jrobinsonsrr

My 88 C4 will not idle below 800 rpm. Does not throw a code. Distributor is set by ecm. Throttle body is clean and all vacuum hoses are new. Runs fine otherwise. Any ideas?

ls777z

Just a thought but is there anything in the way of the linkage? I saw a video that the high idle was caused because the linkage was rubbing against a hose stopping it from returning all the way back..

FAUEE

IAC valve stuck? Maybe try an IAC relearn?

aklim

Get a SCANNER. Check what the ECM is commanding the idle to be. That is where you start. If the ECM wants 800, it will get 800. Check what the IAC counts are when everything is off but the motor. It doesn't matter what YOU think is going on as much as what the ECM thinks is happening. It could think the temp is low and the RPM needs to be up. That is where the CTS value comes in. It could be an air leak which would be showing with a 0 IAC count.

mazdaverx7

There are a few things to check. I would remove and clean the idle air control valve. They can stick when the get carboned up and cause the idle to hang.

There's a chance that the coolant temp sender for the ecm is failing, causing the idle to be slightly increased as if the engine were in cold start enrichment.typically though, the idle would be higher than 800 rm, but this is an option and the sender can be removed and tested.

Another thing to consider is the onset of worn throttle shaft bushings. An air leak past the bushings resulting in a slightly higher than normal idle or a lingering idle.

cdm747

If all the above items check ok, make sure someone did not adjust the idle stop screw on the driver side to where the throttle blades are not closing completely.

Hot Rod Roy

Wrong! Unless your engine has been replaced with a later model engine.

jrobinsonsrr

IAC is clean and clear. Cleaned with the throttle body. Linkage is free and clear. Low speed idle screw is still sealed from factory, don't think it has been changed. This is mentioned in the FSM as a last resort to idle setting. Where can I find a Scanner? My FSM does not mention a relearn on the IAC. Where can I find the instructions for this? Is it part of the Scanner docs?

aklim

From what I see, this isn't meant to be set since it should come "close enough" from the factory. It assumes everything is stock and is working in perfect order hence the lack of need for adjustment.

I would get a scanner either as a used unit like a Snap On MT2500 or a bluetooth dongle using the ALDL Scan (Free) program for android. That will let you see what the ECM is seeing as opposed to getting a code because a value is way out of whack.

I think they are talking about the adjustments that the ECM is making in the future. I wouldn't bother with it at this juncture. First of all, you need to see if your tach is accurate as per the ECM. I would read around to find a scanner. You will have to put out some money first but it will be worth it in the future. Or you will be spending more time and money kicking around trying to guess what is wrong before you run out of money.

xrav22

When my idle shot up to 1000 all of a sudden it was because my tps was loose and turned forward making the computer think foot was on gas. It was
.76 volts. Put it back to .54 and all was good.
Do a idle procedure first.
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Good Luck

aklim

That process will ASSUME everything else is in spec and somehow the screw got mistakenly set. Tach is not always accurate, leaking occurs, timing might not be right and sensors are spot on

xrav22

Well I would think that if everything else was working then the idle procedure will help set the idle.
You have to start somewhere. Then see if any codes come up.

aklim

"IF" being the operative word. And IF everything else is working, how did we get to this point? The set screw doesn't usually come out on it's own. That is why I said to scan and see what the ECM is thinking is going on. Assuming he does have high idle, it often is an air leak but before I chase the hoses for a problem, I'd like to know if I am on a wild goose chase or the problem is elsewhere. Honestly, i don't understand why people will buy hundreds of dollars worth of tools but are so resistant to a scanner.

jrobinsonsrr

Purchased an MT2500. No codes are present. Sensors are in range. Desired idle runs 100-200 lower than actual. I am going to set timing and if that does not work, I think the next step is the idle set.

aklim

What is the timing at? Is the tach accurate? What is the IAC count at when it is warmed up to stable operating temp and everything but the motor is off?

aklim

How did you verify the sensors were reading right? Did you scan and do an IR on the intake and the CTS? Your desired idle was lower than actual. Actual being what the scan tool said idle was or the tach?

CorvetteRules

If you want to eliminate the CTS unplug it and see if the idle is lower. Although I can tell you when my CTS was bad it showed up as a code 15 when scanned.

jrobinsonsrr

IR of block at temp sensor agrees with dash and scan. Idle rpm and Scan idle rpm are equal, though Scan idle is in finer increments than dash. Scan idle rpm was higher than desired idle rpm on Scan tool. TPS voltage on Scan was equal to actual read from DVM at TPS and in range. IAC read in correct increment on Scan tool as referenced in SM. O2 reading on Scan read primarily Lean, but did flip to Rich momentarily as if reading was close to edge. Spark angle in degree read around 27 in idle though emission label said it should be 16.

jrobinsonsrr

I am not getting any Codes. I can try unplugging it tomorrow. Any others I should unplug to see if it changes?

aklim

When you had everything but the motor off at operational temp, how many IAC counts?