My 89 idles at 1200 rpm in Park when first started. Even after getting warmed up, it will only drop to 1000 rpm in Drive at stops with foot on brake. I have disassembled and cleaned the Throttle Body and IAC, checked for disconnected vacuum hoses without success. The Cruise Control works fine so I am assuming my vacuum system is OK.
Any suggestions on what to try next?

 

You can have vacuum leaks even if the hoses are connected. I would run a propane torch over all hoses. UNLIT, just gas. RPM spikes, you have a leak.

As to RPM, how do you know what the RPM is? C4 tachs are notorious for reading wrong.

Do you have a scanner to check your idle speed as to what the ECM sees? Any modifications?

 

Fully warmed in gear should be right at 600. You either have a leak, or need to perform a minimum idle air adjustment. Has the factory cap for the throttleblade screw been removed? This would mean adjustment has possibly been mis-adjusted.

See here:
http://www.chevythunder.com/do_it_yo...ge.htm#Setting minimum air

 

Tried the propane method with no RPM spikes. Used a whole bottle. I tend to believe the RPM reading because in Drive, it reads 900-1000 rpm and if I don't hold my foot on the brake the car will move forward a brisk rate. I have a Tach/Dwell meter with an RPM gauge I could try but I really think the readings are fairly accurate.

 

Since you took your IAC off, I would go through the set up procedure and also set your tps sensor.

Scott

 

The factory cap is still in place. I should have mentioned that when I said that I cleaned the IAC, that was done when it was still installed in the housing that mounts to the underside of the throttle body. I removed the housing and cleaned the IAC pintle as best I could through the housing orifice. I'm thinking the next step is to go back in and remove the IAC completely to ensure it is really cleaned and not siezed up.
Any thoughts on that?

 

Scott:
See further comment below. IAC was never completely removed. Comments?

 

Pull the IAC, and clean and check for function. Put a scanner on it and watch both IAC, and TPS function. Also, put a finger on each injector too feel for a click. A bad injector could cause a lean condition and high idle.

 

Causes of high idle:
Vacuum leak - Sucking sound.
Bad IAC valve - It's a vacuum leak. You'll hear it.
Low Fuel Pressure/bad injectors - running lean
Junk Throttle Position Sensor - I believe the 89 still had an adjustable position TPS. It might be loose and skewed.

Anyways, my guess would be a vacuum leak caused by the Idle Air Control valve.

 

OK. I will pull the IAC and clean. Don't have a scanner or know anyone around Ottawa Canada with one. Tell me more about the finger on the injectors. When should i feel a click? Does it only happen once,etc?

 

The injectors operate continuously while the engine is running. Once per revolution, so that would be 600 clicks per minute at 600 RPM, or 10 clicks per second.

A mechanic's stethoscope is the best way to check them (and it's useful for lots of other things).

 

There you go, make an appointment with your doctor to do what common sense can accomplish. His 8 year degree is superior to a 6 year degree, so what he says vs a lifetime of experience of the sbc is superior. Geez man, really?

 

I completely removed the IAC and cleaned it this time. There was lots of black carbon caked on it. I checked the pintle dimension for 1 1/8". Checked again for obvious vacumm leaks and reinstalled IAC and TB. No improvement in Idle RPM. Should also mention that the car still stalls when shifting into gear (D or R) when warm. I have learned to apply gas and brake together to get underway. When it does stall (warm), it is often difficult to restart and I have to hold the gas down to the floor. Are these related problems?

 

Such as? Listening to the neighbors fighting and make up sex? Why do you think I bought the wife a Master Cardio Stethoscope when she graduated for her RN class? Why do you think I bought the wife an electronic stethoscope when she graduated NP class? To listen to the engine? The electronic one can record sounds.

 

OK. That is a good first step. Did you remove the IAC housing and clean all the passages of the TB? Did you use new gaskets? Those paper gaskets are like condoms. You don't reuse your own or your buddy's.

Refresh my memory:

1. How is your timing and what is it set at?
2. Do you have a scanner to read the IAC counts?
3. What is the status of your injectors? New? Original?
4. Have you checked the TPS for smoothness all the way to WOT?
5. Fuel pressure good? Does it hold pressure after shutdown?
6. What does the scanner say your real RPM is?

 

Can your doctor hear a bad bearing in an alternator or A.I.R. pump? Can he hear solenoids operating while the engine is running? I don't think a medical stethoscope would be suitable for those kinds of applications, but I guess your lifetime of experience allows you to check these things without tools. I suppose you have calibrated fingers? Do you send them out to a metrology lab periodically to make sure they're within specs?

You have a lifetime of experience but the OP may not...

Here's a fun test for your calibrated fingers: Disconnect an injector and start the engine. I believe you will find that the vibration of the other injectors will make the disconnected one FEEL like it's operating normally, when it obviously is not.

Try it, you might like it:

http://www.harborfreight.com/mechani...ope-41966.html

 

with the minimum idle adjustment. I had to remove the cap to adjust mine and it did fix the stalling problem I was having. Throttle bodies wear and mechanicals change over time.

 

I agree it works well for stock applications but when things change, stock goes out the windown. Just get a scanner and set it via the IAC counts assuming timing is right and TPS is good.

 

I am searching for a scanner that will work on an 89 but don't have one as of yet. In the mean time, after the last IAC removal and cleaning, the idle returned to 600 rpm and everything worked well (no more stalling when putting it into gear). I have driven the car 4-5 times since then and the in some instances, the old fast idle returns, and then after a few miles, it will reset to 600 rpm so something is definitely intermittent. I suspect the IAC because when I cleaned it, I found it very stiff and difficult to compress with my thumb. I'm not sure it moved much at all but do not know what a good one would feel like. Should the pintle have compressed easily under thumb pressure? Is there a good check for IACs when they are installed to see if they are functioning properly?

 

IACs are what is known as stepper motors. The ECM applies pulses to it that cause it to "step" once for each pulse (vastly simplified description). It turns a fraction of a turn for each step. If you look at the pintle shaft you will notice that it is threaded like a screw. When the stepper motor turns the pintle gets "screwed out" or "screwed in", depending on the direction the motor turns. When you push on the pintle you're forcing it through the "nut" inside the motor. I screw it in with my fingers.

The internal design varies depending on the brand so some feel different than others. I have pushed the pintle in on one I have and I couldn't push the other one in and had to screw it in.

That's a tough one. The only thing I can think of is that when you short terminals A & B of the ALDL connector the IAC will continuously try to extend. If you disconnect the cooling fan relay and turn off the AC so the HVAC fan doesn't run you can hear the IAC clicking as it tries to extend.

Even off the car is difficult, as it requires two square waves that are 90° out of phase with each other. The phase determines the direction the IAC turns. This kind of signal is not easy to generate. Probably the easiest way is to use a stepper motor driver IC. I saw a YouTube video several years ago where some guy programmed an Arduino microcontroller board to exercise his IAC. Here ya go: