dzierke

I'm asking for advice on solving the mystery behind my cars high idle problem. I've parked my car for a few months to do some much needed restoration on the interior.

I thought that I'd take my 86 C-4 out for a drive last weekend after installing new plugs. The car fired up just fine and I pulled it out of the garage to warm up. Once the engine temp reached about 180 degrees, the idle went from 800 rpm's to over 1200 rpm's. As the engine temp climbed to the range of 225 degrees, the electric fan engaged and cycled the temp down to 205 degrees. At this point the engines idle had increased to about 1400 rpm's. I shut it down for about 5 hours to cool off. I started it again, The idle started out at 800 rpm's but once again it climbed after hitting operating temp as it did the 1st time. I checked the OBD-1 for codes but nothing came up. Is there any sensors that could cause the idle to climb like this?

I also wanted to mention that I replaced the 02 sensor and pulled my batteries cable to clear any settings on the ECM today. I ran a new idle test. Unfortunately, no change. It still idles high when warmed up. Anyone ever had this happen?

I'm stumped!!

CREWZIN

There's an IAC (Idle Air Controller) on the pessenger side of the Throttle Body that supposed to control everything. Did you try and disconnect that, and plug it back together? Sorry, can't help any more than that.

dzierke

I replaced the O2 sensor after the idle problem began. It hadn't been replaced in 4 years and I'm do for an up comming smog test.

As for the engine idle speed, It starts out at about 800 when the engine is cold. This should be normal since the cold injector is kicking in. Normally the engine would slow down to between 650 and 700 rpms when it's warmed up. Once the engine temp reached about 180 degrees, the idle goes from 800 rpm's to over 1200 rpm's.

I also removed the idle air control valve and cleaned out the throttle body. The valve it self was replaced a few years back after a code issue. Is it possible for the I.A.C. valve to cause the high idle problem even though I'm not showing an ECM code?

dzierke

Can't anyone offer some advice before this post gets dumped where no one can see it?

Randy93

Sounds like either a vacuum leak or a sensor disconnected or a ECM problem. Is it throwing any codes?

Sorry just saw that it is not throwing a code, sounds like a vacuum leak though. That will make the car idle high.

TIMSPEED

Did you make sure to set your base idle and everything?
If you need a hand, just let me know. There's nothing I can do to my vette, so, why not work on someone else's

dzierke

Hey Tim,

Can't say that I even thought of setting my base idle. It just started acting up after sitting in my garage over the winter season. Frankly, I think that my car's just pissed off at me for not driving her year round like I once did. Can't find a code for that one though. LOL...

I'm going to try to tackle it more this comming weekend. Heck, I don't even have the seats in it at the moment. I'll send you an I M if I can't get it sorted out soon.

Thanks, Dan

CE_Vetteboy

sounds like the engine (ECM) is not going into closed loop mode after warm-up. you said the o2 sensor has been replaced, how long ago and where did you get it? I've had this problem before and it turned out to be a faulty o2 even though it was new. you have about a 1 in 3 chance of getting a good one if it is an aftermarket part. GM ones tend to work better. It could also be the trottle pos. sensor (TPS) giving a false reading to the Idle air control valve (IAC). I believe at idle you should read 0.7Volts at the TPS and your base idle should be 400 RPM with the IAC disconnected, and a jumper in your ALDL connector. (to force the IAC closed).

Caboboy

If the car's been parked a while and there weren't any mods/repairs to cause the problem, I'd look at the T/B getting stuck on the old varnish & not closing all the way. Other things aren't going to change their specs when they're just sitting there not being used

Try pushing on the linkage a bit with your finger to seat the blades and see if that helps before doing a bunch of other stuff

Oh yeah, the TPS voltage should be at .545........setting it to .70 on a nearly stock motor can increase idle speed, not decrease it.

flylowguy

I would be checking the vacuum hoses. They are apt to deterioriate with disuse and they probably wouldn't throw a code.
But the fact that the idle goes up when the operating temp is reached is another clue. So maybe the new ox sensor is no good. It's the last thing you did, so it should be suspect just on general principles.

azzkikr

Sounds like a vaccuum leak to me... mine just cracked a small vaccuum line connector under the plennum. It looks easy to replace if I can get my fingers in there. Also, you mentioned the car has been sitting awhile during an interior restoration... if you had your battery disconnected for a period of time I think the ECM has to "re-learn" the fuel curves or something to that effect. My money is on a vaccuum leak, everybody else's ideas sound good too. Hope you figure it out.

CE_Vetteboy

If you've already cleaned the throttle body then coking shouldn't be a problem, (thanks for the correction on the TPS voltage). I would check the TPS though. My '87 sat for a couple of years and it was outta whack. without scanner values suggesting other sensors would be a long shot. My guess is probably the O2 sensor because the idle goes up as the engine warms up. the engine will only go into closed loop based on the engine temp and the O2 reading, and your getting to temp...

weightlossstar

1986L Corvette—When reinstalling the Throttle Body always disconnect the bottom heater hose (Especially if its new) Bolt the Throttle Body, free of any tension otherwise a good chance of a gasket vacuum leak, causing your rpms to stay between 10 & 15 /.Also use ACDelco idle air control valve, better chance of not being defective!
Thankfully I discovered this after only 3 attempts in 93 degree weather. New hoses avilable @ https://shop.advanceautoparts.com/p/...653/19590295-P

weightlossstar

1986L Corvette—When reinstalling the Throttle Body always disconnect the bottom heater hose (Especially if its new) Bolt the Throttle Body, free of any tension otherwise a good chance of a gasket vacuum leak, causing your rpms to stay between 10 & 15 /.Also use ACDelco idle air control valve, better chance of not being defective!
Thankfully I discovered this after only 3 attempts in 93 degree weather. New hoses avilable @ https://shop.advanceautoparts.com/p/...653/19590295-P
​​​​​​​

weightlossstar

1986L Corvette—When reinstalling the Throttle Body always disconnect the bottom heater hose (Especially if its new) Bolt the Throttle Body, free of any tension otherwise a good chance of a gasket vacuum leak, causing your rpms to stay between 10 & 15. Thankfully I discovered this after only 3 attempts in 93 degree weather. New hoses avilable @ https://shop.advanceautoparts.com/p/...653/19590295-P Please share info on other pages because this rpm problem can drive corvette owners NUTS!

weightlossstar

1986L Corvette—When reinstalling the Throttle Body always disconnect the bottom heater hose (Especially if its new) Bolt the Throttle Body, free of any tension otherwise a good chance of a gasket vacuum leak, causing your rpms to stay between 10 & 15. Thankfully I discovered this after only 3 attempts in 93 degree weather. New hoses avilable @ https://shop.advanceautoparts.com/p/...653/19590295-P Please share info on other pages because this rpm problem can drive corvette owners NUTS!

martinsx7

I am having the same problem and it is driving me nuts. Replaced Idle air control valve. Pulling codes 22 and 54 any ideas?

aklim

This is a TPS and fuel pump voltage low. Do you have a scanner? I'd get one. I would want to see what the TPS really reads from the ECM perspective and what the fuel pump voltage is with respect to the DVOM and go from there.