Idle too fast

Hi,
Got a 1994 C3500 with the TBI 454 engine and automatic OD transmission. The truck runs fine, no engine problems while driving, but the idle is too high - around 1200rpm. This happens when both cold start or after being driven and warmed up - no difference. I checked for leaks on the vacuum lines, none found. Is there an adjustment on the throttle body? What other items can I check? Thanks

Have you tried spraying carb cleaner around the base of theTBI while it is running? Sometimes this shows up a leaking TBI mounting gasket. The idle on the TBI is controlled by the IAC(Iddle Air Control) which is a valve located on the TBI that is controlled by the ECM. There IS a set screww on the TBI to help set the base idle, but it is not to be used to set the normal idle. I would guess an air leak or the IAC.

You have a vacuum leak somewhere on the tbi unit or a sticking IAC

Thanks for the input guys. I checked the TBI with carb cleaner and couldn't detect any stumble. Checked the hoses again with no sign of vacuum leaks. I pulled the IAC and visually inspected but since I don't know what it should be doing I don't know if it is bad. Should I just go ahead and purchase a new one? I would like to fix the problem but don't want to waste money on parts I don't need.
Thanks

Clean all the black stuff off of the IAC valve and all the black crud inside the tbi where the IAC goes into . Also hold open the TBI throttle Cable and spray a ton of cleaner in there also. When you go to restart it might be a little hard (long crank time ) but it will start. REV it up a few times to help blow out the crap then put it in drive and just sit there a few mins for the IAC to relearn the idle
post it that helps or not

You should check your TPS. I recently had a problem with high idle and took the TPS off and measured the ohms. It started at 1K and showed that the pot was non-linear when I moved the throttle position from closed to wide open. The resistance would increase and than decrease and then increase. I assumed that some of the windings had shorted. I replaced the TPS and the idle came back down to ~500 RPM.

OK, today I removed and cleaned the IAC valve and cleaned the throttle body. Still no change. I replaced the air filter just to make sure it was alright, still no change. I checked the IAC measurement and found it was 1 1/8" from mounting base to end of plunger which I'm told is the correct length. Should the plunger be able to move in and out by hand when I remove it? What exactly is its function in relation to the seat in the throttle body? I assumed it would move to allow air in the orfice therby constantly maintaining a smooth idle but I cannot move the plunger by hand and I don't want to force it and break it. What's my next step?
Thanks

The IAC is an electric motor that moves the plunger in and out of the opening to control idle useing airflow. you should not be able to move the plunger by hand. With out a scan tool to get the iac readings i cant tell you if its good or bad

You CAN move the IAC by hand. It's part of the installation procedure. Put down pressure on the plunger with your thumb and wiggle back and forth. It will retract down shortening the length. The 1 1/8" is the maximum starting length measurement when installing to make sure the seat in the TBI isn't damaged when you screw it in. If it won't move by hand (unless it's already at its maximum retraction), it's bad. You can also hook up a 9v battery to it to verify that the motor is moving the plunger in and out, however, that does not guarranty that the ECM is moving it the proper distance. After installation of a new one or after changing the length of the original IAC, you start the engine and let run until up to normal temp. It may idle very high. AFTER you shut off the ignition, the ECM resets the IAC. A bad coolant temp sensor, the one at the front of the intake manifold can also cause high or low idle conditions. The ECM monitors the variable signal from the sensor to determine engine temperture, and then changes the IAC valve for idling in either hot or cold conditions. The temp sensor could be bad making the ECM thinks the engine is always cold, thus keeping keeping the idle high.

Rpreston has a good point about the temperature sensor. You can use an ohm meter and measure the resistance of the sensor and then correlate the resistance with the outside temperature when the engine is cold using the manual listed values.
I also replaced the gasket under the TBI which helped. I have had two TPS wear out in 90k miles which resulted in a bad reading to the ECM.
Harry

Thanks guys
I replaced the TBI gasket for good measure - the old one looked OK but it was $4 for a new one. Found one suspect vacuum line which I repaired just in case. Readjusted the throttle stop out some, it had been run all the way in by the previous owner. A test drive indicated a great improvement. I am going to monitor the fix under all conditions and see if this took care of the problem. Thanks for the help and I will repost the outcome.