Stumble at idle

[clutterhead]

I have a 92 C1500 with a 5.7 TBI.

I'm having this issue where it stumbles and makes a popping noise out the tailpipe, like a soft misfire. Not a backfire though. It also never stumbles to the point where it dies, and it's only at idle. Anything passed like 1/8 throttle and it has perfect response. WOT pulls seem normal to me. I've tried to chase this problem for years, and I've never been able to fully solve it. It runs really rich. It stumbles in both open and closed loop. Right when I fire it up it stumbles all over but again never dies. I notice that driving it cold for a few minutes is the only time it doesn't sputter and pop when I first come to a stoplight. It will purr and fire on all cylinders. But after about a minute or so it goes into closed loop and it starts sputtering again when I'm stopped. When at a stoplight, I've watched my BLM slowly drop to about 110-115, and that's when it starts acting up on me in closed loop. As the numbers drop down, the sputtering starts out the tailpipe.

Things I've done:

-CTS was replaced with a new pigtail wiring harness.

-All secondary ignition components have been replaced including ignition coil with known good. Cap/rotor/plugs new within a few months all OEM. Distributor is a few years old, swapped with known good and no change.

Verified square wave coming from ICM and signal coming from computer. I don't have a powerful O-scope I could use, but a pocket one where I can at least see a consistent waveform. Haven't verified the pickup coil signal itself while running. The ICM signal is consistent so I assume that means my PU coil isn't dropping out. I can feel a consistent magnetic pulse when I turn the distributor by hand out of the car. I ohm'd the coil and it is in the proper range. Can't verify for magnet cracks, and no evidence that the magnet is grinding like a bad bushing would cause. The aftermarket distributor has this stamped piece over the top where you cant really see the magnet. And still, I swapped in an OEM distributor with OEM ICM/PU coil and no change.

-Timing is verified, and the chain was replaced last year with a high-end series Cloyes set. Harmonic balancer was also previously replaced.

-IAC was swapped for a known good, no change.

-Map sensor was verified good. IIRC 1.2V at idle, 4.9V KOEO. Tried a known good too.

-TB was rebuilt a few times since I've owned it. FPR is new with good spring, along with fuel pump assembly/filter, and PCV valve is working correctly.

-Can't say I have any vacuum leaks, at least none I can find. I have a smoke tester and was not able to find any. Intake gaskets are new. No vacuum in the crankcase when checking for internal intake leaks. I think I tried isolating the brake booster once before with no change too. I'll try again. Also tried capping off the purge valve. No change.

-O2 sensor is fairly new, and I can verify its signal. Switches between .1 and .9mV consistently in closed loop

-TPS verified good throughout throttle range, .6V closed. Reads "1" on scan tool at idle.

-No knock counts reported.

-EGR is new, and it is currently blocked off either way. Capped at the EGR solenoid and a block off plate installed where the EGR bolts on.

-Computer was swapped with a known good. No change. Ohm'd out most wires from sensor connectors to computer and get consistent readings. Checked all grounds I could find. Replaced some eyelets on them too including the one by the thermostat.

- Checked for exhaust leaks. I cant find any. Went as far as holding my hands all around the manifolds when I just start it from cold because I know it can be hard to catch them sometimes. It does have new manifold gaskets. I bought the gaskets to do the Y pipe either way and plan to change them soon just in case.

- Did a compression test a while back. All consistent numbers around 130 or so IIRC. Leak down test had minimal blow-by. Valves have also been recently adjusted.

- Has new (used) OEM injectors in it from a known good car. These are the proper injectors for the 5.7, stamped numbers verified. Spray pattern looks great. I haven't checked fuel pressure since I installed the pump, which was around 13 psi IIRC. I will verify this again soon.

The injector swap was the only thing that made the issue any different, so I want to say its a fueling issue somehow. It didn't solve my issue, it actually made it sputter more when cold which it didn't do as much with the original injectors. MY BLM actually improved a bit too in closed loop. Still, it's hard to believe two sets of OEM injectors have a varying degree of issues to cause this though. I know the injectors going bad on these isn't that common. I plan to get my original ones cleaned professionally, but my gut feeling is it won't fix the problem.

If I think of anything else I've done I'll update, but at this point I have no idea where to go with it. I would consider a mechanical engine issue, like maybe a burned valve or a leaking HG, but I did the leak down test and I have no symptoms of HG issues throughout the 4 or so years the truck has been having this problem. Only thing I can say is the sputter is worse than it ever was before warm or cold.

 

[Schurkey]

I have a 92 C1500 with a 5.7 TBI.

I'm having this issue where it stumbles and makes a popping noise out the tailpipe, like a soft misfire... ...and it's only at idle. Anything passed like 1/8 throttle and it has perfect response.

It runs really rich...

...I've watched my BLM slowly drop to about 110-115, and that's when it starts acting up on me in closed loop. As the numbers drop down, the sputtering starts out the tailpipe.

HOW do you know it runs "rich"? Do you mean the computer is in lean-command?

the chain was replaced last year with a high-end series Cloyes set. Harmonic balancer was also previously replaced.

Was the damper checked to verify that true TDC shows alignment of the timing mark on the damper and the timing indicator "0"?

Was the cam timing degreed, or was it just shoved in "Dot-to-dot"?

-O2 sensor is fairly new, and I can verify its signal. Switches between .1 and .9mV consistently in closed loop

What are the cross-counts?

-TPS verified good throughout throttle range, .6V closed. Reads "1" on scan tool at idle.

.6 seems high. "1" I don't know about.

- Did a compression test a while back. All consistent numbers around 130 or so IIRC. Leak down test had minimal blow-by.

130 is lower than I'd like, at least at my altitude. What is your altitude? Was the battery fully-charged (cranked quickly)? Throttle blocked at least part-way open?

Valves have also been recently adjusted.

Any chance you didn't get "zero lash" correct? How much preload did you apply?

I haven't checked fuel pressure since I installed the pump, which was around 13 psi IIRC. I will verify this again soon.

Good plan.

I have no idea where to go with it.

Connect a scan tool, verify every sensor and computer output. Assure there's no leakage at the EGR/EGR blockoff plate.

 

[clutterhead]

HOW do you know it runs "rich"? Do you mean the computer is in lean-command?

I took it as running rich based off the BLM being low and you can smell it. I know a lean running engine can smell too, but it smells like a classic rich running engine. I guess that would be more accurate to say that it is lean command. My interpretation is the computer is trying to subtract fuel.

Was the damper checked to verify that true TDC shows alignment of the timing mark on the damper and the timing indicator "0"?

I did not verify the damper other than lining the tab to the "0". Never did a true TDC procedure with a TDC bolt. I know I compared the balancer to the old before installing.

Was the cam timing degreed, or was it just shoved in "Dot-to-dot"?

Put dot to dot.

What are the cross-counts?

i would have to set the scanner up and get that data back to you, but I do remember them crossing fast. Enough that it made me disregard it.

.6 seems high. "1" I don't know about.

I know some TPS have slotted bolt holes for fine tuning, but I know this one doesn't. I have filed in some slots on TPS before if I wanted to fine tune the voltage. What should a normal to you voltage be? The "1" I take it as the computer seeing that as its "0". When I changed the ECU out I saw the 1 on the scanner for the first time. I've seen a lot of cars do this where a number other than 0 is the closed level to the computer, and WOT could be something like 90. If these engines are supposed to truly have 0 or its interpreted as something other than closed throttle, I guess that could be something. The voltage sweeps very consistent as I move the throttle plate to WOT.

130 is lower than I'd like, at least at my altitude. What is your altitude? Was the battery fully-charged (cranked quickly)? Throttle blocked at least part-way open?

I am at sea level. Guess I ruled that out because it was consistent across all 8.

Any chance you didn't get "zero lash" correct? How much preload did you apply?

I set it by pulling up on the pushrods till I got zero lash, then gave it 1/2 turn. When I got the engine running, I backed the nut off each rocker till I heard the clicking, then tightened 3/4 turn.

Good plan.


Connect a scan tool, verify every sensor and computer output. Assure there's no leakage at the EGR/EGR blockoff plate.

I've tried so many times to go at this problem, I keep looking at every sensor on live data thinking I missed something, and every time I'm annoyed that the data looks ok other than my BLM haha. Now that I think of it, the only other data PID that never seemed right was my IAC in the 30s. I believe that is the computers interpreted position of the IAC pintle? I know I read once that it should be around 15, but I was never able to achieve that. I've tried a few IAC valves in it's lifetime with me.

I did think about the EGR plate as well. I have a gasket under the plate, and I've ran the EGR as the block off plate with no vacuum line before fabricating a real block off plate. The EGR is blocked off because I had this problem that when you give it gas from a dead stop, it would choke and almost die and go nowhere before recovering. Saw on the scan tool that the EGR solenoid was being commanded in the 30% range right off idle, and I took that as it opening the EGR too aggressively and almost killing the engine. I never looked into why that was happening. I've tried both the positive and negative pressure valve, and bought NOS GM genuine off eBay with the proper stamping on the body. I've come across old threads where people eventually solved their problem as being the EGR, but never figured out what happened past capping it off. With the scanner I could actually see it was the EGR solenoid getting a signal to pull it open seemingly, way too much off idle. Those command values seemed kinda aggressive to me, but it's all relative I guess. Maybe thats normal. I was always interested in knowing what goes into the computer calculating how much to open the valve because that could possibly lead me somewhere with this problem, but I'm not sure. Engine load etc. I assume, but nothing I can think of that would be the "smoking gun" in being related to my issue.

 

[1997]

wow, way to stick with it.

when my 95 was running similar to what you describe, it was the 02 sensor. Scanner showed it was working as advertised ....

maybe time to try another one, OEM/professional AC delco would be my choice.....

good luck ...

 

[clutterhead]

wow, way to stick with it.

when my 95 was running similar to what you describe, it was the 02 sensor. Scanner showed it was working as advertised ....

maybe time to try another one, OEM/professional AC delco would be my choice.....

good luck ...

Tried that at one point. Even had a 3 wire o2 to make sure it was giving the best shot at running stoich. A few years back it failed smog for excessive hydrocarbons. Put the 3 wire in and it passed. That might be able to get me by, but I'm really tired of the stumble at idle, and i'm sure it failed for high HC because of the soft misfires at idle. As of now it has a GM genuine one wire back in it that was bought not too long ago. Goes into closed loop fairly quickly for being one wire.

 

[Jeepwalker]

You've checked a lot of things. Here are a few things that come to my mind. Maybe you've done them and didn't list ...but here they are:

1) Vacuum Gauge? Have you put a legitimate vacuum gauge on it to see what it's doing? What's it read? Needle steady or dancing around? One of the first things I had to do when I bought my truck 20x years ago (used) was remove the intake and fix a vacuum leak. Watch a couple Y/T videos how to check vacuum.
2) Temp sensor? Again, what's it ohm out at cold (ambient temps), and truck warm/hot? Use an ohm meter and cross-check with a table.
3) Barely-good sensors? I've also had sensors which were on the edge of 'good', as mentioned above, which caused problems.
4) Vac Leak Check? Have you tried putting a piece of hose on a small LP torch and wave it around specific components in the engine compartment, particularly at the base of the throttle body, vacuum areas, intake corners?
5) More Vacuum: Unhook all extra vacuum hoses and run, see how it performs.
6) Grounds: Ensure your ground is good. Batt->Body, Batt->Engine and so on.
7) Spark plugs/coil: Sounds obvious, but I'd ohm them out again. I've on two occasions installed a set of spark plugs and had a bad one. Took forever to find the problem b/c we were sure it *wasn't* the plugs ....how could it be? One time I had to take the vehicle to a Ford garage and have them hook it up on an old-school analyzer. Yep, new plug was bad! Dought! Ohm them out. Same with the coil.
8) Compression: Yeah I was going to say 130# of compression isn't what I'd call stellar. That's why I'd check the vacuum. How many inches mercury at idle and off-idle? If it can't atomize the fuel very well, that could be an issue. I would run the engine a few minutes, then pull the plugs (Ohm them out again, check for problems) and while at it do another compression test. 4-5 puff's per cylinder. It should run ok at 130...but it's on the low end of "ok" ...I'm not even going to say 'good'. Not trying to call your baby ugly, but generally a person would rather see 145+ (150+ even better).
9) Fuel Pressure: Did you mention what the fuel pressure was??
10) What happens if you temporarily unhook all the key sensors? I mean it should run ok in open loop.. Does it?
11) Computer: Have you tried a different computer? Just asking...
12) Distributor: You did good by measuring the distributor output. But still, i'd consider pulling it out, taking apart, and having a look inside. Eyes are for real. Mine was totally goofed up from corrosion. The star had cracked and a few edges broke off the pickup coil. I can't believe it even ran! Its fairly easy to do. See pic below (not mine ..but how mine was).
13) O2 sensor. Because they're pretty cheap and the O2 controls the rich/lean, I'd probably buy another one. Just me...
14) Cracked distributor cap, or faulty rotor? These are hard to spot sometimes. Esp hairline cracks. I assumed you've tried a couple.. ?? Might want to do a continuity check for all cap posts in case there's a crack on one where you can't see it.
15) Faulty ignition wires? Again, back to the basics ...it only takes one. Even new wires could be faulty in one spot. Ohm out each wire. Or put a plug at the end of each. Find a very dark spot at night with no moon. Open your truck hood (engine running). Look closely for sparks jumping.
16) Ignition module: I assume you've tried a couple ...now I can't remember what you wrote. But I've read reports of import ignition modules being crappy. Consider a NAPA Echlin.

Just thinking out loud, throwing a bunch of mud on the wall as they say. Again, maybe you've done some or most of the items listed above. These hard-to-find misses are the pits!! Love to see your dedication and persistence ...


This an example of how the dist looked when I took mine apart! Actually the reluctor on mine (different than pictured) was even worse, partly busted, and the pieces literally crumbled off when I touched them! Ozone (from the "sparking" process) creates a corrosive environment inside the dist, which eventually gets to the parts. And those wires from the pick-up commonly crack as well. Checking the lower dist gear bushing is a good idea too. Replace with quality Standard parts (not the cheapest parts from the orient).. lol.

You must be registered for see images attach

______

 

[clutterhead]

You've checked a lot of things. Here are a few things that come to my mind. Maybe you've done them and didn't list ...but here they are:

1) Vacuum Gauge? Have you put a legitimate vacuum gauge on it to see what it's doing? What's it read? Needle steady or dancing around? One of the first things I had to do when I bought my truck 20x years ago (used) was remove the intake and fix a vacuum leak. Watch a couple Y/T videos how to check vacuum.

I've been meaning to. I own one, but all my tools are in storage and some are still hiding. I will either way because I know it has its value even if I smoke tested the intake. I did check for intake leaks though. Pulled the PCV, plugged the other side of the breather, and felt if the crankcase had vacuum. Didn't feel anything.

2) Temp sensor? Again, what's it ohm out at cold (ambient temps), and truck warm/hot? Use an ohm meter and cross-check with a table.

I ohmed it once a while back and the numbers were good when comparing to the chart. The scan tool I have will also read what the coolant temp is KOEO and it always lines up to the outside temp. It climbs steadily as the engine warms up and I never see any deviation in the data with the truck running. I also replaced the pigtail harness going to it. It surged on me one day and quit. Moving the connector fixed it so I spliced in a new one. Verified wiring from sensor to the ECU.

3) Barely-good sensors? I've also had sensors which were on the edge of 'good', as mentioned above, which caused problems.

I always hope for it to be something like that, but the data I see doesn't have any intermittent glitches or anything. I shake the harness and wiggle connectors as I watch live data and it doesn't change. I replied to Schurkey that I did remember that my IAC counts were kind of high and I never really figured that out. Don't know how detrimental it is.

4) Vac Leak Check? Have you tried putting a piece of hose on a small LP torch and wave it around specific components in the engine compartment, particularly at the base of the throttle body, vacuum areas, intake corners?

I have done that, used carb cleaner, and I own a smoke machine. Never find anything.

5) More Vacuum: Unhook all extra vacuum hoses and run, see how it performs.

Did this too a while back. Disconnected and capped all lines to TB. No change.

6) Grounds: Ensure your ground is good. Batt->Body, Batt->Engine and so on.

I tell myself I got every one of them, but I plan to go back and check again.

7) Spark plugs/coil: Sounds obvious, but I'd ohm them out again. I've on two occasions installed a set of spark plugs and had a bad one. Took forever to find the problem b/c we were sure it *wasn't* the plugs ....how could it be? One time I had to take the vehicle to a Ford garage and have them hook it up on an old-school analyzer. Yep, new plug was bad! Dought! Ohm them out. Same with the coil.

I hear you on that. I literally ohmed everything secondary ignition-wise. Thats how bad I've been trying to find this problem. Only car I've ever ohmed every plug on before installing new haha.

8) Compression: Yeah I was going to say 130# of compression isn't what I'd call stellar. That's why I'd check the vacuum. How many inches mercury at idle and off-idle? If it can't atomize the fuel very well, that could be an issue. I would run the engine a few minutes, then pull the plugs (Ohm them out again, check for problems) and while at it do another compression test. 4-5 puff's per cylinder. It should run ok at 130...but it's on the low end of "ok" ...I'm not even going to say 'good'. Not trying to call your baby ugly, but generally a person would rather see 145+ (150+ even better).

I can do another compression test sometime. I was going off numbers in my head so I could be wrong on the 130. I think with the MAP voltage being around 1.2 at idle it translates to about 20 in. Hg

9) Fuel Pressure: Did you mention what the fuel pressure was??

I know I took it a while ago and it was around 13psi. I plan to check again this weekend.

10) What happens if you temporarily unhook all the key sensors? I mean it should run ok in open loop.. Does it?

It seems to run somewhat better when I unplug the MAP. But no, even in open loop it has these soft misfires. The odd thing is it isn't always sputtering in open loop. It sputters when you start it, but if I drive for a bit, before it goes into closed loop for the first time it'll idle smooth at a stop. Then you drive it a little longer and it goes into closed loop. Next time you stop, within a 10 second window it starts sputtering and soft misfiring out the tailpipe. Taking off from the stop I feel no hesitation and my BLM values go back to a normal range. My fuel trims are only bad when I come back to a stop.

11) Computer: Have you tried a different computer? Just asking...

Was able to try another. No change.

12) Distributor: You did good by measuring the distributor output. But still, i'd consider pulling it out, taking apart, and having a look inside. Eyes are for real. Mine was totally goofed up from corrosion. The star had cracked and a few edges broke off the pickup coil. I can't believe it even ran! Its fairly easy to do. See pic below (not mine ..but how mine was).

I did think of that. As many parts as this thing has gotten, I really hate being a parts changer. I also understand I don't have some nice Sun brand distributor diag machine from the 80s, and that things get weird sometimes with mechanical ignition systems when running. Visually it all looks ok. I swapped in another used distributor and it didn't seem to change anything. I cant really see the star on mine. Its United Motor Products and it has some sort of stamped piece over it to where the only thing I can see are the very tips of the magnet.

13) O2 sensor. Because they're pretty cheap and the O2 controls the rich/lean, I'd probably buy another one. Just me...

It's seen a few. A three wire and a recent new OEM one wire sensor. The ECU side of the harness has around 500mV bias voltage KOEO too.

14) Cracked distributor cap, or faulty rotor? These are hard to spot sometimes. Esp hairline cracks. I assumed you've tried a couple.. ?? Might want to do a continuity check for all cap posts in case there's a crack on one where you can't see it.

Also seen a few. Always inspect as best I can for carbon traces etc. No arcing I've even been able to see.

15) Faulty ignition wires? Again, back to the basics ...it only takes one. Even new wires could be faulty in one spot. Ohm out each wire. Or put a plug at the end of each. Find a very dark spot at night with no moon. Open your truck hood (engine running). Look closely for sparks jumping.

Replaced them recently after last doing them years ago. High quality Denso plug wires, with Delco plugs gapped properly. Made sure not to cross wires 5 and 7. Ohmed each one before I installed. I know thats not the best test for a plug wire, but it's something.

16) Ignition module: I assume you've tried a couple ...now I can't remember what you wrote. But I've read reports of import ignition modules being crappy. Consider a NAPA Echlin.

Tried about three. Granted they were used, but one was GM genuine and the other funny enough, was an Echlin. All tested good waveforms.

Just thinking out loud, throwing a bunch of mud on the wall as they say. Again, maybe you've done some or most of the items listed above. These hard-to-find misses are the pits!! Love to see your dedication and persistence ...

I appreciate it. Anything helps. Especially when I don't really personally know anyone that I can talk this stuff with. Just me and flashlight holders

This an example of how the dist looked when I took mine apart! Actually the reluctor on mine (different than pictured) was even worse, partly busted, and the pieces literally crumbled off when I touched them! Ozone (from the "sparking" process) creates a corrosive environment inside the dist, which eventually gets to the parts. And those wires from the pick-up commonly crack as well. Checking the lower dist gear bushing is a good idea too. Replace with quality Standard parts (not the cheapest parts from the orient).. lol.

You must be registered for see images attach

______

 

[clutterhead]

Just wanted to stop in and say I'm still active in this thread, I just havent had much time to work on it more than where I left off. I'll update when I do.

 

[clutterhead]

So I have an update:

Still haven't been able to put a vacuum gauge on it, but I think I narrowed down my problem. At least one problem. I swapped my original injectors back in the TB. The car ran much better. It still have some soft pops, but now it only happens in closed loop. Thinking back, this was the original scenario of the problem I had when I first noticed this happening years ago. So it's at least back to its best worst state haha.

I didn't get to look at data passed rebuilding the TB, it got too late. But I know it's a closed loop problem. It doesn't stumble or pop whatsoever when you first turn it on and it can sit for minutes idling perfectly. Go down the road a minute or two, and it starts acting up when I'm at a stoplight after a few seconds. So the o2 sensor woke up and it's now isolated to a closed loop problem. I think my next step is to get the original injectors cleaned professionally because that variable made a drastic improvement.

Here is where I'm a bit confused. I did an idle relearn procedure. I followed this guide, which seems very well written: IAC reset

I notice the guide says the TPS should be at .6V. I know it was mentioned in the thread that it's pretty high. I was able to get a known good GM original TPS. That one reads .49V. Which should I use, and why does that guide say .6V is proper?

Another head scratcher is I was able to test my fuel pressure again. KOEO its about 7psi. I know that's not right. I know it's supposed to go higher than the running pressure initially. I think thats why it takes a while to turn over on it's first start. I unplugged the harness that is from the relay to the first connector before the harness that runs down the frame to the pump. I have 8V at the harness. The thing is that when the truck turns over, I get 13.6V at the pump, and I get about 11-12psi running that stays steady. Not sure what thats about.

If I had to guess, I'm thinking the relay may be worn and the contacts won't close fully till the alternator comes into play with the engine running. To test this I might jump the oil pressure switch connector. I don't have the sensor anymore, I swapped in a mechanical oil pressure gauge a while ago. I know that the system has the oil pressure redundancy for the fuel pump operation if the relay goes bad.

I'll post with any more updates as the come.

 

[Schurkey]

I swapped my original injectors back in the TB. The car ran much better... ...I think my next step is to get the original injectors cleaned professionally because that variable made a drastic improvement.

"I" would dump a bottle or two of Chevron Techron Complete Fuel System Cleaner in the tank, and buy only Top Tier fuel. See if the injectors clean themselves with some added detergent in the gasoline.

I was able to test my fuel pressure again. KOEO its about 7psi. I know that's not right. I know it's supposed to go higher than the running pressure initially.

True on a system where the fuel pressure regulator is vacuum-regulated. Not true on TBI where the fuel pressure regulator has no vacuum signal.

But 7 psi is low. Spec is 9--13, and "everyone" seems to think the higher-end of that spec is better.

I think thats why it takes a while to turn over on it's first start.

Low priming pressure, seems reasonable.

I unplugged the harness that is from the relay to the first connector before the harness that runs down the frame to the pump. I have 8V at the harness. The thing is that when the truck turns over, I get 13.6V at the pump, and I get about 11-12psi running that stays steady. Not sure what thats about.

What is no-load battery voltage?

If I had to guess, I'm thinking the relay may be worn and the contacts won't close fully till the alternator comes into play with the engine running. To test this I might jump the oil pressure switch connector.

Or swap relays. I get extras from the Treasure Yard.

A voltage-drop test with the relay engaged would tell that tale, too. Excess voltage drop = failing relay contacts. But that assumes that the battery voltage is sufficient.

I don't have the sensor anymore, I swapped in a mechanical oil pressure gauge a while ago. I know that the system has the oil pressure redundancy for the fuel pump operation if the relay goes bad.

That's gotta change.

Put a brass tee in the block, route the OEM switch and wire harness to one outlet of the tee, put the aftermarket pressure gauge into the other outlet of the tee. Simple, easy, cheap.