clutterhead
Newbie
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.
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.