99 4.3 Rough Idle, Low Temp, Bad Gas Mileage


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dr_lee29
12-15-2008, 07:15 PM
Hi I have a 99 S10 4.3 4X4 with some problems, wondered if anyone out there could help.
Its idle is rough, but it doesn't seem to have lost power on the highway. I noticed really poor gas mileage (don't have exact numbers but I just burned through over a quarter of a tank in 20 miles). The temp is reading about 150 even after several miles on the highway. The oil pressure gauge is pegged at 80! The vacuum seems good, it reads 19 off the manifold at idle. The SES light just came on BUT, I've never been able to read SES lights on this truck, there's something wrong with the service port and the scanners just say error or no power (the truck has been this way for the last 6 years).
OK I started with replacing the thermostat, temp sensor, distributer rotor, IAC. I cleaned out the EGR valve and PCV. I bought one O2 sensor to check the function of the truck's O2 sensors (I swapped them out one by one checking idle after every swap). The spark plugs look clean, no carbon, no oil.
Anyone have any ideas? I wonder if the main computer is dead? The oil pressure and low temp is just really strange.

MT-2500
12-16-2008, 09:54 AM
You need to check for codes and make sure engine is coming up to operating tempt.
If no scanner read out check fuses.
X or W code engine?
Cigar lighter fuse to pin 16 on diagnostic connector.
Post back code no.

dr_lee29
12-16-2008, 11:18 AM
Thanks for the help!
Its an X engine.
I've checked the fuses in the fuse box located on the drivers side door these all look fine, is there another set of fuses that would run the diagnostic port? Are you saying the cigar lighter fuse runs the diagnostic connector? Or I can run power to the port by jumping a line from the cigar lighter to pin 16? If so, where can I get a diagram of the port to find pin 16? Again, I really appreciate the help!

MT-2500
12-16-2008, 11:47 AM
Thanks for the help!
Its an X engine.
I've checked the fuses in the fuse box located on the drivers side door these all look fine, is there another set of fuses that would run the diagnostic port? Are you saying the cigar lighter fuse runs the diagnostic connector? Or I can run power to the port by jumping a line from the cigar lighter to pin 16? If so, where can I get a diagram of the port to find pin 16? Again, I really appreciate the help!

http://www.troublecodes.net/OBD2/

Fuse 2 cigar lighter on end of dash.
Put a test light on it and check for 12 volts both sides.

Yes you can run a fused jumper wire to pin 16 if all else fails.
Get power on it and check codes and then clear codes and see what comes back.
Post back code no.
Good Luck
Good Luck.

dr_lee29
12-16-2008, 08:01 PM
I jumped a wire from the cig lighter to pin 16, and I was finally able to check the codes. I have a random cylinder misfire on cylinder 4. Any advice?

MT-2500
12-17-2008, 04:10 PM
How ramdom?
Can you feel it miss?

dr_lee29
12-17-2008, 05:06 PM
No, I can't say I "feel" it miss in the sense that there is a rhythm to the miss. If anything it seems like the truck is just idling low and rough.

So let me step back just a bit as well. When this whole thing started (rough idle, SES light, low temp), I noticed a little bit of coolant on the front of the block and thought the LIM was bad so I replaced it. After I got it all buttoned back up, there was the same problems as before but now the low temp is much more pronounced (150 instead of 180).

I just can't figure out the low temp thing, unless there is a problem with the computer (oil pressure gauge is pegged at over 80 constantly).

MT-2500
12-17-2008, 05:43 PM
Make sure engine is getting up to 195 running tempt.
Check for vacuum leaks and egr and pcv valve sticking.
Check fuel presure for up to par.
And go over all computer readings for up to par.
Double check plug and wire and cap and rotor on no 4 cly.
Run a compression test.

dr_lee29
12-17-2008, 10:11 PM
I'm positive the engine isn't reaching 195 degrees. I don't know how to make it get warm either, I thought the new thermostat would fix that but it didn't. I'll double check the wiring. Where is the best place to get a vacuum reading?

MT-2500
12-18-2008, 10:04 AM
I'm positive the engine isn't reaching 195 degrees. I don't know how to make it get warm either, I thought the new thermostat would fix that but it didn't. I'll double check the wiring. Where is the best place to get a vacuum reading?

Some times new stat's are bad.
Take a temt reading of engine after warm up.

Take Vacuum reading with a vacuum gauge hooked to mainengine intake vacuum line.
Check vacuum at idle and at 2K rpm and post back reading.

dr_lee29
12-18-2008, 10:30 PM
Checked the vac tonight. Off of the manifold, I get 19 at idle, 21 at 2000 rpm. The vac seems to react well also. When I snap rev the truck the vac drops to 0 then raises quickly to 20-21, decel it raises to 26 then drops to 19. I can't complain about those numbers.
I don't have a cylinder compression gauge so I haven't tested compression, but that would come out in the vac numbers wouldn't it? Besides the truck seems to have power, it just sucks down the gas and idles rough.
I checked the plug wires, they all seem good. I turned off the lights and watched for sparks and didn't see anything. I'm leaning towards buying a coil, anyone have any other suggestions?

MT-2500
12-19-2008, 10:09 AM
Checked the vac tonight. Off of the manifold, I get 19 at idle, 21 at 2000 rpm. The vac seems to react well also. When I snap rev the truck the vac drops to 0 then raises quickly to 20-21, decel it raises to 26 then drops to 19. I can't complain about those numbers.
I don't have a cylinder compression gauge so I haven't tested compression, but that would come out in the vac numbers wouldn't it? Besides the truck seems to have power, it just sucks down the gas and idles rough.
I checked the plug wires, they all seem good. I turned off the lights and watched for sparks and didn't see anything. I'm leaning towards buying a coil, anyone have any other suggestions?


Vacuum does look good.
Make sure the engine is getting up to 195 degrees running temt.
How old are the plugs and wires?
Does it have good hot blue spark to all spark plugs?

What is the gas milage MPG in town and on the road?

dr_lee29
12-19-2008, 12:50 PM
The plug wires are probably pretty old, I'll just change those out tonight, its worth doing anyway. New coil while I'm at it.
I don't know the exact gas mileage because I haven't filled it back up to check how much gas I've used but I'd guess I'm hovering around the 10 MPG mark, just about half what I normally get. That's mostly in town driving, just a little bit of highway.
So while I'm under the hood tonight, I thought I would take a look at the oil pressure sensor, where is it located?

MT-2500
12-19-2008, 01:03 PM
You really need to check MPG.
You have never answered the question of.
We need information and answers to be able to help.
How is the engine tempt?

Good time to replace plugs with ac delco plugs when repacing wires.
Good Luck

dr_lee29
12-19-2008, 04:59 PM
The engine is still running cool even after a new thermostat and temp sensor. I'm at 150 - 160.
I'll fill the tank when I pick up the plug wires and such.

MT-2500
12-19-2008, 05:27 PM
You need to get that engine tempt up to 155 degrees for it to run right and get good gas mileage.
Try another thermostat to start with.
Good luck

dr_lee29
12-20-2008, 08:53 PM
Changed the plug wires and the coil. I can't believe they used rivets to hold the coil on! What kind of stupid engineering is that? So after grinding down the rivets and knocking them through with a punch, I put on the new coil with the proper fasteners (they're called bolts and nuts, GM!). Regardless, the truck runs exactly the same, rough idle. I'm about ready to slide a stick of TNT in the gas tank.

dr_lee29
12-20-2008, 08:56 PM
By the way, I know that it needs to get to the proper temp in order to get good gas mileage, but it idles rough cold and hot.
In case anyone is keeping track
New LIM gasket, temp sensor, thermostat, IAC, plug wires, rotor, coil, cleaned PCV, EGR, checked O2 sensors. Are there anymore Made in China parts I can replace on this heap!

dr_lee29
12-21-2008, 04:20 PM
Its official, I checked the gas mileage today and I'm getting about 10 mpg.

MT-2500
12-21-2008, 06:13 PM
Its official, I checked the gas mileage today and I'm getting about 10 mpg.

Cold running will do that.
Has it still got miss fire?
Get that engine tempt problem fixed.
No amount of throwing parts at it is going to fix it untill you
Get and do some proper testing.
Have it put on a good engine capable scanner and checked out also make sure fuel pressure is up to par.
Read out the engine running data in scanner.
AND GO FROM THERE.
GOOD LUCK

dr_lee29
12-26-2008, 01:01 PM
Well, I changed the thermostat again and the spark plugs (probably should have been done a while back). The truck is still running too cold (max of 180, but most of the time 150-160). Its like the thermostat is open all the time, I can't hardly believe that I had 3 bad thermostats in a row. There has to be something else causing the truck to run cold.
The spark plugs looked pretty good overall, there was a little carbon, and they were a little wet (gas not oil). Which makes sense to me, the truck is running cold and its sucking down the gas to get it to temp, thus the bad gas mileage, rough idle. I just don't know how to get it to warm up, other than dynamite in the tank which very well may be my next move.

MT-2500
12-26-2008, 01:06 PM
How are you checking you running tempt?
Check your actual running temp and check the pcm coolant sensor reading.
Do they match?

dr_lee29
12-26-2008, 01:10 PM
Almost forgot, I cleared the codes (random cylinder misfire, misfire #4). I want to see if it will throw the same codes again.
Thanks again for the ideas, keep sending them my way.

dr_lee29
12-26-2008, 01:12 PM
Yeah, its really cold, I stuck a thermometer in the radiator and sure enough its at 165.

MT-2500
12-26-2008, 01:44 PM
Yeah, its really cold, I stuck a thermometer in the radiator and sure enough its at 165.


Rad stay cold in cold weather.
You need to check the engine block tempt.
What is the PCM tempt sensor reading?

dr_lee29
12-27-2008, 06:57 PM
I had cleared the codes and the SES light came on again today so I read the codes again and now I only have a misfire on cylinder 4. I really wonder if the injector is bad in that cylinder? Is there a way of testing the injector?

MT-2500
12-27-2008, 07:12 PM
Get it on a good engine capable scanner that has the built in injector balance test in it.

dr_lee29
12-28-2008, 02:20 PM
Today after church I pulled the upper air plenum to look at the spider. Both the spider and the manifold are a little wet. I snapped some pics, let me know what you think.
http://s487.photobucket.com/albums/rr233/dr_lee29/?action=view&current=P1000104.jpg
http://s487.photobucket.com/albums/rr233/dr_lee29/?action=view&current=P1000106.jpg
http://s487.photobucket.com/albums/rr233/dr_lee29/?action=view&current=P1000109.jpg

dr_lee29
12-28-2008, 02:26 PM
sorry I messed that up, here we go again:
http://i487.photobucket.com/albums/rr233/dr_lee29/P1000109.jpg
http://i487.photobucket.com/albums/rr233/dr_lee29/P1000106.jpg
http://i487.photobucket.com/albums/rr233/dr_lee29/P1000104.jpg

dr_lee29
12-28-2008, 02:28 PM
Some of the wetness seemed more like oil than gas but its hard to say. The bottom of the spider is wet with gas but I don't know if thats normal?

dr_lee29
12-28-2008, 02:48 PM
One more question, if the injector at cylinder #4 is bad, should I just change that injector or opt to change the whole spider? Its expensive but I don't want to keep doing this every weekend.

MT-2500
12-28-2008, 04:56 PM
Sorry.
We can not tell from the pictures.
What are you going to do now?

You should have done some proper fuel pressure testing and a injector balance leak down test first.


Good luck

dr_lee29
12-28-2008, 08:54 PM
Yes I know I should have done some fuel pressure testing but I don't have the proper gauges to do those tests. I thought that perhaps if there was a leak on one of the fuel lines I could see the gas on the manifold. Is the spider always wet like that or is that normal?

I cleaned the tips of the injectors and put the thing back together. It still runs the same, misfire on cylinder #4 SES light.

I'll have to take it to a mechanic because I don't have the gauges to do the rest of the diagnostics and by the time I buy all the gauges I'll have spent more than the mechanic fees. I've been taken by a few mechanics over the course of owning this truck so I'm reluctant to visit them.

dr_lee29
12-28-2008, 08:59 PM
By the way, what should I expect in terms of diagnostic fees? Can I get an estimate up front?

MT-2500
12-29-2008, 10:11 AM
Yes I know I should have done some fuel pressure testing but I don't have the proper gauges to do those tests. I thought that perhaps if there was a leak on one of the fuel lines I could see the gas on the manifold. Is the spider always wet like that or is that normal?

I cleaned the tips of the injectors and put the thing back together. It still runs the same, misfire on cylinder #4 SES light.

I'll have to take it to a mechanic because I don't have the gauges to do the rest of the diagnostics and by the time I buy all the gauges I'll have spent more than the mechanic fees. I've been taken by a few mechanics over the course of owning this truck so I'm reluctant to visit them.


The injector unit and manifold is always black and nasty looking.
If there is a external leak it will be bright and shinny around the leaking spot.

When the upper intake is off I always hook up the fuel lines and prime up fuel pressure and watch for leakage.

Leakage will show up with a fast leak down on fuel pressure test.

Find a good repair shop and have them test it out.
A good repair shop will give you a estimate of diagnostic cost.
A real good test out can run into a hr or more labor.
Check around Ask around
Word of mouth.
Family friends coworkers neighbors business people and delivery people.
Mailmen and parcel delivery people get around and notice a lot of stuff.
Even a good parts house knows what shops are good and not good.
Chamber of commerce and better business bureau and city hall.
If you find a good referral to a repair shop go look them over and talk to them.
Look for a clean looking busy place with nice people running it.
Ask a few questions and ask about their qualifications and training.
Not all places have trained tech/mechanics.
If they do not find a place that does.
Even all dealers do not have all trained techs/mechanics.

dr_lee29
01-07-2009, 11:47 AM
Took the truck to the mechanic yesterday. After an hour of looking they called me and said they still can't figure it out and wanted to know if they could keep looking. I told them they could. How long should I let them go before I give up on them?
One thing they said is that the truck only misses at idle, at 1500 RPM the miss goes away. I agree, the truck seems to run fine on the road, the idle is bad and the gas mileage is low, not to mention it never gets warm. They were wondering about the distributor? Any thoughts?

MT-2500
01-07-2009, 12:19 PM
Took the truck to the mechanic yesterday. After an hour of looking they called me and said they still can't figure it out and wanted to know if they could keep looking. I told them they could. How long should I let them go before I give up on them?
One thing they said is that the truck only misses at idle, at 1500 RPM the miss goes away. I agree, the truck seems to run fine on the road, the idle is bad and the gas mileage is low, not to mention it never gets warm. They were wondering about the distributor? Any thoughts?

That is hard to say.!!!!!

Did you check them out?
Have they got good trained techs?
How much equipment have they got to test it.
How much are they charging?

What test have they run on it?

dr_lee29
01-07-2009, 03:11 PM
Well, I didn't get a list of tests that they ran on it but here is what they would like to do:
Clean the Mass Air Flow sensor (about $30)
Change the distributor and rotor (about $90)
Change the intake air sensor
And they want to change the lower manifold gasket because they say it has a vacuum leak on cylinder #4.
All told they want something like $450 to do the repairs.
This all sounds suspicious to me because the LMG is new! The rotor is new! And they just want to change the distributor because it looks old (its about 1 year old).
My question is, could a vacuum leak on the LMG cause a misfire on cylinder 4 without causing a misfire everywhere else? Is there a diagnostic test that they should have done to find this leak?

dr_lee29
01-07-2009, 03:12 PM
Well, I didn't get a list of tests that they ran on it but here is what they would like to do:
Clean the Mass Air Flow sensor (about $30)
Change the distributor and rotor (about $90)
Change the intake air sensor
And they want to change the lower manifold gasket because they say it has a vacuum leak on cylinder #4.
All told they want something like $450 to do the repairs.
This all sounds suspicious to me because the LMG is new! The rotor is new! And they just want to change the distributor because it looks old (its about 1 year old).
My question is, could a vacuum leak on the LMG cause a misfire on cylinder 4 without causing a misfire everywhere else? Is there a diagnostic test that they should have done to find this leak?

dr_lee29
01-07-2009, 03:16 PM
I looked at the vacuum on the engine and it looked fine to me, 19 steady at idle, about 21 steady at 2000 RPM, dropped with accel, raised with decel.

MT-2500
01-07-2009, 04:25 PM
Well, I didn't get a list of tests that they ran on it but here is what they would like to do:
Clean the Mass Air Flow sensor (about $30)
Change the distributor and rotor (about $90)
Change the intake air sensor
And they want to change the lower manifold gasket because they say it has a vacuum leak on cylinder #4.
All told they want something like $450 to do the repairs.
This all sounds suspicious to me because the LMG is new! The rotor is new! And they just want to change the distributor because it looks old (its about 1 year old).
My question is, could a vacuum leak on the LMG cause a misfire on cylinder 4 without causing a misfire everywhere else? Is there a diagnostic test that they should have done to find this leak?


All of that does not sound quiet right.

Did you check them out first?
Have they got trained techs?

Maf sensor can be cleaned with a shot of MAF sensor cleaner or just unplugged for test.

Was fuel pressure tested good?

Have you got the temp back up to 195?

Any missfire codes?

Vacuum redings are good.
A injector balance test would be a good test to run.

dr_lee29
01-07-2009, 04:54 PM
I'm new to this area but I did check out the group. They are certified mechanics and suppose to be specialists in diagnostics. They were the only garage in town without a bunch of negative comments posted online.

I replaced the injector on cylinder #4, so I know at least that injector is good. I'm going to pick up the truck tonight without letting them do their suggested repairs. I guess I'll bite the bullet and buy a fuel pressure gauge and probably a distributor cap (can't hurt anything its part of a good tune up). But I think I just lost the $150 in diagnostics fees.

MT-2500
01-07-2009, 05:19 PM
I'm new to this area but I did check out the group. They are certified mechanics and suppose to be specialists in diagnostics. They were the only garage in town without a bunch of negative comments posted online.

I replaced the injector on cylinder #4, so I know at least that injector is good. I'm going to pick up the truck tonight without letting them do their suggested repairs. I guess I'll bite the bullet and buy a fuel pressure gauge and probably a distributor cap (can't hurt anything its part of a good tune up). But I think I just lost the $150 in diagnostics fees.


150$ in diagnostic fee should have found the problem.
Did they say for sure the MAF sensor and or dist and vacuum leak was the problem?
Has the temp engine running problem been fixed?
Is online the only place you checked them out?

dr_lee29
01-07-2009, 08:57 PM
Picked up the truck, $170 for diagnostics. I have a list of the tests that they ran.
The fuel pressure test says 54 psi no drop after 10 min. This sounds low to me, I expected 60?
The IAT failed the smoke test. (I put a new IAT in just a few weeks ago).
10" vac at PCV return hose
Bank 2 fuel trim always neg even at 2400 rpm
#4 compression 130 Dry
#4 running compression 0-50 fluctuates
Miss goes away after 2300 rpm

MT-2500
01-08-2009, 09:47 AM
HAS THE ENGINE RUNNING TEMPT BEEN FIXED?

I always check engine vacuum at a main engine source.
Like the brake booster hose.

Uneven fuel trims could be caused by engine miss.
Also 02 sensors.
On some engines I have had a 02 sensor throw a miss on no 4 cylinder.
Back in 95 trucks they had a 02 sensor wiring problem that would cause a cylinder 4 miss.
There was a TSB on it that required a 02 sensor wiring replacement.

Yes you are right on fuel pressure.
LOW FUEL PRESSURE WILL GIVE ALL KINDS OF PROBLEMS.

You need to get the fuel pressure up to par to start with.

The only true test on a fuel pump is to check the direct fuel pressure from fuel pump.
A quick test is to block off the return line and see if fuel pressure comes up to 75-85 lbs.
But do not run the pump at full pressure very long.
Also when testing fuel pump and pressure you need to tape a gauge to outside windshield or outside mirror and drive it on the road for 20 -30 minutes
until the pump gets has run a while to check for a pump fading out after hot.

Hard to start cold and fuel pressure testing guide line.
Check cold start fuel pressure.
Check engine running fuel pressure.
Check engine running on the road fuel pressure.
Then shut it off and watch for fast leak down.
Pinch off at rubber part or block off return line and check full pressure.
If pump has full pressure with return line blocked and low pressure without it blocked most usually the fuel pressure regulator is leaking or not holding pressure.
Later V6 and V8 gm engines should have 60-66 fuel pressure.
Cold start should be 64-65 lbs of pressure.
If you do not have full fuel pressure on a cold start the injectors will not squirt fuel.
Do not leave home without it.
Engine running should be 60-66 lbs pressure depending on engine load.
Full pressure with return pinched off should be 75-85 or more.
And if it has a fast leak down after shut off you have a leak in system.
Post back fuel pressure readings.

dr_lee29
01-08-2009, 11:41 AM
The temp has not been fixed. I drove it today to work and it never got to a proper running temp. But this seems like a secondary problem because the idle is rough immediately upon starting the truck. Of course the low temp could be the cause of the bad gas mileage though. I just can't figure out what would make the engine run cold. The PCM says its cold (165 last I checked), the gauge says its cold, the fact that I can't get the cab roasting hot says its cold.
I'll try to check the fuel pressure tonight. I'm a little confused though. How do I check the direct fuel pressure? I know there is a valve on the fuel lines on top of the block where I can attach a gauge, but where (how) do I block the return line? Do I do this at the tank or on top of the block (these are metal lines)? Then for the rest of the testing I assume I attach the gauge to valve on the metal lines and run the gauge under the hood to the windshield?
Again, thanks for your help.

MT-2500
01-08-2009, 11:49 AM
The temp has not been fixed. I drove it today to work and it never got to a proper running temp. But this seems like a secondary problem because the idle is rough immediately upon starting the truck. Of course the low temp could be the cause of the bad gas mileage though. I just can't figure out what would make the engine run cold. The PCM says its cold (165 last I checked), the gauge says its cold, the fact that I can't get the cab roasting hot says its cold.
I'll try to check the fuel pressure tonight. I'm a little confused though. How do I check the direct fuel pressure? I know there is a valve on the fuel lines on top of the block where I can attach a gauge, but where (how) do I block the return line? Do I do this at the tank or on top of the block (these are metal lines)? Then for the rest of the testing I assume I attach the gauge to valve on the metal lines and run the gauge under the hood to the windshield?
Again, thanks for your help.

The tempt problem needs to be fixed for it to ever run right.
If it has a good thermo stat there is no reason for the temp not to come up to 195 degrees.
Also you will need fuel pressure up to par.

Direct fuel pump pressure can be checked by hooking a gauge onto the fuel filter in line or blocking off or plugging the fuel return line.
Down between frame and engine there should be a rubber part of fuel and or fitting where you can block it off.
Let us know what the fuel pressure test show.
Good Luck

dr_lee29
01-08-2009, 07:09 PM
Well I haven't checked the direct fuel pressure because I don't have a fitting to get the gauge on the fuel filter and I can't find a rubber line to pinch off the return line.
But I did check the fuel pressure on top of the block. Its about 54 at start up and hovers around 50-53 at idle. When I shut off the truck it jumps to 60 and stays there for at least 15 minutes. I really expected a higher pressure than these readings.
I drove around with the gauge and at about 2000 rpm I get 54-56 psi, if I tromp the gas I can get about 59-60 but never higher and it won't stay at that pressure very long before it gets back down to 55-56. When I let off the gas pedal it drops to 50 then creeps up to about 52.

MT-2500
01-08-2009, 07:30 PM
Well I haven't checked the direct fuel pressure because I don't have a fitting to get the gauge on the fuel filter and I can't find a rubber line to pinch off the return line.
But I did check the fuel pressure on top of the block. Its about 54 at start up and hovers around 50-53 at idle. When I shut off the truck it jumps to 60 and stays there for at least 15 minutes. I really expected a higher pressure than these readings.
I drove around with the gauge and at about 2000 rpm I get 54-56 psi, if I tromp the gas I can get about 59-60 but never higher and it won't stay at that pressure very long before it gets back down to 55-56. When I let off the gas pedal it drops to 50 then creeps up to about 52.


Yes fuel pressure is way to low.
It is not going to run right to you get the pressure up.
And could be the engine miss on no 4.
A bad fuel pressure regulator can lower pressure.
Need the return line locked off to test fuel pressure regulator.
But only true fuel pump test is to block off return or test direct fuel pressure.

dr_lee29
01-08-2009, 07:34 PM
Well on second thought, Haynes actually says I should expect 58-60 without the engine running and 3-10 below that at idle so I'm within the good range?

By the way, I replaced the distributor cap, the old one did have some corrosion on the posts so it was probably a good thing to replace but it didn't fix the problem.

MT-2500
01-08-2009, 07:53 PM
Well on second thought, Haynes actually says I should expect 58-60 without the engine running and 3-10 below that at idle so I'm within the good range?

By the way, I replaced the distributor cap, the old one did have some corrosion on the posts so it was probably a good thing to replace but it didn't fix the problem.


Mr Haynes is full of crap on fuel pressure specs.

Your factory specs is 60/66,
Engine off you need 65 lbs of fuel pressure.
Engine cranking you need 65 lbs.
Engine under load you need 65 lbs of pressure.
Engine idle or no load the pressure regulator will cut it down to around 61 -62 lbs.

But for the fuel pressure to regulate fuel pressure right you need 95 -100 lbs full fuel pump pressure to deliver right regulated pressure and and fuel volume.

dr_lee29
01-08-2009, 08:25 PM
Still having trouble finding a rubber line to pinch, as far as I can tell these are hard plastic then metal lines from the tank to the spider.
Does it tell us anything that the pressure is just too low and doesn't bleed off after the engine is shut down? Does that lean towards a pump and/or a filter as opposed to the pressure regulator?

dr_lee29
01-09-2009, 09:20 AM
One more thing, would fuel pressure cause a big drop in gas mileage?

MT-2500
01-09-2009, 10:11 AM
One more thing, would fuel pressure cause a big drop in gas mileage?

Yes low fuel pressure will caus the computer to richen up fuel mix.

MT-2500
01-09-2009, 10:21 AM
Still having trouble finding a rubber line to pinch, as far as I can tell these are hard plastic then metal lines from the tank to the spider.
Does it tell us anything that the pressure is just too low and doesn't bleed off after the engine is shut down? Does that lean towards a pump and/or a filter as opposed to the pressure regulator?

Blocking return line separates fuel pressure regulator from weak fuel pump.
With Return line blocked and fuel pressure comes up to around 95 lbs the fuel pressure regulator is bad.
If no more pressure with it blocked /pinched off the fuel pump is weak.
If you can not find a place to block/pinch/plug off return line.
You can hook a gauge onto the in fuel line at fuel filter to check direct fuel pump pressure.
Sometimes a rubber hose on the end of into filter line will help connect a gauge direct to it.
It should have around 95-100 direst pressure from fuel pump.
But do not run fuel pump at full longer than required to test it.

Pump has to be capable of full pressure and fuel volume to hold regulated pressure to engine.

The later 4.3 engine have to have right pressure and fuel volume to run right.
Even sometimes a couple of lbs low will give problems.

dr_lee29
01-09-2009, 10:08 PM
Alright, I checked the direct fuel pressure. I rigged up a fuel line directly to the output side of the fuel filter and turned the key on, I got 90+ PSI. I was afraid to leave it too long, but I think I could have had a little higher.
So I replaced the fuel filter since I was down there already. The old filter was pretty hard to blow through, I'd say the pump was having a hard time. After I put the new filter on, I tested the pressure on top of the block. This time the gauge needle was shaking violently between 50-55 psi, even when I gave it gas, the pressure was about 56-58 and shaking like crazy. Is this the pressure regulator symptom?

MT-2500
01-10-2009, 10:25 AM
Alright, I checked the direct fuel pressure. I rigged up a fuel line directly to the output side of the fuel filter and turned the key on, I got 90+ PSI. I was afraid to leave it too long, but I think I could have had a little higher.
So I replaced the fuel filter since I was down there already. The old filter was pretty hard to blow through, I'd say the pump was having a hard time. After I put the new filter on, I tested the pressure on top of the block. This time the gauge needle was shaking violently between 50-55 psi, even when I gave it gas, the pressure was about 56-58 and shaking like crazy. Is this the pressure regulator symptom?


Yes it is a good indication of bad fuel pressure regulator.

I would repace it and retest fuel pressure.

Cut open old fuel filter and check for a lot of black stuff in it.
A lot of real black stuff can be from a fuel pump going out.

Fuel pump pressure test should be done on a fuel pump that has run for 20-30 minutes to.
Some pumps will pump good untill hot and then stard getting weaker pressure.
Let us know how it goes.
Good Luck

dr_lee29
01-11-2009, 04:20 PM
Alright, new pressure regulator in, buttoned back up. On start up the truck raced for about 10 seconds then came back down to a low rough idle. I drove it around the block and the light came back on.
Fuel pressure
Key on, engine not running 59
At idle 51
Vacuum 18 cm Hg at idle, 21-22 @2500 RPM, drops to 0 with accel, goes to 24-25 with decel.
I haven't read the codes but I'm betting it will be a misfire on cylinder 4.

dr_lee29
01-11-2009, 04:22 PM
I forgot, the fuel pressure doesn't bleed down quickly but the fuel pressure isn't high in the first place.

MT-2500
01-11-2009, 06:42 PM
I forgot, the fuel pressure doesn't bleed down quickly but the fuel pressure isn't high in the first place.


Fuel pressure is still way low.
Run some more test on fuel pump.
Check voltage and ground to fuel pump
Run fuel pump for 20-30 minutes and see what the pressure does.

Is the miss on no 4 cyl a dead miss?
Can you feel the miss on engine?

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