1988 5.0 TPI manual. I've been having an issue where the idle increases from 800ish to 2k once the car is warm, someone's when I'm driving. If I let my foot off the gas, the car will decelerate till it hits about 2k and it holds there. Just replaced the EGR and checked for vacuum leaks. Seems to be very intermittent. Timing and TPS are spot on, and I don't see the throttle cable getting hung up. I do notice a little delay when I hit the gas though. I've got an IAC on the way, will this solve my problem?

 

So I replaced the IAC yesterday and it drove great for about an hour, and then it started idling high again. I was just sitting at a stop light with the clutch in and the revs rose on their own to about 1500 and stayed that way for a while. I don't know when but they did come back down on their own eventually. No error codes set. I can't drive the car until this is fixed so please help!

 

I'll admit I've never ran into this before. I'd be incline to think vacuum leak at the TB. (shaft bushings, blades not closing from carbon etc) Past that if you can get your hands on a proper scanner, check the IAC steps. See if they go up when the engine starts to hang up revs.

 

Agree with getting a scanner on there if you can, it really makes it obvious what the problem is. It may be the same thing that I posted about in this thread

.https://www.thirdgen.org/forums/tpi/722259-idle-problem.html

 

Can I buy a scanner like this or do I need to get it into a shop to figure out exactly what's wrong? I'm going to replace the TPS and check all the wiring. What did your scanning find was the problem?

 

You should be able to buy one, or even make one if you're electronically inclined. You may be able to scrape by without one but I found it quite helpful. On mine you could see the throttle reading jumping around erratically when it was exhibiting the issue and I could see that when it thought it was off idle that it was commanding the IAC valve to open a bit.

 

What kind of scanner am I looking for? Like an actron scanner? I've already got a OBD code scanner but I'm clearly looking for something more

 

I will look into this scanner, but in the meantime...

Does this sound to anyone like a particular sensor might be failing? I'm planning to put in a new TPS and I'll check all wiring and connectors to be safe, but I'm failing to see what could cause this to only happen intermittently.

 

Code scanners won't get you any further than a paper clip will on this vintage of ecm. Before I went to EBL I was using an old snapon scanner like this. http://www.ebay.com/itm/MT2500-Scan-...-/261986792446 A GM tech1 tool is probably the most useable if you can find one at a good deal. Many change hands for more money than that snapon scanner is.

As far as what "sensor" is going wrong remains to be seen. It may not be a sensor at all, thus the scanner to help point you in the right direction.

 

when the idle is high what happens when you quickly blip the throttle?

 

When the idle rises, blipping the throttle doesn't do much of anything. The car sits right where the idle is set, between 1500 and 2000. I don't understand why, but the only way I've been able to make the idle sit back down is to run it through the procedure of resetting the idle (jumper the aldl, remove the iac and start the car). Then after I plug the iac back in the idle typically stays where I set it at least for a few minutes before it acts up again. I'm almost leaning towards a fault in the ECM because I would think by now I'd be getting an error code but I'm not.

 

I'm having the exact same problem, I had started a new thread on it today. Mine is a Formula 350 TPI. Sounds exactly the same. I can make it go away by resetting IAC, but it always seems to come back. With mine, it might be running perfect, idling great, then shut the car off and immediately restart it and it jumps to 2100 RPM. You can look at my thread on everything I've done - but don't count on the new IAC solving the issue, it didn't work for me.

 

Have you tried replacing the TPS? I have run into the similar symptoms as well when the TPS started to fail. I have had to replace my TPS twice now, both times it showed .54 when I replaced them.

 

I haven't replaced it because I have been able to set it the 0.54 volts and it seems to increase correctly as the throttle is opened. I may try replacing it this weekend not sure what else is left! It seems like it's something that is heat sensitive, because I can usually get it set either when the car is hot, or when it's cold, but then it acts up in the opposite scenario. Thanks for the tip!

 

I agree with the heat sensitive part. I noticed the wires for my IAC, TPS and MAF all run underneath the plenum and don't have any heat wrap. I'm going to wrap mine up and see if the problem goes away

 

My problem seems to have gone away. I replaced the throttle body gasket, and wrapped all wires running across the intake manifold with high temp heat shield. I've had it out for about a week with no issues.