88 350 TBI cold start stumble.

[Steven Petersen]

Still fighting this thing. I got rid of the idle surge, it was the heat stove flapper in the air cleaner being stuck open. But I’m still getting 4 or 5 stumbles when it’s on fast idle 1200-1250rpm. After that everything is perfect. This morning I disconnected the evap canister from the TB and plugged the vacuum port. Still stumbles. All ignition parts are new Delco. New fuel filter. No changes in scanner readings when it stumbles. I have a new TPS arriving today, and new EGR valve and MAP sensor coming next week. Anything else to try?

 

[PlayingWithTBI]

No changes in scanner readings when it stumbles.

Can you data log and then look closely at the data? Here's a running average history table from Tuner Pro RT where I caught SA @ 4800 - 6,000 where I never ran that engine that high (well not 6,000 anyway). It turned out to be the ICM and coil sending a DRP (Distibutor Reference Pulse) sporadically to the ECM which thought it was running that fast so, it changed timing and fuel to match that speed. These pulses were random and so fast I didn't see it happen real time. This was while cruising at ~2200RPM with the engine warm but, who knows yours may be doing it cold?

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[Steven Petersen]

I’m not sure if my MT2500 is capable of doing that. I’ll need to find out. As far as ICM and coil I already replaced both with Delco.

 

[Steven Petersen]

Just installed NOS Delco TPS. Voltage measured .62-4.48. Have to wait till tomorrow to get a true cold start and see if it still stumbles.

 

[Schurkey]

No changes in scanner readings when it stumbles.

Fix the O2 sensor circuit. Might be as simple as replacing the O2 sensor itself.

 

[Steven Petersen]

Fix the O2 sensor circuit. Might be as simple as replacing the O2 sensor itself.

I changed about a year ago. Think I should try another one?

 

[PlayingWithTBI]

I seem to remember from another thread your cross counts were better after you fixed the heat riser pipe?

 

[Steven Petersen]

I seem to remember from another thread your cross counts were better after you fixed the heat riser pipe?

Cross counts are within spec now. I feel like fixing the butterfly so it pulls air from the heat riser fixed the surging during warmup. But still have that annoying stumble. Only at high idle 1200-1250.

 

[Supercharged111]

How cold is cold? When my '88 got down near a 0 degree cold start it was a pig for the first 10 seconds. I'd have to give it gas for a bit before it could catch itself. Everything after that was fine. Parents' 87 G20 van was the same way. Both parked with the nose facing the lake (Superior) so that didn't help. In the days of carburetion that would sometimes spell a no start.

 

[Steven Petersen]

How cold is cold? When my '88 got down near a 0 degree cold start it was a pig for the first 10 seconds. I'd have to give it gas for a bit before it could catch itself. Everything after that was fine. Parents' 87 G20 van was the same way. Both parked with the nose facing the lake (Superior) so that didn't help. In the days of carburetion that would sometimes spell a no start.

I think it was 16 this morning when I started it.