Hello everyone,
I'm in desperate need of help again with my stick shift 91 RS with TPI. When the engine warms up the idle starts jumping up and down for some reason. I've changed the following:
-Idle Air Control Valve with Pigtail
-TPS Sensor with Pigtail
-Temperature Sensor
-Ignition Control Module
-Spark Plugs and Wires
-Oxygen Sensor
-I also smoked the vacuum lines and checked for leaks. I did find one and fixed.

Now the car is running a whole lot better but it still idles funky when the engine warms up.


Should I start checking my fuel lines next?
Maybe a bad Fuel pressure regulator?
Also posted a video.....

 

Adict - The video is very helpful. It was not apparent that the "CHECK ENGINE" light was on while the engine was running, so I'll presume it was off. If the light is on with the engine running, the cause needs to be investigated first.

It is important to be sure the throttle body bores, throttle plates, and IAC air passages are clean and free from varnish and contamination. If the TB hasn't been cleaned, now is the time.

Also, since your external vacuum lines have been checked and verified, it would be helpful to disconnect the vacuum line to the brake power booster and cap it, then restart the engine to see if the condition still exists. If the idle variation is solved, start looking at the brake booster. If not, reconnect the booster line and proceed.

The next step will be isolating the EGR valve, a.k.a., the internal vacuum leak. Remove and cap the vacuum line from the plenum to the EGR solenoid. Restart the engine to determine if the idle variation still occurs. If not, the EGR solenoid may be leaking internally, or the ECM is cycling the EGR at the wrong time. If the condition still exists, keep the EGR vacuum disconnected and capped temporarily while you proceed with the diagnosis, but remember to reconnect the EGR vacuum line when finished.

You'll need a multimeter to perform some voltage and resistance checks, and a tachometer (other than the one in the instrument panel) will be helpful.

The first check will be the base voltage to the TPS - It should be the gray wire. Voltage should be 5VDC referenced to chassis/engine ground. If the base voltage is correct, measure the TPS output voltage with the throttle closed. It should be below 1.0VDC. Test it with the KOEO (ignition on, engine not running), then test it again with the engine running. The output voltage should remain stable while the engine is running. If the TPS output varies while the engine is running, check the base voltage with the engine running to verify whether it is steady.

The next step will require getting the engine to normal operating temperature (to 190°F or higher coolant temperature) then shutting it off. With the engine off, turn on the ignition but do not start the engine. Wait about ten seconds in this mode, then disconnect the IAC electrical connector. Restart the engine - It may require that you apply a little throttle with the IAC disconnected to start the engine. Once running, the idle speed should remain steady with the IAC disconnected, and the engine RPM should be about 550. If the RPM at this closed throttle position is higher than 550, the throttle stop screw may have been moved. If the engine is at full temperature, adjust the throttle plate stop screw to achieve an idle at 550 RPM, ±50 RPM. Idle speed should be steady. If not, about all the common potential problems are exhausted, and there may be vacuum leakage, fuel pressure modulation, or other issues at play.

 

Thanks for the quick response Vader.
I don’t have a check engine light on which is a good thing.
I did clean the throttle body blades and cleaned the IAC air passage. I also removed the brake booster vacuum line and capped it off, still no fix. Next I disconnected the vacuum line that goes to the EGR, capped the line and problem still continued.
Voltage on the TPS grey wire remained at about 5vdc.
Lastly I turned ignition switch for 10 seconds then disconnected the IAC. Turned engine on and RPM’s jumped up to a steady 4000 in neutral. I was only able to adjust it down to about 3000 as the screw was retracted to the point of no contact. After connecting the IAC back the engine barely was able to stay alive with the same idle surging up and down.

 

Reconnect the IAC and jumper the ALDL pins A and B, wait ten seconds, then pull the IAC connector. Shut off the ignition, pull the jumper from the ALDL, and try adjusting the throttle stop screw again.

Apparently, I forgot that critical step in my initial reply. Sorry for the confusion.

 

Just took a look at my ECM and noticed its off a 1986 IROC. Did Chevy make 5 speed IROC's in 86? Do I have the wrong computer in this car thus causing all my weird idle?

These are the numbers on the ECM Label:
16127491 (top left)
ANTS (top right)
86ANTSK100780749 (Barcode)

 

I also noticed im missing my MAF sensor. WTF? Comp says TPI is from a 86 so it should have a MAF right?? Yet no codes......
I did find a the MAP sensor for the RS still hooked up. This is driving me nuts.