1988 5.7L TBI Low Idle

[posk5blazer]

Hey Everyone,

Thanks for looking. Just got the truck running after a no run situation off the fresh rebuild. 170 psi all around, swapped ign coils and cleaned plugs. Now it runs beautifully except for the fact that it dies around turns and wont idle 50% of the time, and the other 50% it sits there idling around 350-450 barely staying alive. Pulls a code 42 est but no service engine soon light while running. disconnected battery and code was gone but came back. O2 sensor jumping from all around in between 150-800mv, coolant temp reading, closed loop, everything else seems normal.

Will not start unless i give it gas and crank for 5-9 seconds, retain foot on gas for another 30 seconds and thats when I find the low idle, will stall usually after a minute or so. Runs beautifully if I keep my foot on the gas.

Any ideas?

Thanks A Lot

 

[smier]

Double check for vacuum leaks and that all the vacuum lines are hooked up correctly. Check your IAC (Idle Air Control) because if you need to hold the pedal to the floor to get it to start it sounds like it's not getting enough air thru the throttle body. Which will also affect your idle. I'm having a bit of a brain fart right now, so sorry for lack of correct terminology, but double check that the throttle plates and linkages are moving freely and aren't binding or stuck. Remember, the ECM controls the injectors and how much fuel they deliver via readings from the TPS (Throttle Position Sensor), Oxygen sensor, and the CTS (coolant Temperature Sensor, the one in the intake manifold). All of them need to be hooked up functioning properly!!! You really only directly control the air flowing thru the throttle body... Hope this makes sense. What's the injector spray pattern look like? Nicely defined cone shape or is this thing just dumping fuel and running really rich and flooding on you?

 

[smier]

CODE 42

Trouble Code 42 indicates that there may be a malfunction in the Electronic Spark Timing (EST) system.

During cranking, the timing is controlled by the ignition module while the ECM monitors the engine speed. When the engine speed exceeds 400 RPM, the ECM sends a BYPASS signal to the ignition module which switches the timing to ECM control. The ECM calculates what the timing should be then "tells" the ignition module via the EST circuit.

An open or ground in the EST circuit will stall the engine and set a Code 42. The engine can be re-started but it will run on ignition module timing.

The conditions for setting this code are:

System in BYPASS mode but the ignition module is still controlling timing
** - or -
Engine speed > 600 RPM with no EST pulses (ECM controlled timing) going to the ignition module for 200 msec.

Typical causes for this code include:

1) BYPASS line is open or grounded
2) EST line is open or grounded
3) PROM or CALPACK not seated properly in the ECM
4) Poor connections between ignition module and ECM
5) Poor routing of EST harness and/or poor quality ignition wires (EMI induced electrical noise)
6) Faulty or incorrect ignition module
7) Faulty ECM

 

[posk5blazer]

Thanks a lot, plenty of useful info. I appreciate your time spent. Yes, I work with carbs all the time and I recognized this to seem to be a lack of air, it's doing the same thing as if you are choking it out. Also, my spray pattern is cone-like and pulses quite quickly, too fast to really see a distinct pulse. I will check all my wiring in the est circuit and ign circuit. i have taken out and cleaned the iac, how might I check it, or should I just replace it for shiggles?

Once again thanks a lot!

 

[AJBack]

My 87 is doing the same thing. Did you ever figure out what was wrong with it?

 

[496truck]

There's a procedure for setting BASE IDLE too. Do some searching here, it has come up recently.

 

[garlicbreath]

Hmm, It's gotta be OBAMA's fault!

Seriously though, I have an 87 2wd Suburban that is doing the same thing after replacing the throttle body mounting gasket.

AND my brother in laws 87 4wd K5 is doing the same thing too.
Low idle, dies at lights etc.

 

[British Slave]

Take the paper clip and open it up and form it into a big "U" shape. Insert the clip ends into the ALDL in the 'A' and 'B' pins.
Turn on the ignition, but don't start the engine. Wait 30 seconds. Now, go remove the connector from the IAC. Remove the paper clip.
Start engine. You are now going to adjust "minimum air". There is a Torx screw on the side of the throttle body. This is what needs to be turned to adjust minimum air, or more commonly known as "idle speed". It comes from the factory with a protective metal cap over it. If the cap is still there, use a small punch to knock it out. Set the idle speed to 450 rpm, rotating the Torx screw clockwise to raise rpm, and counter-clockwise to lower rpm. Once the idle rpm is set, turn off the engine.
Re-connect the connector onto the IAC. Start engine. Idle speed is now once again governed by the ECM, but your idle should be smooth and steady, approximately 600 rpm in Drive (for unmodified cars).
If you set an SES light by having the IAC disconnected, then after shutting down the engine disconnect the negative battery terminal. Wait 5 minutes. This will clear the ECM of all trouble codes. Re-connect the battery and drive the car for 20 minutes to allow the ECM to relearn your driving style.