Random Cylinder Misfire...

My truck has been periodically missing out - but then seemed to go away. I kept driving it waiting for a Check Engine light to help my diagnostics. Well it came on today with a P0300 (Cylinder Misfire Detected - Random) code. I recently changed plugs, wires, cap and rotor. What is next?

What year do you have?

What plugs did you use and how much gap did you gap them too?

Did you install a new fuel filter lately?

Changed gas stations recently?

OMG not the P0300 That is a GM techs nightmare.
It could be just about anything from the change over in gas from summer to winter(if you live in that type of area) to a piece of carbon on a plug.
Did you use all AC Delco parts?? Please say you did
Do you have any of the scan tool readings or was it just a code reader??
Fireplug

It's a 1996 S-10, 4.3W. I cannot recall what brand I used - but it was not the cheapest available. I gapped the plugs to the spec under the hood too. I have changed the fuel filter withing the last 3 months (along with the fuel pump).

I change gas stations all the time - but it's been doing this for 3-4 tanks now.

I have all the DTC info and Freeze frame data, but not with me now...

I just inspected the coil and it has some severe discoloration around the housing near the "iron bar" that wraps around it. Almost like it's been arcing to ground??? Too bad I cannot watch it while I'm driving! Could it be a bad coil?

I thought P0440 (EVAP system) was a GM tech's nightmare!!!

It's a 1996 S-10, 4.3W. I cannot recall what brand I used - but it was not the cheapest available. I gapped the plugs to the spec under the hood too. I have changed the fuel filter withing the last 3 months (along with the fuel pump).

I change gas stations all the time - but it's been doing this for 3-4 tanks now.

I have all the DTC info and Freeze frame data, but not with me now...

I just inspected the coil and it has some severe discoloration around the housing near the "iron bar" that wraps around it. Almost like it's been arcing to ground??? Too bad I cannot watch it while I'm driving! Could it be a bad coil?

Aha! This is have heard of before!

Yes, it is probably your beast! Replace the coil and try that out.

The arcing can definitely be the problem.

I would try a new distributor cap with brass contacts

A very easy test is to take a small spray bottle with water in it and MIST the entire system while looking for any arcs. Also look at the ing wires and see if there are any white patches spots on the wires . That is a tale tale sign that they are arcing to ground.
Fireplug

Dannk
yep
po440 and put that stupid evap machine and do all the stupid tests and then get paid .3 time for and hours work and then find a loose gas cap.
I just lived for a evap code.
Fireplug

From past reading, most people with this problaem have the arcing coil pack.

Best to look at it at night with no lights on and look for the blue light from the arcing.

Good point.
I found what works nice is to take a mixture of washer fluid and water and mist it on .Then I day The mixture was too much washer fluid and it flashed (can we yell FIRE) It did no damage to the car because it was just a little puff. I did find the problem and had to change my shorts after I was done.

I replaced the Coil Pack and it seemed to fix the problem. $32.50 aint bad! It actually runs a lot better now too! It was not real easy, but I probably saved $100-$250 by doing it myself. You have to remove a bracket to remove a bracket to remove a bracket the coil is fastened to. Thanks GM for designing POP RIVETS into the coil pack bracket assembly....freekin interns.

Good to see you got it fixed.

Well, I was wrong - again. A few days after my last reply, it started missing out again. I have been putting up with it but now I have to get it fixed (I need a dependable car for the winter). For review, I have replaced the following within the last 2 months:

1. spark plugs
2. cap and rotor
3. plug wires
4. coil pack

When it's real bad the Service Engine Soon light will come on with a code of cylinder misfire detected (random) P0300. It will go off on it's own. It really only misfires under normal driving conditions. When I mash down the gas it seems to run very strong. It really feels like crossfiring in the distributor BUT it's a brand new cap???

PLEASE HELP!!

Sounds like you should do some fuel system diagnosis.

Fuel Filter
Fuel Pressure test etc...

It has been getting progressively worse. I finally broke down and took it to a dealer to confirm my suspicions. They have diagnosed it as faulty injectors/poppets. At $91 a piece times 6, plus labor - they want $1200 to replace all injectors and poppets.

Has anyone else had this prognosis? Is there a "work around" - maybe some type of LETHAL fuel injector cleaner? The truck is only worth about $3000. I will do the work if neccessary and have already found a Delphi replacement injector assembly that elimates the poppets for about $300.

Please help - I need my winter vehicle!!!!

It has been getting progressively worse. I finally broke down and took it to a dealer to confirm my suspicions. They have diagnosed it as faulty injectors/poppets. At $91 a piece times 6, plus labor - they want $1200 to replace all injectors and poppets.

Has anyone else had this prognosis? Is there a "work around" - maybe some type of LETHAL fuel injector cleaner? The truck is only worth about $3000. I will do the work if neccessary and have already found a Delphi replacement injector assembly that elimates the poppets for about $300.

Please help - I need my winter vehicle!!!!

If you pay them 1200 for this, it will be dumbest mistake of your life.

They don't have a clue what the problem is and they know it.

Have you even check the fuel pressure like I asked?

Oh yeah,

I snaked my fuel pressure guage up to my windshield and drove around the neighborhood - with it running very poorly - while watching it closely (I almost took out a mailbox). It was right on the money at about 53-60 psi the whole time. There were no abrubpt changes, but it did vary about 5-7psi as RPM changed...

As a side note, I recently changed the fuel pump and filter about 6 months ago. The gas guage has not been the same since either. When I fill it up, it only goes to about 7/8 full. This repair did cure my slow start condition though.

Check the fuel pressure with the ignition to the first on position and the truck not started again.

Check the fuel pressure with the ignition to the first on position and the truck not started again.

I feel pretty confident I do not have a fuel pressure issue. I dumped 3 bottles of STP fuel injector cleaner into my tank and it's been running great since. I went and bought a $21 bottle of GM fuel system cleaner and put that in my tank when it was almost empty too. It has been running flawlessly since....Sounds like a fuel injector/poppet problem - right????? I am still open to ideas, because I know this problem will be back eventually.

I feel pretty confident I do not have a fuel pressure issue. I dumped 3 bottles of STP fuel injector cleaner into my tank and it's been running great since. I went and bought a $21 bottle of GM fuel system cleaner and put that in my tank when it was almost empty too. It has been running flawlessly since....Sounds like a fuel injector/poppet problem - right????? I am still open to ideas, because I know this problem will be back eventually.

Could have been a poppet valve sticking.

It is not smart to run such a high concentration of fuel additives light that.

It will foul the plugs.

I'm going to murder GM
I went through this whole ordeal. Just last night I got p0440 (which yahoo.com led me to this site.) after having swapped the plenum, and MAP.
http://www.mastrianni.net/stevespictures/steve/images/repair.htm
check out my site. I havn't added p0440 to it yet.

--Steve

OK. I am ready to give up on this. I am out of ideas. Here is what I have done so far - regarding my random cylinder misfire issue..

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)

This last thing with the fuel injection system was supposed to fix my alleged sticking poppet valves. I paid $80 at a GM dealer for a "professional" diagnostic and that's what they came up with too (along with about 3 other people I had spoken with). Well, guess what - its runnin like crap again.

My symptoms are like this - Under normal acceleration I feel the engine intermittently losing power, mostly in first gear. If I am cruising at interstate speed, I feel a miss about every 1-3 seconds. This is most severe between 1500-2500 RPM. It does not feel like a single problematic cylinder. It feels like an evenly dispersed, random misfire. If I drive in first gear at about 2000 RPM for about 1/4 mile, the SES light will come on with a code of "Random cylinder misfire detected". (THANKS GM/OBDII FOR NARROWING IT DOWN).

I am about ready to set this "truck" on fire and watch it burn...burn...burn...Somebody, please help.

Wow the Dealer did not take your truck for a ride with the scan tool hooked up and take a cylinder reading while it is doing the problem??? I would go back to the Dealer that you paid money to and demand that someone good who knows how to check drivabilty problems take it for a test drive (at no charge to you because this is a come back problem)with you and have the Tech Tool 2 hooked up so he can get a true picture of what the heck is going on. I try to take every car owner on a test drive so they know that I really want to fix there problem and just talking to the owner about the problem helps most times to pin point when it does it so i can see it happen and then fix it.
I got every drivabilty problem that everyone else in the shope could not fix. And I blame a lot of no fix problems on the flat rate pay system. These Techs dont get paid if they are not turn a wrench

OK. I am ready to give up on this. I am out of ideas. Here is what I have done so far - regarding my random cylinder misfire issue..

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)

This last thing with the fuel injection system was supposed to fix my alleged sticking poppet valves. I paid $80 at a GM dealer for a "professional" diagnostic and that's what they came up with too (along with about 3 other people I had spoken with). Well, guess what - its runnin like crap again.

My symptoms are like this - Under normal acceleration I feel the engine intermittently losing power, mostly in first gear. If I am cruising at interstate speed, I feel a miss about every 1-3 seconds. This is most severe between 1500-2500 RPM. It does not feel like a single problematic cylinder. It feels like an evenly dispersed, random misfire. If I drive in first gear at about 2000 RPM for about 1/4 mile, the SES light will come on with a code of "Random cylinder misfire detected". (THANKS GM/OBDII FOR NARROWING IT DOWN).

I am about ready to set this "truck" on fire and watch it burn...burn...burn...Somebody, please help.

I will make a few generic suggestions, forgive me if they don't seem applicable to your exact make/model. Some of these may be outdated.

Check for loose mounting screws that hold the distributor module to the base of the distributor, and that the wire connections are tight.

Check the little wires that attach to the module as the come through the distributor housing for bare spots etc.

Check to see if the Di-electric (White?) grease is all gone between the bottom of the distributor module and the distributor base. If it is all gone I'd re-apply more.

Check for any moisture under the distributor cap or in the distributor.

Check to see if there is excessive slop in the distributor shaft (where the rotor attaches). Don't ask me what excessive is, but it shouldn't be very much.

This may help ... It is some insight to a P0300 Engine Misfire code...it's what the dealer would do...

Circuit Description

The Crankshaft Position (CKP) Sensor is the primary input to determine if misfire is occurring. Engine misfire is detected by monitoring crankshaft speed variations between cylinders. If a crankshaft deceleration occurs during a combustion or power stroke, the Control Module (PCM/VCM) will compare this change in crankshaft speed to the previous cylinder. If the crankshaft speed change is more than a maximum allowable speed, the misfire is detected. Misfire may occur in a specific cylinder or all cylinders randomly.

When an engine is misfiring, brief decelerations in crankshaft rotational speed will be detected by the CKP. The Control Module determines which cylinder has misfired based upon the Camshaft Position CMP Sensor input. Misfire data is stored for each cylinder in separate accumulators. After 100 combustion events, the misfire totals are compared to a calibrated maximum number. If the misfire is excessive, a this DTC will set.

Conditions for Running the DTC

Important: If start-up ECT is below -7°C (20°F), misfire detection is delayed until ECT is more than 21°C (70°F). If start-up ECT is more than -7°C (20°F), misfire detection begins after a 5 second delay.



No active VS sensor DTCs
No active TP sensor DTCs
No active MAF sensor DTCs
No active CKP sensor DTCs
No active CMP sensor DTCs
The fuel level is more than 10%.
The engine speed is between 450-5000 RPM.
The system voltage is between 11-16 volts.
The throttle position is steady within 2% for 100 ms.

Conditions for Setting the DTC

The VCM detects a deceleration in the crankshaft speed characteristic of either an emission type misfire or a catalyst damaging type misfire.

Action Taken When the DTC Sets

If the VCM determines that the engine misfire is significant enough to have a negative impact on emissions, the VCM turns ON the MIL after the misfire has been detected on 2 non-consecutive trips under the same operating conditions. If the misfire is severe enough that catalytic converter damage could result, the MIL flashes while the misfire is present.

Conditions for Clearing the MIL/DTC


The Control Module turns OFF the MIL after 3 consecutive drive trips when the test has Run and Passed.

A history DTC will clear if no fault conditions have been detected for 40 warm-up cycles (coolant temperature has risen 22°C (40°F) from the start-up coolant temperature and the engine coolant temperature is more than 70°C (158°F) during the same ignition cycle).
Use the scan tool Clear Information function.

Diagnostic Aids

The Misfire Index counts the number of misfires. The scan tool can monitor the Misfire Index. There is a current and history misfire counter for each cylinder. Use the current misfire counter in order to determine which cylinder is misfiring or use the history misfire counter for misfires that are not currently present.

Many different condition could cause an intermittent misfire.
Check for the following conditions:


Check the IC control circuit for a intermittent short to ground.
Check the spark plug wires and the coil wire for the following conditions:
Ensure that the spark plug wires are securely attached to the spark plugs and the distributor cap.
Check the wire routing in order to ensure that cross-firing is not occurring.
If the misfire occurs when the weather is damp, the problem could be due to worn plug wires.
Check for contaminated fuel or a low fuel level and the following conditions:
Check the fuel condition and quality. Dirty or contaminated fuel could cause a misfire condition.
If the fuel level is low, the fuel pump may draw air into the fuel rail, causing a stumble and possible misfire condition. Check the fuel trim numbers in the freeze frame to determine if this has occurred. It would be likely if the short term fuel number was above + 20.
Check HO2S for abnormal voltage readings.
Check for a vacuum leak as a possible cause of the engine misfire.
Sticking intake or exhaust valves can cause a misfire when the engine is cold.
An intermittent may be caused by any of the following conditions:


A poor connection
Rubbed through wire insulation
A broken wire inside the insulation
Thoroughly check any circuitry that is suspected of causing the intermittent complaint. Refer to Intermittents and Poor Connections Diagnosis. See: Diagnosis By Symptom

Test Description
The numbers below refer to the step numbers on the diagnostic table.


The misfire is considered random on all cylinders if, while viewing the misfire counters in the misfire data list, the misfire seems to move to different cylinders.
The misfire is considered consistent if the misfire is occurring on the same cylinder(s) consistently.
When checking the spark at the spark plug wires, the spark should be consistent. A few sparks then nothing is no spark.

TonyMazz
where did you get that info in your post. It looks like its right out of the GM Manual? Did you find it on the net and if so do you have a link for it??

TonyMazz
where did you get that info in your post. It looks like its right out of the GM Manual? Did you find it on the net and if so do you have a link for it??


Actually it's through a paid subscription to AllData.com. It IS from the GM manual as AllData has the exact same service info that any manufacturer does...including service bulletines etc.

I pay 24.95 for the year and it's an ulimited use...great deal.

www.alldatadiy.com..

It is specific to a vehicle, and in my case it's a 99 blazer but you can use it with any vehicle....pretty cool tool...it pays for itself 100 times over when you fix something your self at a rate of $80/hour savings :-)

TonyMazz, do you have similar detailed information for troubleshooting a P0440 evap system failure code?

I will make a few generic suggestions, forgive me if they don't seem applicable to your exact make/model. Some of these may be outdated.

Check for loose mounting screws that hold the distributor module to the base of the distributor, and that the wire connections are tight.

Check the little wires that attach to the module as the come through the distributor housing for bare spots etc.

Check to see if the Di-electric (White?) grease is all gone between the bottom of the distributor module and the distributor base. If it is all gone I'd re-apply more.

Check for any moisture under the distributor cap or in the distributor.

Check to see if there is excessive slop in the distributor shaft (where the rotor attaches). Don't ask me what excessive is, but it shouldn't be very much.

Thanks Rick Norwood for the good advice. As you mentioned the distributor area, I have had problems with the mounting screws that hold the distributor cap down...So I checked it again today by taking it apart (cap and rotor removal inspection). Everything looked great. No corrosion, some dirt particals but nothing big.

My mounting holes on my distributor base have both cracked and I have "rigged" a washer/bolt combination to hold it down - it has been like this for 3 months. I am in the process of locating a new distributor, BUT I did not see anything suspect.?

TonyMazz - I will re-post after making the dealer get me the misfire counters for each cylinder. I had no idea the computer logged this. Why did'nt they look at this before? It's one of the first things I would do for P0300!! That is GREAT info, thanks...

Again, I have a 1996 4.3 W vortec in a S-10 SS truck. When it runs good, it's somewhat peppy. Nothing like my 1997 Formula WS6 though.

I just got back from the dealer after taking a ride with the tech and his Tech Tool 2. As I watched the misfire counters, cylinder 6 was showing LOTS of activity. Cylinder 5 had about 1/3 as many misfires, but 1-4 were perfect. Cylinder 6 is definitely very suspect at this point. The tech swears it's a fuel issue. After being scolded for buying an aftermarket fuel injector assembly, he said he thinks I may have a bad injector - again??? I have only had this new injector assembly for 3 weeks! It ran great for 3 days after installation, and then it was back to the old P0300 blues. Any ideas? PLEASE HELP.

How is the fuel pressure again?

How is the fuel pressure again?

I just checked it again. While driving, I was running at about 50 psi. If I gun it, it rises to about 58 psi. I saw it as low as 48 psi too. Is this OK or is it too low? I just paid $280 for a new Federal Mogul fuel pump about 8 months ago.

Also, my gas gage has been reading incorrectly since I have had the new fuel pump...3/4 tank is as high as it will go.?

does it hold pressure after shut down?? and you running pressure looks low to me

I just checked it again. While driving, I was running at about 50 psi. If I gun it, it rises to about 58 psi. I saw it as low as 48 psi too. Is this OK or is it too low? I just paid $280 for a new Federal Mogul fuel pump about 8 months ago.

Also, my gas gage has been reading incorrectly since I have had the new fuel pump...3/4 tank is as high as it will go.?

What is the pressure when the key is on but the engine is off?

What is the pressure when the key is on but the engine is off?

It does hold pressure after being shut down. I will have to check the exact number when I get home...What should my operating/running pressure be?

The dealer said "it better be at least 55 psi"...but he has been VERY wrong about several other things too.

Please starting listening to us.

CHECK THE PRESSURE WITH THE KEY TO THE FIRST POSITION.

We want to know this.

Please starting listening to us.

CHECK THE PRESSURE WITH THE KEY TO THE FIRST POSITION.

We want to know this.


I will have to check the exact number when I get home.

I will post the pressure reading with the key in the first position.

Sorry to reply to an old thread, but........

Are you losing coolant? When the intake gasket went bad in my Impala, it would leak into the cylinder and foul my plugs. I had to clean them every week or two or I would end up getting a p0300. The rear cylinders are the most affected by Chevy's intake gasket plague.

It took a while for me to find the problem.

I had same problem replaced fuel pressure regulater and upper plenum gasket cured problem

Sorry to reply to an old thread, but........

Are you losing coolant? When the intake gasket went bad in my Impala, it would leak into the cylinder and foul my plugs. I had to clean them every week or two or I would end up getting a p0300. The rear cylinders are the most affected by Chevy's intake gasket plague.

It took a while for me to find the problem.
After going through the same problems I switched to a MFI assembly problem solved...thanks to the input on this forum!!!!!!!!!!

After going through the same problems I switched to a MFI assembly problem solved...thanks to the input on this forum!!!!!!!!!!
I had asked this in your thread, but I don't think you saw it... Did you have to buy anything other than the parts listed on the TSB I sent you? If I have problems with mine, I'm going to upgrade and I hate starting a project and then realizing I need more parts.

I had asked this in your thread, but I don't think you saw it... Did you have to buy anything other than the parts listed on the TSB I sent you? If I have problems with mine, I'm going to upgrade and I hate starting a project and then realizing I need more parts.
I ordered what they offered...The assembly, bracket, and seals.

Check your distributor I had a problem like that and also changed plugs, distributor cap and rotor and finally distributor. Also check if all injectors are working.

Danke,

I realized that the last post you sent out was back in Feb 2005 but unfortunately, I bought the exact same '96 Blazer as yours and am having exact same problem as yours. I was already spending money on various parts but have decided to stop second-guessing and draining my bank account into this junk piece of crap truck.

My question to you is if you were ever able to find the root cause or not. If you still haven't, I'd like to work with you because I've been reading all of your posts and your situation is truly same as mine. I started with the fuel pressure, ign coil, tps, plugs, wires, cap, rotor, same story but stopping short of replacing injectors since you tried that and didnt' work.

I'm curious if you replaced your catalytic converter or not. I read in another forum that if you had a bad fuel pressure regulator, the leaked fuel can foul out the converter, thus creating backpressure which keep the idle valve stuck open, thus causing rough idle and excessive fuel (flooding).

I also read that another possibility is the Crankshaft Sensor do cause all of those symptoms.

Even the stupid EGR too.

This Blazer is the most ridiculous vehicle every produced, next to those dreaded Ford Bronco II which is another SUV from hell cuz I had one and it wasn't fun dealing with it.

I'm kind of hoping that you're still around because I've spent as much money as you've spent and I'm tired of it. I'm very interested in hearing any new update from you since yoru last post in Feb 2005.

I'd be grateful. By the way, I'm from Illinois and I saw that you're from Indiana.

--Ralph

I noticed you mentioned that you switched to MFI injection. Doesn't the switchover from CSFI to CMFI requires you to change the PCM computer and the injection wiring harness? Or are the existing PCM/harness compatible with the MFI? And also, where did you find your MFI from and for how much?

I'd appreciate the info a lot. I think all Blazer owners should get together and head down to GM headquarter to hold a "million man march" but instead call it "Pissed Off Blazer Owners March".

Thanks a bunch!

--Ralph

I have a 97 Blazer that I've had for 5 years now. It's been great. But the last few months, I've also gotten that miss fire. I've also had a hard start problem. If the truck sits for 24 hrs or more, it will have a real hard time starting. I checked today when it wasn't starting, and I did not have spark at the coil. After trying for 20 minutes or so, it finally started.

For the miss fire, I read in the Haynes manual, that the cam sensor could cause that, due to it helps the timing of the injector.

I had same problem replaced fuel pressure regulater and upper plenum gasket cured problem


a vaccume hose leaking will cause your motor to run like crap also, not sure if its yer problem, but seen vaccume leaks cause strange problems. always something to check.

This might be a stupid question, but have you listened for vacuum leaks under the hood?

I have a similar problem with my 1996 silverado with 5.7 vortec. The misfires come and go but seem to be the worst when it is damp out. If I disconnect the wire to the egr valve the misfires go away and the truck runs fine. I tried replacing the egr valve but that didn't fix it. Have you solved your problem yet?

I am having the EXACT same problem whith my 1997. P0300. Two new sets of Bosch wires, plugs, cap, rotor, Fuel filter.

Am now thinking fuel system. I notice the missing gets progressively worse as my tank gets empty. I replaced the pump about a year ago and its noisier than the original.

Luckly I get my parts at a discount but what a pain in the a$$ to change the pump...

i have a 97 blazer with the SAMe problem..

Except when I get my cylander misfire it is either random or cyl 6... And I lose vaccum pressure and natually I HAVE NO BRAKES WHEN THIS HAPPENS!!!!

I start losing power and then the engine light comes on, it boggs like its going to die, i lose vacuum pressure, then if i pull off to the side of the road it boggs for a few, then the engine light stops flashing and goes on solid then im fine. It only happens when Im going up a long big hill and it revs around or over 2200RPMS.. It happened 5x on the same hill! I cant make it up this hill when it happens and i usually have no brakes to stop or slow down for traffic infront of me. Its so scary. If anyone knows anything or can help me it would be great,

i changed my plugs, wires, and coil pack... No help..

HELP ME PLEASE!!!!!!!!!!!!!

You already have your own thread. There's no need to post in old ones.