In reading up on how IAC works, there is lots to it.

I have an issue on my latest project ... from cold start, the IAC is keeping the engine idling at 1200rpm. Did a test this morning for 900 seconds. Got it up to operating. Closed loop kicked in. BLM's were around 128-132. Working as expected. IAC was keeping engine at 1200 RPM. Desired idle was 800. I did a could restarts ... same thing. Feels like this is some sort of commanded sequence waiting for a test to complete.

Not sure what flag I flipped or what variable I tweaked ... but I'm stumped. I thought it was how I kept open loop by setting close loop temp way high. That is not it.

IT's almost like the MAT sensor has to soak up enough heat before it's warm. Testing last night ... it was after a few test cycles and then it just dropped to 800 RPM and IAC was 25.

Any ideas? I think I am just missing something right in front of me.

ATNX Bin. MAP modified for 2 bar. Idle KPA is 19 right now.

Mark.

 

You mentioned installing 38# injectors, halving the VE table, but left the Injector Constants alone. Above you mentioned increasing the Closed Loop enable to a higher temperature (not sure if you meant you were running OL only, or you just wanted a higher CL enable point), but either way, despite eventually entering Closed Loop, the IAC still held 1200-RPM, and only after a few test cycles the RPM finally dropped to 800-RPM w/25 IAC steps, correct? Hearing all of this, I am torn between a skewed Open Loop fuel multiplier during warmup (did you compensate for that after halving the VE), or its just a sticky IAC pintle...

- Rob

 

More testing. Tried a few IAC, even verified them working outside of the engine in a test housing. Same result in the engine.

Today ... ran the engine in closed loop. Same result. Wondered about the VSS. Back the car out of the garage ... and the idle dropped!!! Tried the test again. Start car in garage (engine is warm now). Idle held at 1200. Back out of the garage ... idle drops. So there is some sort of startup delay looking for a VSS.

ATNX came from a Sep 1989 build car ... very early SD engine. So there could be weird code being the first. In comparing it to other SD code, not much is different, but keep in mind these are only using known data maps.

Anyways ... that is what I learned. Some sort of delay waiting for input.

Mark.

 

If it helps... and as coincidence would have it, just started tuning a member's engine who is not too far from you, with no VSS input whatsoever. IAC was pegged for seven minutes from a cold start, which was the exact time the MAT took to reach 65*F starting from 50*F. Decreased the OL multiplier a tad, and the IAC steps reacted 10* F sooner at a MAT reading of 55* F. Was too much fuel in this case, and of course just extrapolated the colder values preceding 55* F in the same table.

Hope this helps either way...

- Rob

 

I wondered about the MAT temp as the trigger as it seemed a warm engine restart once or twice had a proper idle. ATNX is an early SD chip (sep 1989). Makes me wonder if a later version would act the same.

Mark.

 

A newer version would likely be better. Get either AXXC or AXXD and try it. Common for early releases to have minor issues.

RBob.

 

I tried the later version. Idle control at startup is different. It tried to reduce idle more quickly, but engine wasn't warm. Still stuck higher than expected. I must have tripped over a setting along the way. I did a test with S_AUJP and it brought the idle down rather quickly. So maybe a 5 speed setting?

Just frustrating when you are trying to tune and get side tracked by something like this. Overall the car runs well enough with the Turbo I could let someone else drive it. Nothing too odd about it. Although maybe cold or raining days might be different.

Mark.

 

It might possibly be due to the Minimum IAC Learn setting;.

S_AUJP is set for 140*F whereas ATNX is set for 185*F

- Rob

 

Thanks Rob. I will adjust that and see. I did notice on one or more occasions the idle came down after it was fully warmed up. I am running at 160F thermostat (for now) ... which is less than MAT learn temp. So that might be the culprit. But funny that moving the car (VSS input) also make the idle change ... a red herring as they say.