Wandering idle, especially on start up

[alchemist]

Figured I would ask some of the experienced guys on this forum about something I noticed with my 1990 GM. When I start it up in the morning and occasionally even when it is warm, the idle tends to wander up and down until I give it gas, and then, at idle (e.g. stop lights) it is fine. I'm guessing this is some sort of emission or EEC-IV issue, but not having as much experience on these cars, I don't know where to start. Car doesn't throw codes, so it becomes more of an experience issue.

Would anyone know where to start? IAC? TPS? MAF?

Thanks-

[miamibob]

Clean/replace IAC, clean MAF and see where you are at.

[alchemist]

Sounds good - thanks. GM's OBD-I would pick this up, but I never had much luck with Ford's EEC-III or IV systems. I'm sure you need to take extra special care when cleaning MAF? Alcohol on Qtip method? Cleaning IAC involves using throttle plate cleaner sprayed on the pintle? I guess the absolute best way to see if the IAC is fubar'd is to use some sort of engine tuning program. GM cars, for OBD-1, use programs like TTS Datamaster to log run files and TunerCat to tune OBD-1 cars. What's out there for Ford cars? With a data log program, you can see the actual IAC counts and compare to spec.

Thanks again

[miamibob]

If the IAC is original I'd put a MOTORCRAFT one in and really clean the area it goes into. As for the MAF, use the special MAF spray and DON'T touch the little wires inside!! Parts could be "off" and not throw a code.

[alchemist]

I suppose a complete take apart and clean of the throttle body that the IAC sits in would be recommended as well? Is there a simple write up on this in the forum?

Thanks-

ps - yeah, you're usually not supposed to touch the little wires, but as long as you use delicate care when cleaning, you should be fine. I've cleaned many GM MAF's and there's those three little wires that you have to be extra careful with.

[1990LTD]

Is it a california car? if not, and you didn't do a swap, it's MAP not MAF.


Taking the throttle body off is easy....


Label these things before disconnecting even if your memory is good:

Disconnect TPS, IAC and EGR position sensor harnesses.


Disconnect the green vacuum line going to the EGR valve.

Pop the cruise/TV/throttle cable off of the throttle lever (cruise and throttle cable should be ball-type socket, TV cable goes through the lever) and unbolt the throttle bracket, 2 bolts on the back of the EGR spacer.

Put paper towel or rags under the EGR spacer between the throttle body and plenum....good idea to have new gaskets for everything (tb-egr and egr-plenum and IAC.) Coolant will likely spill out of the EGR spacer when you do this, but not a lot.

Disconnect the 2 coolant hoses going to the EGR spacer and prop them up somewhere so they don't spill anywhere else. Good idea to replace them if you can reach their point of origin.


Just 4 nuts on the throttle body studs left now, unbolt those and off she comes.

[BerniniCaCO3]

granted, some idle wandering IS normal.
For example, when my car first starts up, it revs to 2k for a second, drops down, then might settle back up a bit. It stabilizes, however, within just a couple seconds. If yours wavers much longer, then yeah, cleaning procedures are probably in order and not so hard to do!
Whenever you change the load, it takes a half second to adjust: so when you put it from park into reverse or drive, the engine will idle a little lower. When you first put your foot on the gas, I believe, it closes off the IAC and starts relying just on you.

I'm jealous btw: which 1990 GMs came with MAF stock? Mine's speed density.

[1990LTD]

granted, some idle wandering IS normal.
For example, when my car first starts up, it revs to 2k for a second, drops down, then might settle back up a bit. It stabilizes, however, within just a couple seconds. If yours wavers much longer, then yeah, cleaning procedures are probably in order and not so hard to do!
Whenever you change the load, it takes a half second to adjust: so when you put it from park into reverse or drive, the engine will idle a little lower. When you first put your foot on the gas, I believe, it closes off the IAC and starts relying just on you.

I'm jealous btw: which 1990 GMs came with MAF stock? Mine's speed density.

49 state legal cars were speed density, California had MAF cars starting in 90. Different airbox and snorkel IIRC

[alchemist]

I'm jealous btw: which 1990 GMs came with MAF stock? Mine's speed density. 49 state legal cars were speed density, California had MAF cars starting in 90. Different airbox and snorkel IIRC

Sorry, i just assumed these cars had an MAF but haven't had mine apart that far. GM LT1 motors, from 1993 were MAF-tuned cars. They do have a MAP sensor as well. But don't be so quick to jump in the MAF camp. Many of the tuners around here, over the last 2-3 years, have gone back to preferring speed density tunes, basically taking the MAF out of the equation.