1994 GMC Sierra 1500 Failing Emissions...Low Idle Cyclical Stalling/Surging

...Not much action over on the GMC site, so I thought I'd post my question here...

The truck is a 1994 GMC Sierra 1500 truck w/approx 185K miles on it.

I would appreciate any feedback on my problem...
I have had emissions problems for a few years now as can be seen in the following emissions transcript...

Emissions Inspection 6/4/2002
Hydrocarbons (HC) – Inspection 5.3095 – Requirement 2.7000 FAIL
Carbon Monoxide (CO) – Inspection 35.3126 – Requirement 60.0000 PASS
Oxides of Nitrogen (Nox) – Inspection 0.4853 – Requirement 5.7000 PASS
Carbon Dioxide (CO2) – Inspection 476.3944

Work Performed 8/2002
- Adjusted Timing to 0 TDC
- Replace Oxygen Sensor @ Catalytic Converter
- Replace Spark Plugs
- Replace Ignition Wires
- Replace Distributor Cap & Rotor
- Replace PCV Valve
- Replace Air Filter

Emissions Inspection 8/2002
Hydrocarbons (HC) – Inspection 0.1880 – Requirement 2.2000 PASS
Carbon Monoxide (CO) – Inspection 1.9014 – Requirement 60.0000 PASS
Oxides of Nitrogen (Nox) – Inspection 0.0070 – Requirement 4.2000 PASS
Carbon Dioxide (CO2) – Inspection 73.1697

Emissions Inspection 7/13/2004
Hydrocarbons (HC) – Inspection 6.7361 – Requirement 2.0000 FAIL
Carbon Monoxide (CO) – Inspection 62.4512 – Requirement 60.0000 FAIL
Oxides of Nitrogen (Nox) – Inspection 0.3481 – Requirement 4.0000 PASS
Carbon Dioxide (CO2) – Inspection 599.9200

Work Performed 11/2004
- Adjusted Timing to 0 TDC
- Replace Spark Plugs
- Replace Ignition Wires
- Check and Clean Distributor Cap & Rotor
- Replace Air Filter

Emissions Inspection 7/13/2004
Hydrocarbons (HC) – Inspection 4.6401 – Requirement 2.0000 FAIL
Carbon Monoxide (CO) – Inspection 18.6279 – Requirement 60.0000 PASS
Oxides of Nitrogen (Nox) – Inspection 0.2441 – Requirement 4.0000 PASS
Carbon Dioxide (CO2) – Inspection 609.0446

Still notice low throttle, low RPM miss (seems to get a little worse as engine temperature increases). No miss upon acceleration or at high RPMs that I can notice. After engine warm-up and when truck is parked, holding the accelerator slightly so that engine revs around 500 – 700 RPM (approx), the engine will stall, revs will decrease, engine will recover and surge back to 500 –700 RPM, stall, and repeat in a cyclical manner.

Work Performed 11/2004
- Check and clean EGR valve. Valve seemed to be fine, but cleaned.
- Used four cans of Top Engine Cleaner on TBI on four different occasions with no help, in fact, problem seems a little worse. Checked for vacuum leaks around base.

I'm hoping that someone can give me an idea as to what could be the cause of my problems...I'll gladly reply if further info for diagnosis is required...Thanks

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JStrong -- your problem sounds very similar to mine. See the post "'92 5.7 Surge/Chuggle..." I'll let you know if anybody comes up with a solution. Sevie

Do a leakdown test on each cylinder. Don't focus just on a electrical or electronic problem. It could be a basic mechanical problem. The reason I say this because my 98 5.7 had a bad misfire at idle and low rpm, smoothed out after 1800 rpm and ran fine. would fail emmission test if idle time exceeded 2 min without accelerating. # 5 cylinder was the culprit (after a year and a half of replacing every computer sensor, primary ignition part, fuel controls..etc.) it came down to a basic mechanical problem.Oh and GM (dealership sevice) mechanic's couldn't find problem either, so don't trust every mechanics word.

What was the engine temp at time of test and at what rpms was the test made. I would remove and clean the idle air control valve and clean the throttle body.

JStrong -- your problem sounds very similar to mine. See the post "'92 5.7 Surge/Chuggle..." I'll let you know if anybody comes up with a solution. Sevie

sevie8,

Thanks....

I'll be monitoring your post thread...

Do a leakdown test on each cylinder. Don't focus just on a electrical or electronic problem. It could be a basic mechanical problem. The reason I say this because my 98 5.7 had a bad misfire at idle and low rpm, smoothed out after 1800 rpm and ran fine. would fail emmission test if idle time exceeded 2 min without accelerating. # 5 cylinder was the culprit (after a year and a half of replacing every computer sensor, primary ignition part, fuel controls..etc.) it came down to a basic mechanical problem.Oh and GM (dealership sevice) mechanic's couldn't find problem either, so don't trust every mechanics word.

skipr,

I'm not familiar with a leakdown test...care to explain?

I do not have any misfiring at all or at any RPM, just cyclical idle. Using Top Engine Cleaner several times has seemed to exacerbate the problem somewhat. When I accelerate slowly, or decelerate slowly, the truck will stall to a low idle, recover to a higher idle, stall again, recover, in a cyclical manner so that the truck will surge, slow, surge, slow, etc. when in gear so that my head and that of any passenger will bobble when in this low idle state.

If you were to look at my hydrocarbon emissions profile you would see spikes in hydrocarbons at low speed acceleration and deceleration. Hydrocarbons are well within acceptable limits elsewhere. These spikes are what are causing my emissions failure.

Carbon Monoxide and Nitrogen levels are well within the limits.

I'll keep your mechanical reasoning in mind as I progress...Thanks!

BTW, I haven't mentioned this before...only because I figured it was due to burn spots on my starter...but, I am having problems starting the truck. It may be unrelated, but maybe not. Whether the truck is warm or not, four or five seconds of seemingly torcherous starting is required to turn the engine over. It may be a combination of engine performance at low idle and starter problem because when I try to start the engine the new battery I installed less than 4 months ago seems like it is on its last leg. During the process of starting the truck sometimes it will turnover quickly as if the starter is positioned on a good spot on the starter, but most times it turns sluggishly all through the starting cycle. In all cases it takes a few seconds for the engine to start up.

What was the engine temp at time of test and at what rpms was the test made. I would remove and clean the idle air control valve and clean the throttle body.

Fireplug,

I'm assuming that you mean my engine temperature during the emissions test....If that is the case, my engine was very warm as I have a 45 minute highway drive to the emissions facility. The emissions test has a profile that varies speed and acceleration from stop to highway speeds, down to low speed, back to highway speeds and down again. Some require the tester to slowly take off, stomp on the gas, and fully remove his foot from the accelerator. It is during these slow take-off speeds and when the foot is removed from the accelerator that the high hydrocarbons are present; most notably at lower RPMs.

I've applied 4 bottles of GM Top Enginer Cleaner cleaning the throttle body inside and out thoroughly. I've taken off the IAC valve and cleaned it (it had a build-up of soft carbon on its conical nose) to no avail. I've also viewed the IAC valve through the top of the TBI unit (with the air cleaner housing removed) while in operation and can see it opening up during periods of high RPMs when my wife hits the accelerator...and closes the gap when decelerating.

Any other suggestions?

Here's another idea to check. That engine surge could be from a hole/split in the fuel pump pulsator. Quick test with a gas tank thats about or less than 1/3 full you will or should be able to hear a distinct water fountain sound in the gas tank when the pump is running. If you hear that sound, you've found the problem. If you have to much gas in the tank you have to use a fuel pressure gauge to check for low or fluctuating pressure as the pulsator will be muffled under the gas. Eventually the pulsator hole/split can grow to the point where you won't have enough pressure to start the truck.

Your hard start problem sounds like a combo of two things-- First the fuel pump relay-if it goes out the truck will not start until the oil pressure comes up--about 4-10 seconds. The fuel pump relay bypasses the safety feature of the low oil pressure shutdown of the fuel pump-- allowing the pump to operate before oil pressure comes up. Second-- you probably need a new starter.

Ok lets start over.
First the starting problem
Are you saying it cranks over slowly at times?? And after a lot of cranking it fires(starts)?? And other times it cranks over good and fires with a short cranking time??

Sec.
high hydrocarbons = unburnt gas on decel
Have you checked the EGR valve and make sure it is working ok. A stuck open (even a little open) will cause the engine to surge at idle and if its not working on decel it will not open all the way to recycle the exhaust gasses. ooops just reread your post ok what type of egr valve do you have ? yours should be the vacuum type, when the engine is hot and at idle and the surge is there tap on top of the egr valve and see if there is any rpm change also stick a finger up under the valve and pull open the plunger (it will be hot so be carefull) and see if there is any rpm changes

As for the fuel pump pulsator I have replaced well over a hundred of these things and all of them was due to a long times due to 0 fuel pressure at the start of cranking because of a split in the rubber of the fuel pump pulsator.
For it to cause a driveabilty problem it would cause a lean driving problem.

Here's another idea to check. That engine surge could be from a hole/split in the fuel pump pulsator. Quick test with a gas tank thats about or less than 1/3 full you will or should be able to hear a distinct water fountain sound in the gas tank when the pump is running. If you hear that sound, you've found the problem. If you have to much gas in the tank you have to use a fuel pressure gauge to check for low or fluctuating pressure as the pulsator will be muffled under the gas. Eventually the pulsator hole/split can grow to the point where you won't have enough pressure to start the truck.

Wheeler88,

Thanks...I'll try that.

Your hard start problem sounds like a combo of two things-- First the fuel pump relay-if it goes out the truck will not start until the oil pressure comes up--about 4-10 seconds. The fuel pump relay bypasses the safety feature of the low oil pressure shutdown of the fuel pump-- allowing the pump to operate before oil pressure comes up. Second-- you probably need a new starter.

mbumgua,

I'll check on the fuel pump relay...

Yeah, I'm almost positive the starter needs replacement.

Thanks...

Ok lets start over.
First the starting problem
Are you saying it cranks over slowly at times?? And after a lot of cranking it fires(starts)?? And other times it cranks over good and fires with a short cranking time??

Sec.
high hydrocarbons = unburnt gas on decel
Have you checked the EGR valve and make sure it is working ok. A stuck open (even a little open) will cause the engine to surge at idle and if its not working on decel it will not open all the way to recycle the exhaust gasses. ooops just reread your post ok what type of egr valve do you have ? yours should be the vacuum type, when the engine is hot and at idle and the surge is there tap on top of the egr valve and see if there is any rpm change also stick a finger up under the valve and pull open the plunger (it will be hot so be carefull) and see if there is any rpm changes

As for the fuel pump pulsator I have replaced well over a hundred of these things and all of them was due to a long times due to 0 fuel pressure at the start of cranking because of a split in the rubber of the fuel pump pulsator.
For it to cause a driveabilty problem it would cause a lean driving problem.

Fireplug,

1) In all cases the truck is hard to start, requiring a few seconds of grinding to start. Sometimes it kicks hard at the start of cranking as if it hit a good spot on the starter...most other times it seems like it doesn't want to crank at all as if the battery is dead. At all times it will turn over if given 4 to 5 seconds of crank.

2) I do have the vaccuum type...I haven't had a chance to play with the EGR valve dynamically like you are describing...I have only taken it off and verified that it maintains vaccuum and cleaned it inside and out to remove any rust particles and such to maximize the chance of it closing completely. It appears in operation, by looking at it, to be working properly, although I am not completely confident that it is indeed closing completely to prevent exhaust intake at low RPM. I will try your suggestions...

I am not so worried about the starting of the truck (as long it is not another symptom causing my emissions problem). I can live with these annoyances as I only drive the vehicle on the weekends to move things in, out, and around the house (mulch, wood, dirt, gravel, trash, etc.). I wouldn't be worrying about the cyclical idle problem I am having other than I need to pass emissions in order to keep the vehicle registered. It is the perfect fit for the use I put it through, but would rather not spend alot of money, if I don't have to to keep it for this purpose. I'll keep your fuel pump pulsator explanation in mind, but will probably not follow through with a fix unless I am forced to...

Thanks for your help...

skipr,

I'm not familiar with a leakdown test...care to explain?

I do not have any misfiring at all or at any RPM, just cyclical idle. Using Top Engine Cleaner several times has seemed to exacerbate the problem somewhat. When I accelerate slowly, or decelerate slowly, the truck will stall to a low idle, recover to a higher idle, stall again, recover, in a cyclical manner so that the truck will surge, slow, surge, slow, etc. when in gear so that my head and that of any passenger will bobble when in this low idle state.

If you were to look at my hydrocarbon emissions profile you would see spikes in hydrocarbons at low speed acceleration and deceleration. Hydrocarbons are well within acceptable limits elsewhere. These spikes are what are causing my emissions failure.

Carbon Monoxide and Nitrogen levels are well within the limits.

I'll keep your mechanical reasoning in mind as I progress...Thanks!

BTW, I haven't mentioned this before...only because I figured it was due to burn spots on my starter...but, I am having problems starting the truck. It may be unrelated, but maybe not. Whether the truck is warm or not, four or five seconds of seemingly torcherous starting is required to turn the engine over. It may be a combination of engine performance at low idle and starter problem because when I try to start the engine the new battery I installed less than 4 months ago seems like it is on its last leg. During the process of starting the truck sometimes it will turnover quickly as if the starter is positioned on a good spot on the starter, but most times it turns sluggishly all through the starting cycle. In all cases it takes a few seconds for the engine to start up.

A leakdown test is opposite of a compression test, it uses compressed air to force down your spark plug hole while at TDC, you will need the proper tool for this (just like a compression tester but reversed).It's more revealing than compression test, it pinpoints the leakage either through intake, exhaust or rings.