1990 C4 Corvette, Base Model

All of a sudden, about three weeks ago, the following started:

1) I start the car, cold start, it idles normally
2) After a few minutes of first drive, the RPM increases to around 3,000 in drive, stopped at a traffic light
3) When I am finished, return home, put the transmission in park, engine running, the RPM is about 3,500

Side note, since I owned the car, it revs to 6000 RPM to shift into overdrive. It has done that from the first day that I owned it.

The car has about 111,000 miles on the engine

Does anyone have input to a solution on the idling issue. The car is consistent with this idling issue for about three weeks

 

I'd check for throttle shaft binding and vacuum leaks.

 

Is it actually idling that high or is it gauge error? When stoped in drive does the car walk as if it's idling high? The 1990 and 91 have a known high tacometer reading issue. Do a search and you will find many posts on this. I know this because my 1990 does the same thing and I have just learned to live with it. I'm not sure of the fix for it.

 

In my case, the car does walk, if I understand how you are using the reference. I also use my ears and driving experience to gauge what the car is doing. It is a high idle. Today, Saturday, I used carb cleaner to clean the throttle body. I keep the engine clean, so it does not have any obvious other issues. I am considering a different throttle body. I have not priced them yet. I have new plugs and wires. The distributor cap and rotor must have been replaced prior to my purchase.

Is there a meter or gauge that I can put on to check for errors. My OBD does not fit

 

I did, as I still do look for vacuum leaks. None detected. You mentioned throttle shaft binding. Tell me about that, please.

I notice that the throttle body spring is in good shape, and the mechanism seats back into its idle position with no binding or obstructions. Cleaning the throttle body, inside and out, and ensuring that no obstructions exist. Visually, there is no obvious reason this is happening. Nothing changed prior to this problem developing.

 

Update:

1990 C4 Base Model

I just finished installing the new ERG valve. I buttoned it back up and fired the engine. I am getting the same problem, Only now, it revs up to 3500 right away. I don't hear vacuum or any other defect. Does anyone have fix #2?

 

Pinch off the brake booster hose, maybe a vacuum leak inside it. You might have to have it scanned, coolant temp sensor reading too cold? If it was reading cold, then it would run rich too. One thing you can try is dropping down the ecm and doing the tap test. You can check the ecm grounds there at the connector.

 

Check your IAC. Mine was doing the exact same thing from a bad connection to my IAC stepper. That one drove me nuts until I found it.

I pulled the IAC wires out of the ECU plug and ohmed them to find the bad wire

 

Thanks for the information. I need more detail to carry out this task successfully.

You stated 'Pinch off the brake booster hose, maybe a vacuum leak inside it.'
I understand pinching off the brake booster hose. Are you suggesting that there might be a leak inside of the brake booster? Also, are you saying that the brake booster may need to be scanned? What reading should I be looking for referring to the coolant temp sensor? The car runs strong, smooth, and clean out of the tail pipes.

You stated that I can try 'dropping down the ecm and doing the tap test. You can check the ecm grounds there at the connector.' Tell me more about this, please. I have not heard of this tap test before. Walk me through it, please. I will check grounds for quality.

 

Thanks for the response. My IAC cable is fine, but the clip that holds the wire into the IAC module is broken. Also, one of the black insulators on the plug is broken off, and the pin is not protected. Tell me what reading each wire should have, and I will test them. What should I be looking for? thanks

How to I check to see if the IAC is bad, and would the engine be doing what it is doing if the IAC is bad? Thanks

 

It absolutely could be doing this because of a bad IAC. An IAC stepper should be seen as a consumable part that should be changed out as part of normal preventative maintenance. I like to put in a new one at 50-60k miles but most people never change them. So if yours looks older than that I would swap in a new one just to eliminate it as a potential problem. Then if the problem still persists, the way I found my issue was I did a simple ohm test (not a continuity test). There are four wires I believe for your IAC. With a good multi meter set to ohms put one lead on the ECU end of the first wire and the other lead on the other end of the wire on the IAC plug. You should see about the same reading as if you just touched the leads of your meter together. Probably under 1 ohm. Do this test for all four wires. In my case I found one wire that had over 10 ohms. This was low enough that the continuity test said it was ok but it was in fact a broken wire.

To fix this I simply made a new IAC harness. Four wires and I removed the molex pins from the ECU plug and replaced them with my new wires and new molex pins. Then I installed a new IAC plug on the other end after I routed it through the firewall all wrapped in new wire loom.

But you very well may just have a bad IAC. I would first remove the throttle body and clean all the carbon out. Then install a new IAC and a new TPS while you're at it. If that doesn't fix the problem then move onto testing. Both of these components should be changed out periodically anyway.

 

Thanks, Phobos84,

I will replace the IAC and TPS with new plug connectors. When I replaced the ERG, I thoroughly cleaned the intake plenum and cleaned all of the carbon out of the two passages in the plenum. I will try to pick up the other items today, January 4.

 

In case it's the dash / tach:

 

I went several rounds with mine. Replaced sensors, replaced vacuum hoses, checked cables, tested with meter and scanners plus removed and cleaned throttle body. In a bit of frustration I grabbed throttle body linkage with hand an pulled back (checking for stuck) and it idled down to normal. So I did the un-recommended and removed the plug and backed the idle screw down about 1/2 turn and perfect idle. Is something worn and in need of replacement/adjustment, likely yes. Does it bother me enough to hunt for it now, no.

 

Thanks, dburgjohn,

I looked at that little black plug as though it was a T-Bone steak and wanted to pull it out. I decided not to at the moment. I ordered my parts, TPS and wire plug, and the IAC with a new wire plug. My factory plugs' clips were broken or dry rotted and broke.

I cleaned my throttle body also, without taking it apart. I did the throttle linkage pull and let it snap back to the idle position thing several times to see if it did not seat against the screw fully. That linkage works as it should.

I get my parts in a couple of days, and we will see where that leaves me.

If anyone else had this issue with a fix, please share.

 

I had high idle problem with my 96 coupe, but it was easily diagnosed with a scanner. My issue was failed TPS.

 

Thanks, car addict,

I have parts coming, TPS, IAC. I will install and replace the connectors and see where it takes me.

 

You will need to follow the minimum idle adjustment procedure for a c4.

following this accurately will point you in the direction of where your high idle is caused.

in any event you will need to follow this to get yourself setup with the replacement parts. use 550rpm as the minimum with iac closed.

 

also, seeing as you have a 90, read this:

https://www.corvetteforum.com/forums...im-pintle.html

 

As @car addict suggest the data is a must in times like this. The image below is a screen shot of the most basic of programs. You can see how it would have helped you.
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