Intermittent hot stalling > 2500 RPM idle

[Timeless]

Ohm tested ISM directly and values within 10% of each other. Thermistor test at ECU plug terminals showing positive results but those wires are on the plug that is NOT disconnected. The outer plug is the one that is causing the 2500 RPM and, according to the schematic in the service manual, that controls the ISM.

[Timeless]

Started again with idle ECU plugged and now the idle is 1500. Plugging in the ECU produces original issue: 3500 RPM. Thinking of checking the decel springs next. Anyone have a spare idle ECU?

[82DMC12]

Started again with idle ECU plugged and now the idle is 1500. Plugging in the ECU produces original issue: 3500 RPM. Thinking of checking the decel springs next. Anyone have a spare idle ECU?

Back in May 2022 I had an elevated idle when hot which I suspected could be decel springs. I successfully ruled them out by gluing them closed, temporarily, with a hot glue gun like in the photo below. My actual issue turned out to be misadjustment of the throttle spool and quadrant link.

PXL_20220618_162417582.jpg

[Timeless]

Back in May 2022 I had an elevated idle when hot which I suspected could be decel springs. I successfully ruled them out by gluing them closed, temporarily, with a hot glue gun like in the photo below. My actual issue turned out to be misadjustment of the throttle spool and quadrant link.

PXL_20220618_162417582.jpg

My throttle spool looks ok and, besides the idle/stall issue, the car goes down the road great. I assume you replaced the decel springs.

[82DMC12]

My throttle spool looks ok and, besides the idle/stall issue, the car goes down the road great. I assume you replaced the decel springs.

I did not replace them because the glue confirmed that the springs were not the cause (loose springs allowing the decel valves to open under idle vacuum).

[Timeless]

Installed a 501 idle speed motor - nothing changed. Tried running engine with both idle plugs disconnected and it runs - just at 3500 RPM. Plugging either of them back in and still 3500. Now it will not go back to 1500 RPM. Swapping idle ECU once it arrives from a member here.

[Timeless]

Lambda/02 sensor replaced. The engine now idles around 2000 RPM with the OUTER ECU connection unplugged. It will raise to ~2500 with both idle ECU plugs connected. This is still with the new replacement ISM. In gear (drive or reverse) it idles 750-900 then will stall once hot and the A/C on and/or headlights.

My brand new smoke machine is a dud so another unit was ordered. Planning to check decel springs after a smoke test.

[Bitsyncmaster]

Lambda/02 sensor replaced. The engine now idles around 2000 RPM with the OUTER ECU connection unplugged. It will raise to ~2500 with both idle ECU plugs connected. This is still with the new replacement ISM. In gear (drive or reverse) it idles 750-900 then will stall once hot and the A/C on and/or headlights.

My brand new smoke machine is a dud so another unit was ordered. Planning to check decel springs after a smoke test.

It sounds like the idle ECU is not getting the ignition coil signal so it thinks RPM is zero.

[Timeless]

Swapped ECU and the engine started normally and idled 750-800. As soon as a bit of heat built up the engine stalled. (~1 minute since it's warm S. FL) Shut off and unplugged the ISM plug on ECU - engine did NOT shoot up to ~2500. With the "new" ECU the engine struggles to idle and will stall unless throttle is depressed slightly. Didn't attempt to put into gear since it would most definitely not stay running. Swapped ISM to original and then back to the 501 and that didn't seem to help much. With either ISM connected or disconnected the idle acts the same - maybe a bit better with the new one.

Is it worth checking the decel springs since it will idle as designed some of the time?

[Timeless]

Unplugged the idle ECU completely and the engine will idle near 800 RPM - but rough and will stall after short time. Unplugged the hose to the vacuum solenoid while running and that did not change anything.

I added some silicone spray to the original ISM and verified it is buzzing with the ignition on. Checked both original and new ISM with ignition on and neither of them move.