S-10 2.8 TBI runs rough midway through warmup

[Blackbeard44]

not a fullsize square body truck, its a compact version.

1989 S10 2.8 liter V-6 TBI w/5speed trans
when cold starting it cranks for about 2 seconds than fires up on the high idle (RPM'S unknown) where it stays for approx 1 minute, runs nice and smooth, as it starts to return to normal idle it goes through a phase where its starts running very rough and smells very gassy at the tailpipe, it surges like it is searching for idle for about 1 minute than it clears up and runs beautifully.

somewhere in between high idle and regular idle it looses its tune.

recently bought the truck from a parts store lackey that could not get it running after a motor swap and proceeded to throw the kitchen sink at it and replace every sensor, distrubutor, injectors, fuel pump, temp senders, MAP, the whole works.

i suspect a vacum leak but cannot find one, I can live with how it acts, but would like to make it run properly. whats your suggestions, i think later today I am going to verify the throttle position sensor is working properly, other than that I'm stumped

 

[Blackbeard44]

forgot to include
base timing set 10° advanced per sticker on the shroud.
fuel is fresh.
smog pump has been plugged off and bypassed.

 

[Blackbeard44]

I have an update, I took a look at all of the connectors and found all of them full of crud, especiall the temp sender on the intake, I cleaned them out with a shot of carb cleaner and some air, it runs better warmed up now so I will let it sit until tomorrow AM and see if it can make a clean transition from high idle to regular idle.

TPS checked out perfectly, so I hope the dirty connections makes a difference

 

[gmbellew]

it could also be the IAC hanging up if it is only midway in the idle down process?

 

[78C10BigTen]

@Vbb199 may be of help in this also

 

[Blackbeard44]

IAC has been replaced by previous owner, maybe I should remove it and make sure everything is clean amd the is no blockages, I'll go out to the shop later and verify that, I appreciate your suggestion.

 

[idahovette]

BEWARE of all parts replaced. A lot of "new" stuff isn't any good!!!.......YMMV

 

[gmbellew]

BEWARE of all parts replaced. A lot of "new" stuff isn't any good!!!.......YMMV

good point. stick to ac delco for all the sensors. even then, it's no guarantee...

 

[Blackbeard44]

its got me nervous, but at least all the old stuff is in a box

 

[Blackbeard44]

i took off the IAC and wiggled a paperclip around and there didnt appear to be any build up, sprayed some carb clean and blew it out with air, I didnt want to start it because I am going to let it sit all night and see what tomorrow brings.

 

[Blackbeard44]

quick update, started it up today and let it run for a while, it was alot better but not quite right, it seemed to hold high idle smoother and the transition to lower idle speed took longer, it wasnt so abrupt, the idle slowly dropped over a few minutes, before it just dropped right off and got chuggy and very rich, today it slowly dropped for a while and I though it was gonne be fixed, but twords the end of the warm up cycle it got chuggy and fattened up again.

I'm thinking about replacing the O2 sensor and getting new exhaust installed, it still has a cat and the muffler has started to rot away, called exhaust shop and they are booked out 2 weeks, but I made an appointment.

 

[78C10BigTen]

quick update, started it up today and let it run for a while, it was alot better but not quite right, it seemed to hold high idle smoother and the transition to lower idle speed took longer, it wasnt so abrupt, the idle slowly dropped over a few minutes, before it just dropped right off and got chuggy and very rich, today it slowly dropped for a while and I though it was gonne be fixed, but twords the end of the warm up cycle it got chuggy and fattened up again.

I'm thinking about replacing the O2 sensor and getting new exhaust installed, it still has a cat and the muffler has started to rot away, called exhaust shop and they are booked out 2 weeks, but I made an appointment.

It may have been said before but did you try a new TPS?? (Throttle position sensor)

 

[Blackbeard44]

i tested the TPS first as I had a hunch it was the reason for the motor getting richened up, it tested fine with a clean increase in voltage as the throttle plate opened, and clean decrease as it closed.

im wondering if the coolant temp sensor might be the culprit, I have 2 old ones and I think im going to try the one that still in the old engine.

 

[gmbellew]

also, a bit of a long shot, but check the ICM harness to make sure it isn't near any of the sparkplug wires. the electrical interference can cause extra refererence pulses to the ECM. also unplug the connector and check the low reference (should be black/red) to make sure it is showing as ground. I've had issues with that ground going bad in the harness near the connector and it gives extra reference pulses and gets very rich at idle. a bouncy tachometer is a clue that there is an issue there. harder to observe the tach in the OBD1 data stream because of the slow data speed.

 

[Snoots]

To reset the IAC.

First jumper pins A&B on the ALDL connector.

Then turn your key to ON. Now walk back and put your hand on the IAC.

You should feel and hear it clicking rapidly.

What it's doing is being commanded fully closed.

If it's not clicking rapidly = dead.

If it IS clicking then simply pull the electrical connector from the IAC.

This freezes the IAC fully closed.

Now walk forward and turn the key OFF, then remove the jumper from the ALDL connector.

Now walk back and reconnect the IAC electrical connector.

You have just reset the IAC to a known position for the ECM.

Now drive the car over 35 mph to re-learn idle. The car should idle fine as it is but
gets more accurate the more times you drive it over 35 mph.