OK, I have checked all!! I checked the timing and it is set to 6*, I spent almost 1 hour looking for vac leak and got nothing, adjusted TPS to .56 v and idle still high, between 1100-1500 when cold.

So I was thinking, since the IAC is located under plenum, can I buy a relocation kit and a new IAC to kind test and see if this is the cause?
What I would do is leave the old IAC plugged into the plenum, just unplug the connector and plug in to the relocation kit/wires, then plug a new IAC in and see if it helps?

Vette is a 91 with 88k miles

Thanks all!

 

..... What does it idle at warm ? ... I don't know how you would be able to verify that the original IAC is fully seated and is not still affecting your idle speed ... sometimes sticky throttle blades can cause a high idle ... I use silicone spray ... but usually , a higher than normal idle is caused by a vacuum leak ... check the brake booster for cracks .......

 

When you say original IAC is fully seated, please explain? Is it possible to have a vac leak @ the iac? How can I check? Should I spray carb cleaner under the plenum where the iac is located to see if idle raises when sprayed? On the 91's is it an iac or mat and is there a difference?

Thanks you!

 

I just went thru finding and sealing vac leaks....
It is really easy to miss a leak in a hose.
Get a vac gauge and a vac brake bleeder. Plug off every vac line coming off the plenum and chek vac with and without- a bigger leak will show a noticeable diff. Then apply vac to each line individually-goin to the dash/brake booster etc. and see if will hold vac.

 

..... In your first post , you talk about getting an IAC relocation kit and leaving the original IAC installed while you test it with the relocated one ... How will you determine that the original IAC is fully seated so as to NOT introduce idle air into the manifold while your relocated IAC is being used ? It is possible for the IAC to have a vacuum leak ... it has a gasket and so does the IAC housing ... spray away ... MAT is manifold air temp ......... Do you have an OBD I scanner ? ... with a scanner , you can read the IAC step counts to find out how far open it is ... this would be the easiest way to check it for operation ... you can seat the IAC by doing the minimum idle setting as described in the Factory Service manual .....

 

It's supposed to be a little higher when cold. What's it at when warm?

 

did you disconnect the iac and set the idle??? I usually do this before checking the tps and timing.....also....running a can of tb cleaner up and down the vac lines finds a leak fast.....

 

Hold the phone for a sec here...
The Intake Air Temp (IAT) sensor is located under the rear of the plenum, for which there is indeed a relocation kit available.

The Idle Air Control (IAC) is located in the right side of the Throttle Body (TB). This is the unit that may be faulty, causing high/incorrect idle issues.
There is a specific procedure for adjusting/replacing this unit.

 

He means IAT.

Relocating it will do nothing, as described in his other thread in the General Discussion section.

 

Thank you everyone for your time/reply's!

OK, I was thinking about relocating only to try a new sensor with out having to remove the plenum.

But now I see the IAC can have a vac leak if not seated properly? So to fix this all I must do is try to some how tighten the sensor so it seals against the bottom of the plenum? I think this may be the problem because when I put my ear near the plenum where the IAC is located I thought I heard a vac leak but figured it was just the air being sucked into the plenum, so I continued looking else where for vac leaks but found none.

Next I am not sure and have asked in several places but never got a response, the car is a 1991 dose it have a IAC or is the IAC aka MAT? In a catalog I have from Corvette Central they list a MAT for a 1991 and an IAT for other years?! The car has no mass air if that makes any difference? I am pretty sure it is speed density?

IAC? is this a sensor on the TB? I have to look for it is it near the TPS? or opposite side?

1st I will try to see if the IAC is seated correctly, then move forward hopefully from there?

I am very grateful for everyone taking their time to help me with this. This is my 1st vette, and I am already hoping to buy another in a year or so. I am sorry I didn't have a Vette sooner!

 

forgot to mention....

RPM at startup in park or neutral when cold is roughly 1500 and almost same when warm or hot after driving for 20 min. When in drive rpm about 1000 to 1100 in drive.

 

Also want to be sure I have this clear,

IAT = Intake Air Tem located underside of plenum in the rear.

IAC = Idle Air Control located on throttle body passenger side under TPS w/ 4-wires

 

First off you really need to figure out the difference between the IAC and the IAT, you keep interchanging these two names when posting, this is causing confusion..
Forget the IAT, I have never heard of this sensor causing a high idle like your having , also relocating this sensor does nothing to improve performance as has been stated on here often....The IAC on the other hand is known to cause high idle when dirty or defective..Before removing plenums or doing any other work on the car, it's important to know whats causing the problem, if you don't know what's causing the problem you will be running around in circles replacing parts that don't need to be replaced, and doing work that doesn't need to be done....Slow down, identify the problem, then fix it....

You should check.
1, Idle air control , clean this and the channels where the air travels.
2. Make sure your Throttle position sensor is working and not sticking. Make sure it has the proper range with no dead spots..
3, Check for any vaccuam leaks using carb cleaner or propane torch.
4. Make sure your throttle body is working properly, Butterfly can stick because of, carbon build up, shaft is binding, cable is binding... You need to check these things first to eliminate them as possible problems before moving on to other possible causes...

 

Yes, thanks... So I shouldn't worry about the IAT and concern myself more with the IAC on the TB correct? OK, So 1st I will try cleaning the IAC, to do this should I just spray it with tb/carb cleaner? Also be sure Throttle blade is not stuck. If this dosnt work can I just replace it?

Now, if I am setting min. Idle using the set screw, is it the IAC or IAT that I need to disconnect?

 

Remove the base idle plug.
Connect a paper clip, to your diagnostic terminal under your dash connect to terminal A and B (the top right two pins).
Turn on ignition but don't run the engine.
wait at least 30 seconds and then remove the idle air valve connector (IAC) (the square plug) then turn off ignition and unplug the paperclip.
Now remove the timing connector a single wire near the fuel pump relay.
Start your vette and set the idle speed to 400 rpm in neutral (auto trans) 450 rpm (manual trans) then check your throttle position sensor,
the top two wires it should be set to 0.54volts dc, also confirm with engine off that the tps rises to 4.5 volts at full throttle.
Turn off the ignition and reconnect the iac valve connector and timing connecter and start your vette.
it should go to 1200 rpm then drop slowly to 600 rpm, take the car for a 15 minute drive with the usual stop lights.
It is normal for a little hunting while the ecm relearns, also do a few spirited take offs.
All should settle after 15 to 30 mins normal driving and have a stable idle.

BTW spraying anything on the outside of the IAC does nothing other than clean the outside of the IAC.

 

Churchkey, Thank you too much!!! That clears up much! I was disconnecting the IAT under the plenum when trying to set the base idle! Then I was wondering why it would not move when I turned the min idle screw, but it all makes sense now!! Thank you so much! I now cant wait to get home and try this, now it makes more sense. Also you mention to disconnect the timing connector, this is the same one I removed when setting the timing correct, from what I can remember its near the wiper motor/ brake booster? its a single wire with plastic connector outside the wire loom.

 

Just a FYI my 89 had the similar issues except at first the car wouldn't idle, cleaned plenum TB etc, replaced IAC, did the min idle adjustment set TPS to .54 car started to idle but was high around 1300. I then replaced the ECM and all my issues where solved.

Timing wire is black cylinder with tan/white wire, yes it is same wire you disconnect when setting the timing.

 

Churchkey said cleaning the IAC does nothing , this is not totally correct...Cleaning the body of the IAC does nothing, but you can get a buildup on the pintle( the cone shaped tip ) of the IAC that can cause it to not seat all the way in..This causes air to leak by, which causes high idle..Us a spray carb cleaner or brake cleaner to clean it...Also I just checked and the 1990 Corvette's TPS is none adjustable........WW

 

The vette is a 1991, not a 90. Thank you all everything has been very helpful, and I am hoping this works when I get home tonight.

Like I said previously I was disconnecting the iat when trying to adjust the min throttle setting screw. I have a good feeling this will work.

 

I believe from 1990 on up had a none adjustable TPS, so you just bolt it in and go....WW