I have an 85 Vette that I am tinkering with and I am going to be changing things in the engine. For now the car is staying stock, but I am having issues with High Idle. For the most part the car runs great and it will idle normal for a period of time and then I might take it out for a ride and upon return sitting in the drive way it will idle up to around 1500rpm and she won't kick back down and other times it will run fine. My question is , Do I have a throttle sensor issue , vac issue or other and where do I start?

 

You can search 1985 corvette high idle on google and find results that bring you to this site where someone may have explained it. I find that more accurate than searching the site from the search box.

Basically you're going to need to measure some voltages at the sensors,etc. It's a lot to type out.

 

Disconnect the cruise control cable from the throttle and see if that helps. I had to adjust my cruise control cable to fix a similar problem.

 

I have a 96 coupe auto and just went through something like that. I'm not sure if your year has one but the Idle Air Control valve was dirty and basically needed to be replaced. Bought a new one and cleaned out the passages and rpms were normal again (650rpms). Also, try to see if you have any vacuum leaks. I also found a vacuum leak. Good luck.

 

K, thanx for the info.

 

Thanx, I will give that a try.

 

Thanx, I will check for leaks first then play with the IAC valve.

 

Start with getting yourself a scanner. Not just for this but for future use. Not a code reader, a scanner. I know they say you can short the ALDL terminals and read a code but why? You cannot read the datastream.

First thing to do is to take off your throttle body. Remove the plate at the top, remove the IAC and remove the IAC housing. Make sure you have new gaskets for it. Clean the entire thing with brake cleaner. All passages and the butterflies. A little WD-40 at the joints won't hurt. Clean the pintle tip only. DO NOT PUSH THE IAC in or pull it out or you might be buying a new one.

Before I go anywhere, I want to know what the IAC readings are. If they are at 0, I would like to see whether you have an air leak. When mine read 0, I had to chase the leak till I found it was on a gasket. I scan the values when it is warm and in closed loop with everything off.

Hoses on the manifold are removed one by one and the barb closed off, engine revved and see what happens. If no change on any of the hoses. close off the IAC hole in the TB and see if the car dies. If it doesn't, you might have a broken gasket. Ask me how I know.

If everything checks out, I would proceed to reset the IAC. Turn the adjustment screw in if the IAC readings are high, rev the engine and see what it does. You want about 20 counts. When that is set, check TPS and reset to 0.54V

BTW, I am assuming your timing is on and you have good spark with no leaks in the wires.

 

WOW, lots of helpful info. Thanx a bunch, looks like I've got my work cut out for me, lol

 

Get a good scanner so we have data. As in The Godfather movie, you don't want to come out with just your d1ck in your hand. If you do, I'm certain nobody will want to help you. Lol

 

First thing for me it is a visual and aural inspection, maybe pulling codes. Surely a binding cable is in that look-see.

I also check the vacuum, which may point to a leak, in the cruise or brake diaphragms or any VAC lines. The ECM can cover up errors (trying to compensate) or miss basic problems (no codes) that even a non-ECM engine might have; fuel delivery and vacuum leaks included.

IF there is unmetered air entering the engine, the ECM will add fuel, attempting to achieve proper fuel/air, causing high idle. I suspect, however, that you have a binding cable, as Cliff suggested.

A scanner is rather essential with these cars, but is not the sole tool. The last one I bought is a nice Innova; virtually new and 1/2 price on local CL!

 

ECM will attempt to compensate for unmetered air but the IAC counts will be 0 which hints at it.