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?
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[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!:D

[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!:D

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.

[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.

69237

[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.

69237
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.

[Bitsyncmaster]

I think you should stop unplugging the idle ECU. It's making your results not constant. If your idle motor is buzzing then the idle ECU's drivers are working.

Pull the idle motor and turn the key to on and you should see the idle motor move quickly to about half open (amount depends on your temperature sender resistance) then it should slowly move to open.

[Timeless]

Idle motor not moving at all when ignition is turned on. Original and new 501. Maybe due to ambient temps always warm being in S. FL?
Why would the RPM results be different between both ECUs? The only way to drive the car is with the idle speed control ECU plug disconnected which gives the ~2500 RPM From there, it will idle 750-850 in gear and stall after the engine is hot and the A/C is on.

[Timeless]

Started engine with ISM unplugged and the idle dropped significantly. Upon plugging ISM in the idle will jump back to 2500. Video:

https://youtube.com/shorts/ZrXCWgP6kVk?feature=share

[WHO1DMC]

Started engine with ISM unplugged and the idle dropped significantly. Upon plugging ISM in the idle will jump back to 2500. Video:

https://youtube.com/shorts/ZrXCWgP6kVk?feature=share

I'm not an expert but that ISM sounds like hell I think it's shot.



Dave B.

[Timeless]

Swapped to 501 ISM. Ran with ignition on and it does not move. I waited a minute but nothing - but it is buzzing and buzzing quieter than the original.

Checked vacuum soleniod again just because and it is no longer clicking when the throttle is actuated (ignition on) :what_the:

[Bitsyncmaster]

Swapped to 501 ISM. Ran with ignition on and it does not move. I waited a minute but nothing - but it is buzzing and buzzing quieter than the original.

Checked vacuum soleniod again just because and it is no longer clicking when the throttle is actuated (ignition on) :what_the:

Your idle ECU must be bad if it does not move. Now my ECU will not move without the engine running but the stock one will.

[Timeless]

Bench tested the vacuum solenoid at 12V and it clicks. Reinstalled parts removed and started this morning with the spare idle ECU and new 501 ISM. Takes 2-3 tries with excessive cranking to fire. Engine will not hold cold idle. Keeping foot on the accel pedal will keep it running but it's running more rough even though it was smoother after the new 02 installation.

Next step in 5 days is transport to a local technician specializing in vintage cars and has knowledge of K-Jet. Will report back. :question:

[Ron]

Wondering if there is something odd going on with your WUR/CPR..

[Timeless]

Wondering if there is something odd going on with your WUR/CPR..
I just thought about that last night. Weird that it's running attributes are completely different with each idle ECU.

[Elvis]

Hi, what does this mean ?
~LXA~
Dunmurry | Stuttgart | Leipzig | Tochigi | Fremont | Bratislava | Sindelfingen | Kansas City | Oakville | Coventry

Sindelfingen ? is close to me.


I can repair your ECU

Where are you now ? SWFL ?

[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.
69311
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.
69311

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.