1992 s10 4.3L CPI idles too fast

[LarryGLack]

1992 s10 4.3L CPI 4WD
I always thought it idled a little fast, but not fast enough to cause a problem... but over a couple days last week it got to where it runs at 35 mph with no gas pedal input. I drove the wife's because its hard to stop with the engine running that fast. (auto tranny) I checked the Idle Air Control valve and it was gummed up, I cleaned it and put it back on with no change in symptoms. There are no codes, at least the check engine light is not on. The light turns on with the key and shuts off as soon as the engine starts. I put a new IAC valve on, no joy - same problem. I removed the new one and set it on the engine, turned the key on without starting and observed no movement of the valve in or out. I pulled the plug off the valve and tested the first outside connection I came to, the test light was steady bright, the next connection in line was dark, the 3rd one bright, the 4th dark. It runs fine except for the fast idle. 220K miles. I understand when the key is turned on the valve should adjust itself in and out under ECM control. My question is - is there any ECM input that will stop this initial calibration or is the only option a bad ECM? I did plug and unplug the connectors at the ECM - no change. Everything else on the rig works like its supposed to....

[MT-2500]

Originally Posted by LarryGLack

1992 s10 4.3L CPI 4WD
I always thought it idled a little fast, but not fast enough to cause a problem... but over a couple days last week it got to where it runs at 35 mph with no gas pedal input. I drove the wife's because its hard to stop with the engine running that fast. (auto tranny) I checked the Idle Air Control valve and it was gummed up, I cleaned it and put it back on with no change in symptoms. There are no codes, at least the check engine light is not on. The light turns on with the key and shuts off as soon as the engine starts. I put a new IAC valve on, no joy - same problem. I removed the new one and set it on the engine, turned the key on without starting and observed no movement of the valve in or out. I pulled the plug off the valve and tested the first outside connection I came to, the test light was steady bright, the next connection in line was dark, the 3rd one bright, the 4th dark. It runs fine except for the fast idle. 220K miles. I understand when the key is turned on the valve should adjust itself in and out under ECM control. My question is - is there any ECM input that will stop this initial calibration or is the only option a bad ECM? I did plug and unplug the connectors at the ECM - no change. Everything else on the rig works like its supposed to....

With a high idle and no vacumn leaks and gas pedal not sticking IAC is working.

Remove TBI body and clean carbon from bottom ports and buttery fly and replace TBI base gasket.
9 times out of 10 the base gasket will be sucking air.

Let us know how it goes.

[Tech II]

I always thought the base gasket was a vac leak.....

Just as a double check, spray carb cleaner at the base and see if it affects idle.....if you have access to a scan tool, check IAC counts.....if zero, the PCM has closed the IAC, because of high idle, usually due to a vac leak..........if above zero, then we have another problem.....

[LarryGLack]

Thanks for the quick reply. This one is CPI not throttle body, I have had to fix that gasket in other rigs:-). I don't have a scan tool for OBD1 stuff, Drat.
Upon reflection the +volts I measured using a test light with the IAC disconnected is applied from the ECM. Are these pulses fast enough you would not see the light flicker? Or is it a steady voltage and the low side is switched (pulsed). With the IAC disconnected there would be no volts on the low side. I will check the connector at the IAC and see If I can get access without damaging the insulation.... I did squirt some carb cleaner around the intake but did not see a change - 'course there might be places I could not reach also... an update - i used a paperclip and vaseline to make connection to the IAC while connected. I have the same indication +volts steady light on one side of the IAC and no indication on the low side. I used my finger to plug the IAC valve hole in the intake, but my fingernail would not make a complete seal. It did slow down though not to a regular idle. I guess I'll get a ECM and see if that helps.

[LarryGLack]

I did some further testing and found the check engine light blinks 12 times over and over. This should mean there are no error codes present and the diag. mode is working. It is also (in most models) programmed to extend the IAC to limit the idle. I started it while in diag. mode and the check engine lite blinks fast for open loop operation. The idle is just as fast as ever.... it seems there is an EEPROM that must be programmed for the vehicle it is installed in, so I have to have a tool that will do that If the ECM is replaced. I live out in the woods a long way from any shop that would have one.... I will take it out and look at the insides just in case something will show up.

[MT-2500]

Originally Posted by LarryGLack

I did some further testing and found the check engine light blinks 12 times over and over. This should mean there are no error codes present and the diag. mode is working. It is also (in most models) programmed to extend the IAC to limit the idle. I started it while in diag. mode and the check engine lite blinks fast for open loop operation. The idle is just as fast as ever.... it seems there is an EEPROM that must be programmed for the vehicle it is installed in, so I have to have a tool that will do that If the ECM is replaced. I live out in the woods a long way from any shop that would have one.... I will take it out and look at the insides just in case something will show up.

Why not just replace the base gasket and be done with it?????

[LarryGLack]

How does the base gasket keep the IAC from extending/retracting when the key is turned on without the engine running? I am not sure where the base gasket is located on this engine. It is the CPI version not the Throttle Body one. I also noticed in diag. mode with the jumper on, all 4 connections are on solid on the IAC with it unplugged.

[MT-2500]

Originally Posted by LarryGLack

How does the base gasket keep the IAC from extending/retracting when the key is turned on without the engine running? I am not sure where the base gasket is located on this engine. It is the CPI version not the Throttle Body one. I also noticed in diag. mode with the jumper on, all 4 connections are on solid on the IAC with it unplugged.

Bad Base gasket cause air to bypass the TBI.
You will find the base gasket under TBI body.
With a to fast idle engine running the IAC should extend all the way close.
The goes to a preset position key on engine off.
When started the ECM controls IAC to adjust idle rpms.
If base gasket is letting in to much air the IAC can not control or shutdown RPM.

[Tech II]

I missed the boat on this one, too....

This is CPI, not TBI, so there is no base gasket....

Still should spray carb cleaner around the top of the engine, to check for a possible vac leak.....if nothing, then the operation of the IAC has to be checked out....a Tech I or Tech II can command the IAC open/closed....most techs have the external test tool that attaches directly to the IAC, and you can manually extend/retract the pintle to control idle....so if you manually close the IAC all the way, then you know, the problem is not the IAC if the idle stays high ....is the throttle plate closing all the way?

Has the throttle plate and bore been COMPLETELY cleaned so that it closes correctly? Is the cruise cable keeping the throttle plate open?

[LarryGLack]

The idle does return all the way and the throttle plate is completely closed. I did spray carb cleaner around but could find no change. I removed the ECM and the eprom - (might have been a bad connection) put it all back together and tested it - no change still runs fast. I will check on a tool so I can see what is going on in the computer.

[LarryGLack]

Have any of you had any experience or rumors about an analyzer tool called TOAD OBD1 OBD2 Auto Scanner? I see I have to purchase one in order to work on this thing.... The Toad is about $50 and seems to do it all.

[Tech II]

Went online....if you have a tablet or laptop that is Blue tooth enabled, getting the bluetooth ELM327 and the Torque Pro app(Google store), is very low cost(under $30)......for all the live information that you get, especially the ability to plot graphs(great for P0420, plotting the pre and post O2 sensor)......I have spent upwards of $200 for a scan tool, and it doesn't come close to this(from what I've seen on YouTube).....with a Bluetooth enabled tablet, you don't have any cables involved.....impressive at this cost.......of course you have to have a bluetooth tablet.......if you want to put it on your smart phone, it's a different version of ELM327.......displays on phones are too small anyway....get a tablet....

[LarryGLack]

Hey Tech II: I think my laptop has blutooth, but I don't mind a cable, either way is OK with me. The TOAD is able to do blutooth also but it does not seem to have the codes for this particular OBD1 vehicle. I will check out the torque pro and see if it will run this old rig... I guess If I can't locate a scan tool, i am reduced to changing parts.... (*Ugh*) that will probably cost more than the tool... but on the bright side I'll have lots of spare parts. ;-] I guess its just not old enough, I had a Plymouth Clipper (1952 I think) it got 22 mpg and did not have a computer - it could still be worked on!

[Tech II]

Check this out and then watch Part II.....

http://www.youtube.com/watch?v=bthjTb5ZQDo

[LarryGLack]

Sorry I did not finish this sooner, it was an intake cover leak. The gasket under the cover you remove to replace the CPI spider fuel assy was leaking. New gasket fixed it right up.... This is an example of how not to overthink it!