Hey guys, I just brought my 85 vette out of hibernation and the idle speed is sticking around 1,500 RPM. Once in a while the idle will return to normal but then it rockets up to 1,500 RPM again.

I've always had a slightly 'lopey' idle with this car and it has gotten slightly worse over the last couple of years so this might be related.

Now the question: I disconnected the IAC and drove the car around for a while and the idle speed finally dropped to around 500 RPM *AND* the idle is now just about as smooth as glass. Do you think I've got a dirty or sticking IAC? Can the IAC be cleaned or does it have to be replaced?

Am I wrong in my thinking or diagnostic approach?

 

With a high mileage IAC, carbon can build up on the pintel causing sluggish operation. I won't be able to react fast enough to maintain the commanded idle speed.

You can unscrew it and with a toothbrush and some injector cleaner, clean off the pintel. If you modeately pull on the pintel as you rock it from side to side, you can completely remove it from the IAC housing, which will allow you to thoroughly clean it.

Only thing is that when re-installing it, using a moderate pushing/rocking, you have to make sure the tip extends no farther than 1-1/8" inches.

Another good idea is to clean the backside of the throttle body butterflies. they get a buildup of carbon too, which causes them to stick which causes the idle to be erratic.

Will be a lot easier to remove the TB from the plenum to get to the both of them.

After doing that, let us know how she's running.

Jake

 

I've always heard that one should not attempt to clean the IAC. They are very susceptible to damage that way???

 

Nope, old wive's tale, probably started by someone who sells IACs.

Just use normal care and when spraying the pintel hold the plug connecter end UP so the spray doesn't get into the electronics inside. Even if some does, just shake it out and allow several minutes for the cleaner spray to evaporate.

I (and many other Forum members) have cleaned mine several times.


Jake

 

Jake, a couple of comments/questions:

1) 1500rpm seems awefully high, and I would think that the IAC TB passage could not deliver enough air to achieve this RPM, even if IAC was completely open(ie the pintle has moved inside the IAC valve).

2) If the IAC in an '85 is a stepper motor, I have heard that the pintle will simply stay at the same location in the TB at time the electrical connector was removed?

just my $0.02........

 

I've never tried to find out just how high the idle speed the IAC can obtain. The only way to see would be to close down the butteflies completely to eliminate any airflow the flies would contribute. Of course, if you do that, then you'll have a sticky throttle feel because the butterflies will be in direct contact with the TB bores causing sticksion. You'd step on the gas pedal and nothing changes, step a little harder and you'd blast off like Speed Racer - kind of unsettling.

If you jumper A&B on the ALDL, turn the ignition key to "ON" wait 30 seconds then remove the IAC connector, the pintel will have extended the fully extended position during that 30 seconds; that's why the wait. From that point, the ECM has a new base setting from which to calculate and send stepper counts to the IAC.

Now, with the engine fired, the minimum airflow screw can be used to open the butterflies a tad so that more air is flowing through the flies and the IAC doesn't have to retract as far to maintain the commanded idle RPM.

If you use a torx head tool and start to open the butterflies using the minimum airflow screws, the IAC will begin to retract trying to compensate to the greater butterfly opening. If this is done with the engine running, the IAC count number will probably stall. If you're monitoring it with a ScanTool, Diacom, etc., you'll see that the IAC count will no longer move. A restart is needed for it to reactivate.

All this is the ECMs way of trying to maintain the commanded idle speed burned into the PROM. Ideally, the IAC count number should in the range of 20-30 at idle, closed loop, neutral. This setting will give the IAC more range of adjustment to open further in response to changing engine load conditions, like when the A/C is turned on. If the counts are up over one hundred, then the IAC has less than 60 counts left to work with to compensate for additional loads on the engine. Not enough.

Each time the engine is shut down the IAC is supposed to automatically resets itself. But my thinking is that if carbon is preventing this resetting from happening correctly, this could be part of the problem.

Both the IAC pintel and the butterfly settings work together to determine the actual RPM the engine idles at. Those two are trying to maintain the programmed idle speed that's in the PROM.

You can actually cause the IAC to completely close by setting the miniumum aiflow setting so high that no matter what the IAC does, it can't lower the RPM, it just runs out of its range of adjustability.

This is sort of a work-around (not the way to do it BTW) that some have to resort to when they install a perform cam which calls for a higher idle speed - say 850 - BUT their PROM is burned for 550. Essentially what you'd be doing is over-riding the ECM's programming and its ability to control the idle speed and manually setting the idle speed.

I agree, 1500 is much too high for a street engine to have to idle at. If the engine won't idle at anything below 1500, then there's something else amiss.

So, I'd try cleaning it to see if that works; you'd have to buy a new IAC anyway, so what's there to lose? If you opt not to clean it you'll need to buy a new IAC and if you clean it and that doesn't work, you still have to buy a new IAC.

Cleaning it and getting the IAC counts down to the 20-30 range may just do the trick though. I think it's work a try.

Jake

 

My car will idle at 1500-1600 in cold winter weather. If cleaning the IAC AND the IAC passages doesn't help, you might try checking the engine coolant temp sensor in the front of the intake manifold. Also, a worn TB casting can cause the throttle shaft to hang up and cause a high idle, intermittently.

RACE ON!!!

 

So, if I understand correctly, I'm being accused of being a spreader of "OLD WIVE'S TALES"???

I always thought that was the definition of a gynecologist?

Guess I was wrong...........

 

I have a similar problem with my car. It won't drop back down to idle until after I come to a stop. Sometimes when I am rolling up to a stop and I bump the throttle it will drop down to where it is supposed to idle. I placed another spring on the throttle to see if that would help. THought maybe it wasn't returning to where it is supposed to be. It didn't change anything. I have cleaned the passages and replaced the IAC motor still same symptoms. If the throttle was not closing all the way would the IAC counts be high or low? It is getting annoying when you are coming to a stop and you have to press the brake harder becasue the engine won't idle down to where it should be.

 

Hi.
I have an 85 4+3.
I have had much problems with the idle. First when the engine was stock and later when i had some mods and a performance cam.
I had exact same problem. There are some simple things you can do. Huting idle may come from a dirty throttle body. Not only the IAC part, but bhind the butterflies there are very often carbon deposites. This may in itself be the problem. I had a faulty EGR valve . When the throttlebody and the EGR valve were done . The idle calmed down but not entirely. I had some vacuum leakage. It may be thet the vacuum tubes under the plenum has gone brittle and broken , the rubber tubes to the canister are bad. Check the vacuum part. Some have problem with vacuum leakage between the runners and manifold. This can be checked with start gas or propane that you spray around suspected areas. If the engine increases the rpm then you have a leakage.
The vacuum leakage make false air to come in behind the MAF and then the engine computer will try to compensate with the O2 signal and it cannot understand the amount out oxygen in relation to the air passing the MAF and the engine may have the symptoms you have .

 

small little item on the iac.. when it is removed for cleaning apply 12 volts on the spaded side contacts, this will engage that steppin motor for easier cleaning, reverse current to retract/extend. fully close pintle before installing. to reset follow jake's outline.

 

Naw, you're not a spreader; just someone who received bad info. There's a lot of that going around. LOL


Jake

 

Mine just developed similar problem, along with hard starting. Just when I thought I had it dialed in and running properly

 

I had this situation arise last year, and it was my ECM on the spritz.....the IAC motor wasn't getting the proper command signals (among other signals) from the ECM. Just passing this on as a possibility, but check the other things as mentioned.
Replacement of the ECM is usually the last thing that should be considered.