1989 c1500 fast idle

[RABarrett]

In order to simplify the troubleshooting here, bear in mind that the ECM controls the idle via the IAC. If the problem is extras air, as from a vacuum leak, there are no solutions in the world that will fix it until you address the air leak. Using a scan tool, check the IAC counts. I suspect that they will be close to zero, indicating an air leak. Typically, these are the result of the gasket under the throttle body leaking. Replace, if necessary, then allow the system to recalibrate itself. Ray

[RABarrett]

Food for thought... Code 12 is not an all is well signal. It is the result of no rpm signal to the ECM, which is expected if the engine is not running. There is no all is well signal in GM vehicles, only expected codes, as in the code 12. Ray

[homefree]

The only other things that i would look at now would be:
Coolent temp sensor, and check that plug to make sure that the ends are o.k. and check the wire for brakes. The sensor is on top of the motor by the upper radiator hose. If i remmember right when the truck is running and cold if you pull the plug off this sensor the truck should idle down. If bad i think they cost around $20.00. Next I would take that IAC off the throttle body and make sure that the little pintal is still on the end of the IAC (just to make sure that it didn't fall apart inside the throttle body. Mine did and it was brand new.) When you have that IAC off the throttle body look into the hole where it screws into and check for carbon build up, this area has to be pretty clean so the pintal can work properly. Did you check the EGR valve? It maybe stuck open a little.

[troybuilt]

It sure sounds like a bad o2 sensor again or a real lean condition from a vacume leak. Sensors have a high failure rate when they are not OEM.

[johnnymcbrown]

RABarret said "Code 12 is the result of no rpm signal to the ECM" , Where does this RPM signal come from?
and you said "gasket under the throttle body leaking" , Is this throttle body to the intake manifold or the 2 easy gaskets on top?
homefree said "Coolent temp sensor".., I check the sensor on top of the engine and it responded OK, but the other sensor, on the driver side of the engine around the sparkplugs, It has 12v in the wire going to the sensor and the sensor itself registers some ohm value that varies with heat up..Is that one a temp sensor also?
You also mentioned the EGR valve..I tried to pull a vacuum on the valve itself with a vacuum hand pump , and could not get a vacuum..But I can see the EGR valve lifting up and down slightly when engine vacuum is applied. Any clue there?

[troybuilt]

code 12 is a all clear code, the egr is supose to open and close but that is no big deal because the hose from the egr goes to the back of the TBI. vac. leaks need to be check by unhooking the hose from the intake and checking the various hoses, remember the vac. is created from the intake only when running. you might try checking the intake gaskets too.
you've got a tough one

[toivo]

you need to take the top off the air cleaner and if you hear a very loud hissing sound your iac valve is out of adjustment and no matter what you change it will continue to idle fast. spray carb cleaner on the throttle return spring and make sure it is returning al the way to rest. then you will have to pull the plug that blocks the idle screw adjustment. turn the key to the on position for 30 seconds and then unplug the idle air control valve. turn the key off and hook up a tachometer or scanner and start the engine. adjust the idle screw to bring your vehicle to 600 to 675 rpm in drive with the wheels blocked. then turn off the engine, hook up your iac valve and go for a drive up past 40 mph. then the iac should reset itself. and your idle should return. this is after you check your timimg. good luck.

[johnnymcbrown]

toivo--you said "after I check the timing" I would really like to check the timing, but I understand I am supposed to disconnect some wire before checking the timing. I can't figure which wire??

[RABarrett]

Something to consider here is that code 12 is not an all clear. It indicates that the computer is not getting an rpm signal. This is normal if the engine is not running, but there should be no code 12 when the engine is running. I have repaired my share of problems by considering the 12 as a real problem, especially if the light is on and that is the only code set.

[johnnymcbrown]

About the code 12, I only get that when I jumper A and B at the plug, no "check engine soon" light when running.
That post from <toivo> above about , turning the ignition on for 30 seconds to set the IAC fully extended, "no air", then unpluging the IAC, and starting the engine, effectively removing the EMC's ability to control idle speed, (I think?) BUT.....I need to check the timing first!
I couldn't locate the mistery wire , I'm supposed to disconnect before setting the timing, so I set the timing "as it is" . It did slow the idle down to a reasonable level, BUT the engine wouldn't go, pull, just coughed and sputtered. So I put it back where it was and it will go again, BUT fast idle is back..

[nusch]

If you set the timing without removing the wire it will throw your timing off even more. I replyed to your post http://www.automotiveforums.com/vbul...d.php?t=175021 on where to find the EST wire.

[toivo]

john, i goofed, but, first things first. code 12 indicates your ecm is working, nothing else. now , remove the air cleaner,plug any vacuum lines that you disconnect. with key off, leave the idle air connected, ground the aldl connector,turn ignition switch on and wait 30 seconds. this will let the iac retract. if you put your hand on it you will feel it running. with key still on disconnect the iac valve connector. remove the ground from the aldl and start the engine. engine should be warm when you do this.or let it reach operating temp. apply parking brake and remove the plug from the idle stop screw. leave engine in neutral if manual, put in drive if automatic. adjust rpm to 500 to 550 on an automatic, 600 to 650 if manual. turn engine off, reconnect everything and drive up past 40 mph and eac should recalibrate itself. to time the engine you have to disconnect the tan wire with blk tracer. this wire is either in the relay control pnl. on the firewall or under the glove box in the truck. if you have crossfire injection system the procedure is different. you also did not say if you heard a loud hissing noise when you removed the air cleaner. or if you sprayed carb cleaner around the base of the tbi to check for vacuum leaks. vacuum should read between 18 and 22 iches. if it is less your leaking air or have other problems.let me know.

[johnnymcbrown]

JUST WANTED TO RESPOND AND THANK YOU, I took the Throttle Body off the intake and It was just as you said...The gasket had hardened an a piece of it had "blown out or in" anyway it was gone..Replaced the gasket and ALL IS WELL!!!

[RABarrett]

I am pleased about the air leak under the throttle body. To quote the GM service manual, code 12 is no rpm signal to ECM. That is ALL it means. It has no effect on the ECM. The signal comes from the ignition module, and is used to control the speed of injection. Code 12 is expected id the engine is off, key on, seeking codes. I have repaired too many GM vehicles bearing only code 12 by repairing rpm signals to the ECM to tolerate the code 12 being an "all clear". Ray

[ophawk]

I had the same thing happen years ago. I had the gasket under the throttle body suck in, and that caused a huge vacuum leak. replaced the gasket and it went right back to normal idle.