing. Wrong MAP and MAF sensors would cause the computer to calculate wrong air/fuel mixture and bad timing, which in turn, would cause the engine to run rough due to rich or lean fuel as well as misfires.

I feel that it's best to replace both MAP and MAF sensors since they are related to air measurement that affect the fuel/air mixture.

Yes, I definitely agree that fuel pressure do need to be checked to make sure that it's not fuel pump, fuel filter, fuel injection assembly issues. If fuel pressure is stable then I would rule them out.

I had similar problem with the MAP sensor on my old Chrysler LeBaron years ago and it was very similar in behaviors to my current truck.

While I believe that the common theme of P0300 Misfire Random Cylinder along with either P0171 or P0172 Fuel Trim are rather vague and difficult to pinpoint. I realize that it doesn't alway lead to one exact root cause since there's too many factors affecting the fuel/air mixture since they are all controlled by the PCM computer since it relies on several sensors information for optimal air/fuel mixture.

The biggest reason I decided to go with MAP and MAF option this time was based on the fact that I felt the problem was not a mechanical problem. I felt it was more of a electronic problem, hence sensor problem . I had the impression that one of those sensors was bad, thus throwing the fuel/air calculation off in the computer.

My Opinion on Diagnosing P0300 misfire and P0172 fuel trim error code
Therefore, based on my several researches, I still do believe that it's always best to take a backward approach for this particular situation by starting with with a MAP, MAF, and EGR, one at a time before worrying about spark plugs, wires, cap, rotor, camshaft sensor, etc.

My Past Experience with P0300 misfire and P0335 Camshaft Sensor
However, I had some problem with my truck refusing to fire up and run a few months ago and ran the scanner and found two codes.... yes, the infamous P0300 misfires random cylinders and P0335 Camshaft Sensor failure. Turns out to be the Camshaft Sensor in the distributor. Replaced it and it fired right up on the first start. However, the P0335 Camshaft Sensor error code was absent from my current situation so I ruled that out.

Today's Results of Replacing MAP and MAF sensors in my truck
Started it up......... ran smooooooth........took it for a test drive to my nearest town which is 3 miles away. First two miles were country road leading up to the highway so that gave my truck a chance to warm up completely before I hit the highway. In the past, by the time I get to the highway and start driving on it to town, that's when the problems show up.......the hestitation.....misfiring while the service engine light remains lit.

This time........ my service engine light went away which I noticed when I hit the highway. Drove at 55mph. Ran smooth. No hestitation. The outside air temp is currently 36 degree. Reached town's first stop lights and that's when it normally would idle funny but not this time. Idled very smooth, as it used to be. Drove around the town.......still "feathering" the gas pedal like I had to before this fix.

Headed out of town back on the this time, that's when I would have really felt the major misfirings and hestitations at 55mph........not this time. Set the cruise control to see if I could feel one hint of misfiring. None at all. Still smooth riding.

Came home.....let it sit there in my driveway and idle. Still smooth.

Turned key off......back on.......started up on the first key turn.

At this point, it looks like my problems are solved. However, I will DEFINITELY keep the EGR in mind as I drive around the new few days.

Next Steps
The next biggest test is tomorrow morning or in a few hours from now when my truck has completely cooled down. I'll do a test start to see if it would start on the first keyturn. Will drive my truck around tomorrow at Noon on my lunch break to see how it will perform. Will drive to another town about 10 miles away to see how it would hold up then will hit the Interstate so I can hit up to 75mph to the next exit which will circle me back to my house.

At least, I get to test the truck on various road conditions...... country road..... highway road..... and Interstate road to see how it performs. I'm cautionly optimistic at this point that the MAP and MAF are the culprits. I figured that I would get both parts during one trip to AutoZone since both do affect fuel/air mixture.

I'm hoping that the MAP and MAF sensors replacements have fixed my problem once for all which I why I need to do a couple more test drives just to make sure.

I'm painfully aware, based on other readers' comments, that it's still possible that both MAP and MAF sensors replacements MIGHT NOT solve my problem permanently. They might be only temporary. We'll see how it goes. I'll be glad to post further update one week, two weeks, one month, and 6 months from today to give everyone an idea of how my truck is performing.

So I'm knocking on wood that this is it, just enough to get me through this winter and as much as I hate to say this to you Blazer folks, I will have to err on the side of caution by saying that I will have to get rid of my truck to prevent further financial and emotional drains and get something more reliable as I've had enough problem with my previous SUV, the '89 Ford Bronco II. That's another story.

Main problem was the bad transmission design which was made by Mazda. They left one critical part out which unfortunately trash the tranny out. Had to replace three times and replace the engine one time since it was affected by the transmission. It affected something like '83 thru '92 or something like that when Ford finally took care of it, or did they really? Who knows. Unfortunately, it costed me well over $10,000....... $7,000 for the stupid truck and remaining for the engine/tranny along with mechanic's labor charges. End up selling for $1,000. Major financial loss.

Moral of the story........before you buy a car or a truck, make sure you research the forums to see what other owners have complained about to help you get a better feel of what you might be expecting. I'm not saying that those compliants are representative of entire specific model/make owners population. It's better to be prepared now than to be sorry later. That's the lesson I've learned from my current experience.

I never have bought a new car because I've alway been afraid to end up with one bad financial sour apple. I've only bought used cars to keep the financial risk down.

Good luck to all of you and thank you for you time and help.

And HAPPY HOLIDAYS!!!!!!!!!!

Hope I don't have to come back to this forum with another problem so wish me luck (being sarcastic)! :)

--Ralph a.k.a. Lowrider 71

12-12-2005, 09:30 PM
^^^ Will somebody please sum that up for me?

12-12-2005, 09:36 PM

To make long story short........I replaced my MAP and MAF sensors and they solved my problem.

I basically consolidated several different informations that I've learned from researching the forums and the internet search for those who have unfortunate experiences like mine and don't feel like plowing through several forums like I had to. Just being helpful.

And have a good holidays.


12-12-2005, 09:49 PM
I think that was a whopper post.
MT :grinno:

12-12-2005, 10:07 PM

That's why I decided to start a new thread to keep this specific situation separated from other thousands of postings and multiple threads.

At least this one isn't gonna be long. LOL It really underscored the p*ssed off feeling and the determination to find a solution right away instead of letting my truck sit and rot.

I noticed nobody in any forum really took the time to explain everything so I thought I'd do a favor. So instead of plowing through multiple forums endlessly, I'd make it a one stop shop. I figured if somebody who is clueless as I was and had the same situation would be willing to take the time to read it in details and understand the story behind the problem as well as sharing my experience. Just sparing them pains and further financial losses.

Hope you didn't think my postings were insane or boring. LOL

12-12-2005, 11:46 PM
I respectfully step down from "Post King" and hand the crown to LowRider. Damn boy, you got some lungs on you. Decaf is in the *orange* pot. :lol:

12-13-2005, 01:52 AM
LowRider, Hats off to you. I heard of making long story longer and a short story shorter. But WOW. :lol2: I think you got everyone beat. Glad to hear that you found the trouble. To bad it wasn't more simple and cheeper. But the main thing is that everthing is working in top shape. Right on. :biggrin:

12-13-2005, 08:46 AM
Thanks for posting the fixed and letting us all know how it went. We are glad you got it goin well.
Your post was nowhere close to insane or boring.
You covered everything well.
To many posters hold back so much information that we can not even help them.
Good luck wirh it.

12-13-2005, 11:02 AM
To all recent postings guys,

Thank you very much for the kind compliments. It definitely makes me feel it was worth the time typing out all detailed information.

Drove my truck 20 minutes to my driver's license facility to renew my driver's license and back and not one problem at all. So it's all good now and I'm very RELIEVED!!!! Now I can take the truck for my bi-weekly 4 hours trip to Chicago for business trip with CONFIDENCE. WHEEEEEEW!

Now I can sleep good at nights. LOL

And you all have a GREAT HOLIDAYS!!!!! MERRY CHRISTMAS and HAPPY NEW YEARS!!!!!!!!!!

I will come back and check other postings in Blazer area regularly. You guys are the best!!!!!

Will have to join the Chrylser LeBaron postings as I do have a problem with my LeBaron convertible. It seems that at night when I drive with headlights on and if I come to a stop with my heater fan blowing, my battery charge gauge start to drop and I can hear a clicking noise and my headlights go dim. If I turn the fan off, then it recharges back to normal position. Already put in new computer and alternator which comes with built-in voltage regulator. I'm thinking that it has to come from somewhere else, some kind of regulator.

Thanks a bunch!!!!

--Ralph aka Lowrider71

12-13-2005, 11:58 AM
Nice man, glad you fixed your problem. Good of you to post all the information back too, not a lot of people do the follow up thing.

Just out of curiosity, had you tried cleaning the MAF before you replaced it? When you looked at it did it appear dirty? How did the MAP sensor look?

12-13-2005, 04:25 PM
Good questions you got there. Since I was frustated to the point where I didn't care about cleaning. I just decided to hell with it and just buy the new sensors and replace them and see what happens.

However, the MAF sensor......since it has like a three "strings" elements inside, they looked clean. But I didn't feel like testing it. Just wanted to get it over with instead of spending any more time which I was sick of doing it.

But for the MAP sensor......yeah it was pretty dirty since it goes in the manifold...... the little hold in the sensor had some buildup but not total blockage but sure is black gunk which is carbon buildup.

My gut feeling is telling me that it's probably just the MAP sensor that is at fault, not the MAF. But I played it safe by replacing both since they both measure air and I figured that since I've had so many parts replaced so might as well replace MAF too along with MAP. That way, I KNOW that my truck ain't gonna breakdown on me for a while, knock on wood, but at least will be more less error-prone and easier for me to pinpoint if I run into another problem.

Basically, I've spent about $500 on the truck since I bought it so the way I look at it.......I think it's one hell of a good tune-up which is what it really needed, especially a used vehicle, compared to the cost of a tune-up at a garage which I've heard average around $300 which barely cover anything except for fuel filter, air filter, spark plugs, wire, cap, rotor, that's it. My "tune-up" goes further.

In fact, I do plan on replacing EGR valve next month, O2 sensor(s), and whatever other sensors I have left on this truck. This truck had 142,000 miles when I bought so it makes sense to invest a little on replacing some of the sensors. I do also plan to replace my fuel pump as well as I don't want to take a change of it failing on me while I'm out driving in the cold winter weather. I've heard that fuel pump sensor do have a tendency to fail as well as sending false info to the gas gauge. For now, I'm just keeping an eye on my gauge to see how it behaves for the next few weeks and see how it goes.

I do also plan to replace the oil cooler lines for sure from what I've read, scary!!!!!!!! Don't need my engine to blow!!!! I'll also have to flush my coolant cuz of the way Dex-cool behaves and replace my radiator cap.

I'm also aware of 4x4 issues and ball joints issues so I'm just keeping an eye out and do plan on inspecting them when spring comes.

I figured that probably another $500 - $600 should take care of all known issues and then I should be in tip-top shape.

Have to fix my power windows.....sometimes my driver's side act it become unresponsive while my window is half-way down and I tried to roll it back up. And the rear passenger window power button is not working. Any idea??????? I'm not all too familiar with power window aspect yet but do plan to do some research but any advices/ideas you could provide would be greatly appreciated!!!!

Well guys, I'm off on a 4 hours trip to Chicago so I won't be back til the weekend. Have a good week and I'm looking forward to more postings.

Hasta la Vista!
--Ralph aka Lowrider71

12-13-2005, 05:13 PM
Just wanted to say also. If you can find the right information the MAP sensor on this truck,it is not used as detailed. In this engine the computor uses the MAP to see if EGR worked as it was suppose to. It is not used for fuel control.
There are two kinds of Intake Air Flow Measurements, Speed Density and Mass Air Flow.
On Mass Air Flow systems they do not use MAP sensors. But there is one on this engine but used for EGR diagnostics.
Now you may ask how I know this , I train with CTI and have for severals years now.
I also belong to ITAN.
I also have my ASE.
I have also train with EASTTRAING.
And yes so much money is spent on parts changing. Heck anyone can do that. With enough money and changing of parts anyone might get it right. Understanding how and why something works will only get the job done right, the first time.

12-28-2005, 01:36 PM

I agree that only way to truly fix any problem is to understand how things work first.

I appreciate you explaining the purpose of MAP on my Blazer which is used for EGR diagnostics.

Here's my next challenge for you guys.

Even though I replaced the MAP and MAF sensors which did greatly reduce my misfires and hard to start. However, one minor problem still persist and that has to do with misfires. I even got an e-mail from a Jimmy owner with same engine and same situation as mine.

Why is it that when I first start it up then drive, it'll misfires slightly, but if I shut it down after 15 minutes then wait for a few minutes then start it back up and it run fine. It seems slightly worse if it's damp outside. Could it be EGR related? What part would be highly affected by moisture?

For those of you who are new to this posting, please read my original postings before answering.

For those of you who are already familiar with my postings, any help would be greatly appreciated!

Even if it means explaining how things work and why it would cause it, then I can truly understand more.

Once again, thanks for all your help!!!


02-04-2006, 01:39 PM
Hmmm. Sounds like I've got my problem narrowed down to EGR valve MAP and MAF sensor. Too bad I bought a TPS sensor. But hey. I can use it for troubleshooting future problems.

Last night 1997 Astro Van wouldn't start. This morning it started. I hooked up the AutoTap scan tool on my laptop and I got a P0135 Ho2S Heater Circuit Bank1 Sensor1 failure code. Also I got a P0121 Throttle Position [TP] Sensor Performance failure. Cleared the codes and took it for a test drive.

I got about 2 miles and the Van hesitated and died. I almost got hit by a car when I pulled out onto the road when the van died. Thank GOD the other driver was alert because I had no side impact air bags.

I was monitoring the HO2S Bank1 Sensor1 and it's values dropped as I was losing power. If I let off the gas and peddled the throttle I could get rolling again. I couldn't mash it without stalling.

I hooked up the autotap scanner and it had the infamous P0300 and P0171.
The freeze frame data I had was this:
Fuel System 1 -CL-1
Calculated Fuel Trim B1 -3.1%
STerm Fuel Trim B2 -3.1%
LTerm Fuel Trim B2 8.6%
MAP 10.0 Hg
Engine Speed 574.8 RPM
MAF Air Flow Rate 0.65 lb/min
Throttle Position Angle 0.4%
Throttle Position Angle 301.2%

I'm not sure but I think that MAP is a little low. I could be wrong. But that Throttle Position Angle at 301.2% seems kind of High. And why did it record two of them. One at 0.4% and one at 301.2% ????

I'm going to change the fuel filter and spark plugs for starters because it's been 100,000 miles since I've done that. I also suspect the O2 sensor heater circuit bank1 sensor1 but I have no idea where that is. I think there are 4 02 sensors. I wonder if you can swap the plugs for troubleshooting? I've got to get it started again first. It won't start again. I'm going to let it cool down and try again.

02-04-2006, 01:53 PM
Jimmypodunk, please start a new thread. Posting in old threads causes confusion, and can hinder the process of helping you.

Lowrider71, if you need to update this thread, simply PM a moderator, and someone will reopen it for you.

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