Intermittent hot stalling > 2500 RPM idle

[AugustneverEnds]

When I had my WUR and distributor rebuilt last winter it cured nearly all of the K-Jet issues that had crept in over time, idle hunt, cold starting, stalling. Realistically every D still running K-Jet needs those to be refurbished by now if they never have been.

[Timeless]

Installed original Idle Air Control Valve and module. Tested power and ground to module. Used oscilloscope to verify inputs and outputs from Control Module. System uses inputs from Throttle Switch, Ignition Coil signal and Temperature Sensor to calculate the frequency of pulse width modulated signal to send to the Idle Air Control Valve. Measured PWM signal to Idle Air Control Valve at 12,5 Hz on cold start. Measured resistance value of temperature sensor and found the to be on the Mega Ohm scale indicative of failure. Specified values are 8kOhm-12kOhm. Back probed circuit with a 10kOhm Potentiometer and toggled signal to control module and was able to bring idle down 800 rpm.
scope.jpg
Thanks everyone for your comments. Trip to valley has been started. Bench tested injectors. Any vacuum lines, clamps, etc. that are questionable are being replaced as well as cap/rotor. DPI injector seal kit and valve adjustment afterwards.

[Bitsyncmaster]

Installed original Idle Air Control Valve and module. Tested power and ground to module. Used oscilloscope to verify inputs and outputs from Control Module. System uses inputs from Throttle Switch, Ignition Coil signal and Temperature Sensor to calculate the frequency of pulse width modulated signal to send to the Idle Air Control Valve. Measured PWM signal to Idle Air Control Valve at 12,5 Hz on cold start. Measured resistance value of temperature sensor and found the to be on the Mega Ohm scale indicative of failure. Specified values are 8kOhm-12kOhm. Back probed circuit with a 10kOhm Potentiometer and toggled signal to control module and was able to bring idle down 800 rpm.
scope.jpg

FYI:
The frequency of the stock idle ECU does not change. It runs about 95 Hz all the time. The duty cycle is what makes the idle motor change position. Center pin of the idle motor has 12 volts and one side pin drives to ground when the other side pin floats and vice versa. So the duty cycle you will read depends on which pin your looking at.

The stock ECU will power on, driving the idle motor quickly to about half open then move to full open and slowly close if you don't start the engine.

[Timeless]

All back together. Vacuum lines on replaced, few electrical connections cleaned at bulkhead, new plugs, DPI injector seals, ignition timing set. Smoke test revealed idle control motor is leaking - no other vacuum leaks. Will source a replacement BOSCH 500 ISM.
Car no longer stalling and driving well.

[Rich]

....Smoke test revealed idle control motor is leaking - ......
Car no longer stalling and driving well.

Thanks for sharing the good news.

The idle motor is known for developing a vac leak, usually in or around its elec. connector stub or along the top/inner circular edge of the motor body.

These leaks are easily repaired at no/low cost unless you have another reason to replace it. It seems you already pinpointed a/the leak and it's a minor one if your engine's idling better now.

Carefully pull off the connector plug. Look closely at the motor body for cracks, do the smoke test or pressure test it off the car if you're not sure where it's leaking. Patch the leak after careful wipedown of the area. Black silicone or epoxy work OK. Even non-curing butyl sealant* is fine.

*3M Strip-Calk -08578 (same as you can use to re-seal the vapor barrier plastic sheathing to inside of the door panels)

[Timeless]

Original ISM replaced with a functioning ISM (500) and the hunting has stabilized. I did a quick attempt to seal the original with 3M strip-calk and it did not help. I may do a more through repair to keep as a backup.

[glockworks21]

Original ISM replaced with a functioning ISM (500) and the hunting has stabilized. I did a quick attempt to seal the original with 3M strip-calk and it did not help. I may do a more through repair to keep as a backup.

For what its worth Ive been chasing multiple different idle issues over the last year that ive had the car. I had a intermittent high idle issue and it dropped immediately after unplugging ism like yours. But it was not consistent. After reading your thread i ordered a new ism and installed it (posted another thread about the vent pipe being loose). I will test it over the next few days with cold starts and see how it does. The more annoying idle issue i had involved a surging of about 50-100rpm. it would go up and down every 10 seconds or so after the car was slightly warmed up. I dont know what made me do it but i started adjusting the butterfly valves.(which i really didnt know anything about. I just started twisting them and seeing what it would do) I noticed how it was changing idle and then looked up how they work. I tightened them down and backed them out and equal amount of turns. i messed around with it for about 5 minutes and got the car smooth. I let it cool down and warm up 3 different times today after the adjustments and its solid. NO surge at all. I hope this helps others out in the future. And i really hope im done with idle issues for awhile!


Thanks
Dave

[Timeless]

glock, did you inspect your throttle plate decel springs? There is a great thread here with a brass screw set procedure that many swear by.

[Elvis]

you more or less deactivaeted the ISM by giving it air by the screws.

[glockworks21]

glock, did you inspect your throttle plate decel springs? There is a great thread here with a brass screw set procedure that many swear by.

I could not find the thread. I clearly have more work to do.