89-Fast Idle
#1
89-Fast Idle
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?
Any suggestions on what to try next?
#2
Team Owner
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?
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?
#3
Drifting
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
See here:
http://www.chevythunder.com/do_it_yo...ge.htm#Setting minimum air
#4
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?
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?
#6
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
See here:
http://www.chevythunder.com/do_it_yo...ge.htm#Setting minimum air
Any thoughts on that?
#7
#8
Drifting
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.
#9
Burning Brakes
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.
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.
#10
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?
#11
Race Director
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).
A mechanic's stethoscope is the best way to check them (and it's useful for lots of other things).
#12
Drifting
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?
#13
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?
#14
Team Owner
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.
#15
Team Owner
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?
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?
#16
Race Director
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
Last edited by Cliff Harris; 08-30-2013 at 01:30 AM.
#17
Race Director
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
See here:
http://www.chevythunder.com/do_it_yo...ge.htm#Setting minimum air
#18
Team Owner
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.
#19
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?
#20
Race Director
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:
Last edited by Cliff Harris; 09-07-2013 at 04:41 AM.