High idle cold start --sorry

[glockworks21]

I know this question has probably been asked a million times and Im hoping for a best guess starting point. It seems like there are a lot of things that cause can cause this. I get a high idle when cold, almost 2k rpm. it does it 90%of the time when i start it which is what is throwing me off. sometimes it wont do it at all. sometimes i can throw a rev or 2 into in after 30 second or so and it will drop to normal. it seems to idle fine after that and for the rest of the time i drive it. Trying to do everything myself on this and learning as i go. Can someone give me a "most common" starting point? If that exists? )



Thanks for your time

Dave

[Parzival]

Try this, Start the car and during the high idle, before giving it a few revs to bring it down. Go to the back and press on the throttle arm (where the adjustment screws are for the idle microswitch.) The thinking here is that many cars have tired or worn springs in the throttle body and on that arm. If yours is not returning to the closed position when cold then the butter fly valves will be slightly open and it may cause this condition. If when you press the arm down your idle returns to normal then this is likely you issue, and I can walk you through some ways to adjust it.
This is the easiest thing to check first, do this and report back what you find. If it has no effect we will have a few more things to check.

[Michael]

You are right. A lot of things can cause this. A bit of history on the car might help(has it sat for a period of time and you are waking it up? Does it get driven often and this problem just appeared?)

I would start with the basics. Make sure your throttle cable and spool are free and not sticking. Make sure your idle circuit is working and engaging at closed throttle. Vacuum leaks are always a possibility. Other possible causes are idle air valve or maybe even the CPR but that usually causes a dying problem rather than a high idle.

My first inclination is the idle circuit is not working either because the plate is not engaging microswitch or the switch itself is faulty.

[82DMC12]

Did you see my ongoing thread? I am going through this similar issue and narrowing down the possibilities as I continue to work on it. Maybe you can learn something from it.

https://dmctalk.org/showthread.php?1...intake-rebuild

[glockworks21]

You are right. A lot of things can cause this. A bit of history on the car might help(has it sat for a period of time and you are waking it up? Does it get driven often and this problem just appeared?)

I would start with the basics. Make sure your throttle cable and spool are free and not sticking. Make sure your idle circuit is working and engaging at closed throttle. Vacuum leaks are always a possibility. Other possible causes are idle air valve or maybe even the CPR but that usually causes a dying problem rather than a high idle.

My first inclination is the idle circuit is not working either because the plate is not engaging microswitch or the switch itself is faulty.

Ive revved the engine from the spool when it is at high idle. I have also engaged that switch and it does not seem to do anything to the idle. Maybe replace it first?

[glockworks21]

You are right. A lot of things can cause this. A bit of history on the car might help(has it sat for a period of time and you are waking it up? Does it get driven often and this problem just appeared?)

I would start with the basics. Make sure your throttle cable and spool are free and not sticking. Make sure your idle circuit is working and engaging at closed throttle. Vacuum leaks are always a possibility. Other possible causes are idle air valve or maybe even the CPR but that usually causes a dying problem rather than a high idle.

My first inclination is the idle circuit is not working either because the plate is not engaging microswitch or the switch itself is faulty.

there are 30k plus of receipts done over the last 15 years or so but then it sat for a long time. Ive put about a thousand miles on it since getting it mostly driving around town. I am still trying to go through everything. I have changed the vacuum lines that are visible. i havent done the ones under the intake. Its always started right up and runs well. I drive it at least once a week but the problem has been there from the beginning. it did however go from 1500 to 1800 rpm high idle to about 2k. so it seems its gotten worse.. i started it this morning cold and it idled normally for about a minute then it jumped.. then about a minute or so later it dropped...

[82DMC12]

Ive revved the engine from the spool when it is at high idle. I have also engaged that switch and it does not seem to do anything to the idle. Maybe replace it first?

Disconnect the wires from the back of the microswitch and make sure the switch is actually working with a multimeter set to ohms. See that the switch closes the circuit when you press the switch in. And goes open circuit when the switch is in rest position.

You can also set your meter to DC volts, put the red probe on the battery + terminal (or jump start terminal in the engine compartment) and the other wire on black/green at the idle ECU (black box behind the driver seat, and the black/green wire is on the inner of the two white plugs). It should show 12V when the switch is "on" or in, and 0 volts when the switch is off. This way you know the wiring for the switch up to the Idle ECU is correct. The idle microswitch provides the ground for the ECU and tells it when you are idling.

[Michael]

Yeah definitely just test the microswitch.
.no need to just start throwing parts on it and hope you fix the problem.

You may want to crack the idle computer case (unless it's new) and look at the board. When my idle computer went bad, I was easily able to see a burnt path. That's not to say a bad one will have a visible clue. The only real way to test one is swap it with a known good one but you might "get lucky".

[glockworks21]

Disconnect the wires from the back of the microswitch and make sure the switch is actually working with a multimeter set to ohms. See that the switch closes the circuit when you press the switch in. And goes open circuit when the switch is in rest position.

You can also set your meter to DC volts, put the red probe on the battery + terminal (or jump start terminal in the engine compartment) and the other wire on black/green at the idle ECU (black box behind the driver seat, and the black/green wire is on the inner of the two white plugs). It should show 12V when the switch is "on" or in, and 0 volts when the switch is off. This way you know the wiring for the switch up to the Idle ECU is correct. The idle microswitch provides the ground for the ECU and tells it when you are idling.

I just ordered a new multimeter. I lost or loaned one out long time ago. Never needed it for any car ive had recently. Its seems like its a must have for a DMC though. )

[82DMC12]

I just ordered a new multimeter. I lost or loaned one out long time ago. Never needed it for any car ive had recently. Its seems like its a must have for a DMC though. )

The specialty shop tools I use the most on my DeLorean:

• multimeter
• Actron Dwell and RPM meter
• K-Jetronic fuel pressure test gauge with control pressure valve
• Air compressor
• Camera on a wire (Harbor Freight $85)
• Cooling system pressure tester (NAPA)