If the PCM doesn't see sensor reading when it should it goes into FMEM (Failure Mode Effects Management) and uses values out of lookup tables inside the PCM as substitutes for the missing sensor reading. It should turn on the Check Engine light but the earlier EEC systems didn't always do things quite how they were supposed to.
Having the injectors flushed might be a good thing to try next. Most well equipped shops have an injector flusher.

 

sounds like a problem I had, I cleaned the IAC with some Throtle Body spray, and it got better, for a while, thus pointing at a bad/sticking IAC - worth a shot for the time and price!

 

Copper:

I wonder if you have a cracked head. Cracks will expand when heated up.
Just a thought.

Jay

 

Hi Jay:

I'm quite sure I didn't have a cracked head. The engine has never been overheated before and I had the heads inspected when I rebuilt the engine about 20,000 miles ago. The guy at the head shop said the head was in such good shape they didn't even have to be shaved, just the valves resurfaced. So I don't think it's the head. The car has almost 170,000 miles now and still has all the original fuel injectors (which have never been cleaned) and all the original sensors (except for the O2 sensor I replaced). I checked all the temperature sensors (resistance check) and they all seemed OK, that's why I'm suspecting the injectors. The car still runs fine with pretty good mileage, except for the idle being a little rough, so I am suspecting dirty injectors. I could pull them and send them in for cleaning, which would cost only $70 but I'm too lazy Also, I live in CA where they put all this MTBE crap in the gas. My wife was the original driver for the first 140,000 miles, and she used to put cheap gas in this van, so that made it all the more likely.

MoneyPit:

I cleaned the IAC thorougly, and no change at all. I also put in a few volts on the coil just to make sure it operates OK and it did. So, the IAC is no longer a suspect.

Come to think of it, I never replaced the PCV valve. Since that's a $3 item, I'll do it first. Who knows, maybe I'll get lucky

Thanks and Merry Christmas to all

 

My 92, 3.0 idles rough when hot also. I have changed the O2 sensor, it was bad anyway, cleaned the MAF, IAC, IAT, and throttle body. I have set the throttle plates to specs, set the TPS to specs, and made sure the IAC voltage was correct at idle. The fuel filter is new, and the fuel pressure is correct. I have also tried setting the ignition timing at various settings.

The plugs, wires, cap, rotor, are all fairly new, along with new air filter. All vacuum lines are good also. If I put it under a load, like turning on the A/C, it will almost smooth out. I am thinking injectors also.

Like you, I can hardly feel it in the van unless I am stopped on an incline, but when you raise the hood you can sure see the engine shake.

Off idle it runs perfect, gets around 21-23 mpg on trips, and no codes.

If you find the problem, please post it here. I have had this van for almost 8 years, and it has idled like this all that time.

 

Yardbird:

That's exactly the symptoms I have with my van. If I ever find the culprit, I'll surely tell you.