I recently began having a low idle problem at start-up. Once car has run a while it idles properly at about 850 rpm. When car is started up after sitting a while, it runs below 800 - well below 800 if A/C is on. With AC on, it feels like it wants to die but doesn't + you get the added bonus of something like a belt squeal sound. Without AC on, it just idles low. Either condition produces a lot of vibration in the car. I have replaced the motor mounts, so I don't think that explains the vibration. I have also replaced two vacuum hoses as well as the boot to the air flow meter. The ISV was also removed and cleaned. I need some help, my mechanic wants to check the motor mounts again, which is fine with me, but it idles low so the motor mounts won't get around that problem. I need suggestions - what else goes into the idle control at startup. More electronics, additional hoses (are there only just the two I had replaced?) Why would this problem be so bad when the car is cold then be fine after running a while?

 

It helps to know if you have an early or late car, since the idle circuit is different. I know you mentioned the ISV which goes on the late cars, but sometimes people assume earlies had them too.

 

It's an 86 NA. What about the DME Sensor? Where is that located? Could it be the problem? Price? Easy to replace? Or is that just a part of the AFM? Aftermarket AFMs on Pelican are $220ish - would that solve my problems? As I said, my mechanic replaced 2 vacuum hoses - is that all there is to the idle system?

 

Here's the idle adjustment procedure:
http://www.clarks-garage.com/shop-manual/fuel-08.htm

I think you mean the DME relay. Normally a failure in the DME relay will cause a no-start condition, and I've never heard of it causing an idle issue. I don't the AFM would cause an idle issue either, certainly not anything related to the engine temperature like what you have.

I think the ISV should adjust for the A/C being switched on. It sounds like it's not doing that. I know you said it was removed and cleaned, but still... I would probably look at the belt squeal first, should be an easy fix. The fewer the problems, sometimes the easier it is to diagnose the remaining problems.

Theoretically the ISV should also adjust for any minor vacuum leaks which can cause idle troubles -- emphasis on "minor". If there's a large enough hole in the intake somewhere, there's no way it can work properly.

There are plenty more than 2 vacuum hoses in the engine, and plenty of ways for unmetered air to go past the AFM. Maybe your mechanic was talking about the hoses directly connected to the ISV (but I'm not sure you can really call those "vacuum" hoses).