High Idle IAC TBI
 

90 Silverado TBI is Idling around 1200. Traced to IAC. Did relearn procedure. Idled down to 650. Shut truck off. Restarted and its back to 1200 rpm. Changed IAC. Did relearn. Idle comes down but goes back up after shutdown and restart. Same as original IAC. Installed new Coolant temp sensor and thermostat. Timing is set. Idle stop screw on TBI unit has never been touched. Did smoke test and zero leaks. Excellent fuel pressure and regulator tests fine. Would this be a bad ECU? Thanks

Re: High Idle IAC TBI
 

Welcome to the board.

The ECM is not a high failure rate item.

If you have not checked it for trouble codes that can be done with a paper clip. You may have set false trouble codes if you powered up the ECM with electrical connectors unplugged.

First thing I would do is look at the ECM data and see what it shows for RPM, TPS (Throttle Position Sensor), and IAC count. Good equipment can also show the desired RPM that the system is trying to achieve.

If the TPS voltage is to high the ECM will not send commands to the IAC.

A problem these truck can get is comprised RPM signal due to distributor issues, like loose shaft wiggling sideways, or deteriorating magnetic strength needed for the pickup coil to produce a good RPM signal.

Poor ground problems also seem to cause the system to have trouble reading a good RPM signal.

The TPS voltage can be tested with a meter.

Another possibility that I have run across helping people on the message boards is a failing PROM (chip) in the ECM causing a high idle like you describe.

Re: High Idle IAC TBI
 

Thanks for the response. TPS tested fine and i installed a complete used ECM. Issue persisted. There are no stored codes and no check engine light. Agreed seeing data would be helpful but i do not have that ability. Was hoping somebody had seen the same issue. Distributor issue might make sense but it wouldnt explain why the IAC relearn works but forgets it when engine is shut down and restarted. Goes right back to high idle. Even bad grounds i dont think would explain the forgetting of the relearn. Also smoke tested and did the propane method of looking for a vac leak. Can not find any leaks. Thanks

Re: High Idle IAC TBI
 

On a 1990 you can use WinALDL, but that takes a cable and a lap top.

I have helped several people with this same complaint, and most were a struggle.

This problem seems more common on 2500 series & 1 ton trucks, which a few ended up as PROM failures.

I will think on this and see if I can come up with more ideas.

Seeing the data would really help.

Re: High Idle IAC TBI
 

Did you test the voltage on the TPS signal wire, or test it with an ohm meter?

Re: High Idle IAC TBI
 

Make sure it is not leaking vacuum through the brake booster at times. With a torn diaphragm they can leak at times and seal up again after the brake is used.

Re: High Idle IAC TBI
 

When the engine is shut off the ECM opens the IAC in preparation for the next start.

The wire terminals can loosened up right at the IAC plug. It can cause erratic operation.

Re: High Idle IAC TBI
 

Maybe the computer isn't getting memory power. That would wipe out codes and anything it learns.

Re: High Idle IAC TBI
 

Thanks for the responses guys. Im going to be looking more closely at wiring today. I eliminated booster by plugging the hose. Can rule out PROM as i swapped out the ecm with a used unit from the U pull it yard. As far as codes go i had a 44 or 45(cant remember). Changed out O2 and it hasnt come back so i would assume memory power is not an issue. Will look up how to ohm out the TPS. I tested voltage. The IAC operation isnt really erratic in that the relearn is normal and works every time. And forgets the relearn every time when shut down and restarted. I recently regasketed intake manifold for oil leak and changed TBI base gasket. The high idle was there prior to that. Thanks

Re: High Idle IAC TBI
 

The voltage test for the TPS is a better test so there is no need to test the TPS with an ohm meter.

Did you replace the coolant sensor by the thermostat or the sensor in the head by the spark plugs?

Re: High Idle IAC TBI
 

Yes. I replaced coolant temp sensor on intake manifold. And thermostat. tested coolant sensor anyway and its fine and receiving 5 volts when tested at pigtail. looking into getting this scanned. TPS voltage at idle was .48v. I think thats an acceptable #. Thanks

Re: High Idle IAC TBI
 

Yes, that is acceptable.