At my wits end, please help.

[Dinkster]

1995 Chevy K2500 5.7l manual trans. 200k miles. Engine surges at idle, fluctuating from 500 to 1200. Also after is been running, shut down and restarted, I can feel it buck and jerk at light throttle for the first minute or so. The things I've checked / replaced are extensive, including: TPS, Coolant temp sensor, complete throttle body w/ injectors, TPS and IAC, O2 sensor, entire distributor, cap rotor wires plugs, Ignition control module (AC Delco), MAP sensor ( also AC Delco), EGR valve, ground cables and cleaned up connections for wires, checked for vacuum leaks ( found nothing) vacuum steady at 16-17# idle (I'm at 5000 ft ele), scanners doesn't show anything abnormal, everything seems to be in spec and no codes. The only thing left that I can think of is the ECM and possibly a stretched timing chain excessively retarding the timing to the point the computer tries to over compensate, but that seems incredibly unlikely. When I replaced the plugs, they all were even color and light tan. It also just passed smog though it didn't pass at first but did on the second run after replacing the throttle body ( high hc and co). Somebody please point me in a direction I may have overlooked. Thanks

 

[Schurkey]

1. What is the fuel pressure?
2. What is the ignition timing when the one-wire EST connector is disconnected?
3. What does the scan tool show for IAC counts when the idle speed is changing?
4. What does the scan tool show for O2 cross-counts/O2 voltage swing?
5. What does the scan tool show for engine coolant temperature when the engine hasn't run overnight--just turn the key and don't start the engine? Is it the same or close to ambient temperature?
6. What does the scan tool show for engine coolant temperature when the engine has been running long enough to be at full operating temperature?
7. Look at the magnet on the distributor shaft, under the rotor. Common for them to crack and give goofy signal to the ignition module.

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[Dinkster]

I'll check the IAC, timing and O2 crosscounts when I get back to the shop later. A few things I forgot to add. The fuel pump and regulator are new as well, fuel pressure is set to 13psi (adj reg). The CTS is new and I recall the CTS being accurate to the ambient and engine temp but I will double check. The distributor is new and the reluctor and pick up look perfect. Thing is, it does it in open or closed loop, though worse in open. And the idle smooths out when the AC is engaged, but doesn't eliminate it entirely.

 

[PlayingWithTBI]

Thing is, it does it in open or closed loop, though worse in open.

While idling, take a spray bottle and spray water on the coil, distributor connectors, wires, etc. If it stumbles worse, that's an indication where to look.

 

[Erik the Awful]

Hydroboost or vacuum booster? Most likely hydroboost, but if it's vacuum, block off the hose to the booster and see if it runs smoother (without driving it).

 

[Dinkster]

Ok so O2 crosscounts are good, I watch them climb to 250 then reset to 0 and start over. The CTS was at ambient this morning 71deg and steadily climbed to 195 as it warmed up. IAC is 42-48. Already tried clamping off the booster, no change. But, heres a new symptom. When the vehicle is surging/stumbling, the tach needle starts to bounce erratically, fluctuating from 500 to 750 rpm above/ below the running engine speed. That's leading me to believe it could be the distributor pickup after all. Even though it's only a year old, I'm sure its probably made by the cheapest manufacturer. It does seem to get worse as the day progresses, so the heat soak could be causing the resistance to climb. Any thoughts?

 

[PlayingWithTBI]

The 1st thing that comes to mind is ICM and/or ignition coil but, I guess the pickup coil could be the culprit too. Which ACDelco ICM do you have and, how much did you pat for it? There are counterfeit part out there too.

 

[Schurkey]

Ok so O2 crosscounts are good, I watch them climb to 250 then reset to 0 and start over.

I guess this depends on the scan tool. I've never seen a TBI with crosscounts higher than ~30.

IAC is 42-48.

How does that compare with the spec in the service manual?

When the vehicle is surging/stumbling, the tach needle starts to bounce erratically, fluctuating from 500 to 750 rpm above/ below the running engine speed. That's leading me to believe it could be the distributor pickup after all.

Pickup or module. Potentially both. Or something weird like the signal wire for the tach intermittently grounding-out, causing both tach and ignition issues.

 

[Dinkster]

The crosscounts continually climb. They start at 0 and work their way up to 250, the start back over at 0. They are constantly changing. I did have a sluggish O2 sensor so that was changed a month ago and now seems to be working well. I just replaced the ignition module, first with a genuine gm and now Davis unified ignition so I'm confident it's not a module issue. I'll check the pickup wires for issues but I'm feeling kinda confident it's a pickup issue. Since it does this in both open and closed loop I don't see it being related to the O2 since only the TPS, MAP and CTS are really used when open.

 

[PlayingWithTBI]

How old is your Ignition Coil? A bad coil can/will take out the ICM.