Intermittent hot stalling > 2500 RPM idle

[Timeless]

DMCH stage 2 auto - 7K miles. Recently started diagnosing an intermittent hot stalling issue and a new issue popped up a few days ago. Started the car and as soon as I pressed the accel pedal, the idle shot to 2500 and stuck. Before this the car never did that and has always run well. Checked and lubed throttle cable, checked vac lines & electrical connections and all seems ok. When I unplugged the outer connection to the idle ECU the idle corrected to 750-780. Removed the ISM and it looks clean and "clicks" when shook. Car runs and drives with that plug disconnected.

Water pump was replaced by DMC before I took delivery (2022) so I assume thermistor and it's connection were checked/verified. Can the thermistor go bad that quickly/randomly? Is this a sign of a faulty idle ECU? Anyone have a spare idle ECU I can use for testing?
idle ECU.jpg

[Bitsyncmaster]

Even if the thermistor connection was bad, unplugging the idle ECU should not return the idle to norm.

It sounds like a stuck throttle since it only did it when you hit the gas. A bad thermistor would have done the high idle at start up.

[Rich]

Even if the thermistor connection was bad, unplugging the idle ECU should not return the idle to norm.

It sounds like a stuck throttle since it only did it when you hit the gas. A bad thermistor would have done the high idle at start up.

+1

Sometimes a slowly-worsening throttle linkage/cable/pedal problem can cause an intermittent high idle. And in some of those cases a quick blip of the accel pedal settles it down although this isn't a fix, only another symptom.

Did you verify the throttle plates are closed when you get the high idle condition? Next time it happens push the top of the throttle lever forward to be sure it's bottoming out against the bracket. No need to adjust the throttle setscrews since you know it used to idle OK. Just check it during the next 2500RPM episode to verify the throttle is really closed like it should be.

Did see you lubed one end of the cable already.

[Timeless]

+1Did you verify the throttle plates are closed when you get the high idle condition? Next time it happens push the top of the throttle lever forward to be sure it's bottoming out against the bracket. No need to adjust the throttle setscrews since you know it used to idle OK. Just check it during the next 2500RPM episode to verify the throttle is really closed like it should be.

Did see you lubed one end of the cable already.

Yes verified throttle at bracket. Microswitch newer and engaging. Everything is ok with ECU that connection unplugged. When I plug it back in boom - jump to 2500.

[David T]

Yes verified throttle at bracket. Microswitch newer and engaging. Everything is ok with ECU that connection unplugged. When I plug it back in boom - jump to 2500.

Check the springs on the decel valves. The only way the idle can increase is for the motor to get more air. Either through the throttle plates, the decel valves, the idle motor system, or air leaks. Check the pipe and "O" ring that goes into the bottom of the mixture unit.

[Ron]

Since it jumps back up to 2500 when plugging the control unit back in, it's not mechanical...
I'm wondering if a bad idle speed diode could cause this. Bitsy??

[Bitsyncmaster]

Since it jumps back up to 2500 when plugging the control unit back in, it's not mechanical...I'm wondering if a bad idle speed diode could cause this.
Bitsy??

If it jumps back to 2500 plugging the idle ECU back in then it's an open circuit thermistor. You can bypass the thermistor by shorting the two thermistor pins (or connecting the wires) at the idle ECU.

The diode I never saw why they installed it unless the stock idle ECU can not accept the 12 volts from the idle switch.

[Ron]

If it jumps back to 2500 plugging the idle ECU back in then it's an open circuit thermistor. You can bypass the thermistor by shorting the two thermistor pins (or connecting the wires) at the idle ECU.

The diode I never saw why they installed it unless the stock idle ECU can not accept the 12 volts from the idle switch.

Yeah, the manual says the diode is there to prevent feeback, just wasn't sure of what it might cause (shorted/open).

We thank you, sir!

[Timeless]

If it jumps back to 2500 plugging the idle ECU back in then it's an open circuit thermistor. You can bypass the thermistor by shorting the two thermistor pins (or connecting the wires) at the idle ECU.

The diode I never saw why they installed it unless the stock idle ECU can not accept the 12 volts from the idle switch.

Thanks Dave. I plan to ohm test the pins ASAP.

[Bitsyncmaster]

Yeah, the manual says the diode is there to prevent feeback, just wasn't sure of what it might cause (shorted/open).

We thank you, sir!

It is common to have pull up resistor on a micro input that goes to 5 volts or 3.3 volts so the diode from an external source signal can ground that pull up to signal the micro and the 12 volt signal does not get into the micro input.