C4 Idle Issue

[Ed A.]

Hi Folks,
I'm still trying to adjust the idle on my 1985 L98 Vette, my idle varies from 450 to 950 RPM's and the idle adjuster is just touching the adjustment lever. Last summer I had my car in the shop for a complete tune, plugs, cap, rotor, wires etc. and I had a new IAC valve installed none of this cured my idle problem.
I have since checked the throttle body and elsewhere for vacuum leak and found none, I also cleaned the throttle body and set the TPS to .54 volts. At cold start it idles steady at 950 RPM's until it reaches 145 degrees' when the idle becomes irregular and it gets worse almost stalling as the car gets warmer.
My goal was to reset the IAC valve using the manual method, grounding the ALDL etc. however I can't find a way to disconnect the IAC harness, the lock tab is on the back side of the connector and while I have small hands I can't find a way to disconnect the locking tab without the possibility of breaking it, so I gave up on this method.
I was able to borrow a Tech 1 scanner but I don't have a manual for it and the guy I borrowed it from hasn't used it in 20 years, it has the MSC ( Mass Storage Cartridge ) # 3000003, using the Powertrain option and the Data List option I had no trouble codes but my IAC count was 85 with the engine warm and running and in open loop.
If anyone can offer any advice on the scanner use or how to disconnect the IAC harness it would be greatly appreciated.

[David H.]

Hi Folks,
I'm still trying to adjust the idle on my 1985 L98 Vette, my idle varies from 450 to 950 RPM's and the idle adjuster is just touching the adjustment lever. Last summer I had my car in the shop for a complete tune, plugs, cap, rotor, wires etc. and I had a new IAC valve installed none of this cured my idle problem.
I have since checked the throttle body and elsewhere for vacuum leak and found none, I also cleaned the throttle body and set the TPS to .54 volts. At cold start it idles steady at 950 RPM's until it reaches 145 degrees' when the idle becomes irregular and it gets worse almost stalling as the car gets warmer.
My goal was to reset the IAC valve using the manual method, grounding the ALDL etc. however I can't find a way to disconnect the IAC harness, the lock tab is on the back side of the connector and while I have small hands I can't find a way to disconnect the locking tab without the possibility of breaking it, so I gave up on this method.
I was able to borrow a Tech 1 scanner but I don't have a manual for it and the guy I borrowed it from hasn't used it in 20 years, it has the MSC ( Mass Storage Cartridge ) # 3000003, using the Powertrain option and the Data List option I had no trouble codes but my IAC count was 85 with the engine warm and running and in open loop.
If anyone can offer any advice on the scanner use or how to disconnect the IAC harness it would be greatly appreciated.

Ed

At "cold start" and until warm, engine control by your ECM/PCM is in "OPEN LOOP" mode. "Open loop" is a preprogrammed set of engine operations instructions internal to your engine or powertrain control module. This mode remains in effect until your engine temperature reach a preprogrammed level (yours appears to be 145F) - then ECM/PCM transitions from "open loop" to "CLOSED LOOP".

During "closed loop" operation, your ECM/PCM optimizes air/fuel mixture based on input from Oxygen Sensors in your exhaust pipes. Sensors measures amount of unburnt oxygen in your vehicle's exhaust gases.

Four total O2 Sensors, two on each side of your exhaust pipes. One sensor will be forward of your catalytic converter, and one will be aft of cat.

Your scan tool should be able to identify problem sensor.

Shop manual will have troubleshooting sequence.

Dave

[Gary C.]

Ed,

Found this link to download the Vertronix Tech 1A OBD1 scanner user manual https://www.clubcalibra.net/v6calibr...r%20manual.pdf

Gary
....

[Ed A.]

Hi Gary,
Thanks for your response and the link, I'll give it a try.
Ed

[Ed A.]

Hi Dave,
Thanks for your response and your input. I responded earlier but it got posted incorrectly, but basically I ran two diagnostics tests, one with the ignition on and the second with the engine warm & running and didn't get any troubles codes. I'm not sure what the oxygen values should be but I didn't get a code?

[David H.]

Hi Dave,
Thanks for your response and your input. I responded earlier but it got posted incorrectly, but basically I ran two diagnostics tests, one with the ignition on and the second with the engine warm & running and didn't get any troubles codes. I'm not sure what the oxygen values should be but I didn't get a code?

Ed

Heated Oxygen Sensor range is 0-1132 mV. Should fluctuate constantly when in "closed loop". 10 mV would indicate "lean" and 1000 mV would indicate "rich". All this causes ECM to do short and long term fuel trim adjustments.

Refer to page 6E3-A21 in 1995 Service Manual (Book 2). DTC 13 pages on 6E3-A-42 have info on HO2S with ECM sending a bias voltage signal (450 mv) to those sensors and adjusting fuel trim. Page 6E3-B-13 on "rough idle" has checking ECM grounds/connectors as one step.

Lots of GM's diagnostic references a "Tech 1" tool that can generate voltages to check components. With no DTC triggered seems like chasing a ghost!

Corrosion on Grounds & Connectors?

Dave

Your reader is OBD I ?

[Ed A.]

Dave,
Thanks again for the added info, I did record all the data from the diagnostics checks but I don't have access to them. I'll check them out later and as you know the Tech 1 is OBD1 scanner. My car is a 1985 and I don't have access to a 1995 manual. I'll check into the corrosion issue, but my car is low mileage 30K and stored in a climate controlled garage, however that still could be a problem.

Regards,
Ed

[David H.]

Ed

Sorry, my oldest C4 Service Manual is a 1995.

Dave

[Ed A.]

Dave,
No problem, I appreciate your help.
Ed

[David H.]

Dave,
No problem, I appreciate your help.
Ed

Ed

I've sent you a PM - might help.

Dave