Is my idle issue electrical?

[Rokkinlen]

If your getting vacuum advance at idle, you will be about 500 RPM to high. So I would test you have no advance by pulling the hose off the ignition distributor.

You do not use the mixture screw to set idle speed or idle smoothness. Set it with your meter and fix other problems when set correctly.

My D is a little rough when started from cold, but a little gas helps it start around 775-800rpm. What concerns me more is that as it warms, the rpms go to 1000 and plateau at 1300 or so. The weird thing is that if I turn my AC to Max, Norm, or Bilevel, the rpms go back to 775-800.

I haven't done any other troubleshooting (e.g. unplugging vacuum advance to solenoid) yet, but I can. My D only has 2900mi on it and DMCFL said that I probably should clean my electrical grounds at some point. While I do plan on cleaning them, I really want to fix this idle issue because I've only put 120mi or so on the car since I got it, and this is a new issue.

Any help is greatly appreciated!!

Len

[Ron]

FWIW- Giving it gas (pressing the pedal) does virtually nothing while stating like a carburetor setup.
Anyway, the position of the AC mode selector bringing the RPM down that far suggest it is allowing a vacuum leak when in the other positions. The leak would be an unmeasured air source, so the system would see the lean mixture and increase fuel, which would raise the RPM.
Listen closely to the switch and see if it is hisses. (If nothing, get a kid to listen ;-)
The mode switch is fed vacuum through a line tapped into the left rear of the intake manifold ("horn"). Plug it for another test.

[Bitsyncmaster]

If your problem just started suddenly, (idle 1300 after warm up) then I would suspect you are getting vacuum advance at idle. If it had started getting bad slowly then I would think your problem is elsewhere.

If your advance solenoid connector falls off you get advance at idle on a warm engine. Same problem if your idle switch fails or a connection on it is bad.

My high idle problem was due to bad deceleration springs but I would not think that problem would show up suddenly.

[cpistocco]

If your problem just started suddenly, (idle 1300 after warm up) then I would suspect you are getting vacuum advance at idle. If it had started getting bad slowly then I would think your problem is elsewhere.

If your advance solenoid connector falls off you get advance at idle on a warm engine. Same problem if your idle switch fails or a connection on it is bad.

My high idle problem was due to bad deceleration springs but I would not think that problem would show up suddenly.

I had a problem where my car would stumble/ detonate/ knock from a dead stop. Happened only when warm. After fiddling around, (and with some help from Dave and DMC Humble) I found that my micro switch was malfunctioning. It was allowing some current through while in the off position. This caused my timing to advance at all times....even at idle. I unplugged vacuum hose from distributor (and put a golf tee in the hose). Car ran great after that. I will replace switch this week.

[Rokkinlen]

If your problem just started suddenly, (idle 1300 after warm up) then I would suspect you are getting vacuum advance at idle. If it had started getting bad slowly then I would think your problem is elsewhere.

If your advance solenoid connector falls off you get advance at idle on a warm engine. Same problem if your idle switch fails or a connection on it is bad.

My high idle problem was due to bad deceleration springs but I would not think that problem would show up suddenly.

__________________________________________________ ________

Thanks for the input everybody! As for the AC mode switch, that was actually my first repair on my car a couple months ago. No hissing at all. I had to replace the blower switch a couple weeks later, but the AC work

Just for a little piece of mind, I moved the coolant bottle over and cleaned up the chassis ground underneath it. I have a lot more to go, but figured I might as well bang that out. Then I took off the air filter assembly and adjusted the screw that contacts the microswitch. I adjusted it so that it pushed more on the switch. Didn't seem to make much of a difference, especially after the engine was warm. I have no idea where I left it relative to when I started, so hopefully the next cold start isn't worse than usual. If it is, I should be able to adjust it. Not sure mine is malfunctioning, because I'm not getting any knocking/detonating/etc at warm idle like cpistocco. However, I won't completely rule it out either. If there are any tips on adjusting this screw, the one below it, or any other relevant one - please let me know.

The interesting thing was that after it was warm and idling at ~1500rpm, I pulled the hose (the one in the middle between the electrical connection and the other hose) on the vacuum solenoid, and the rpm went down to spec. I put the hose back on, and it kept idling beautifully. Gave it a little gas, and settled back at 1500rpm.

I pulled the electrical connector off the vacuum solenoid while it was running and it sparks a little, so it's getting power. I didn't notice any change in the warm engine idle when the solenoid was connected vs disconnected electrically.

But because the engine has to be warm to get stuck at 1500rpm, does that mean I can rule out deceleration springs? Wouldn't they keep the rpm high regardless of engine temp? I feel that because the rpm's returned to normal when I pulled that hose off the solenoid, my issue is either the solenoid or the therm vac switch (p/n 102405) like bitsyncmaster said. But I'm the newb, and you guys are the Delorean whisperers. So *please* let me know if I'm looking in the wrong direction.

Josh at DPI said that I should check my idle thermistor, but 1) I don't know how to check it (ohm it out? what value?) and 2) that means taking the intake off which is fine, but I'll need to get gaskets and stuff ahead of time.

And if there's any equipment you recommend for troubleshooting, I'd love to hear what you all use. Sorry for all the questions. TIA!

[Bitsyncmaster]

The deceleration springs would have a constant leak cold or hot but a cold engine requires more throttle to keep it at the set 775 RPM. So the idle motor opens more on the cold engine. On a hot engine the idle motor may be closed as far as it can go so the leaking deceleration springs would now increase the idle RPM because the idle motor can not close anymore.

My car with the bad springs would idle fine at a solid 775 RPM until the engine was warmed up. Then it would idle at 1000 RPM even with the curb idle screw backed out fully.

[cpistocco]

The deceleration springs would have a constant leak cold or hot but a cold engine requires more throttle to keep it at the set 775 RPM. So the idle motor opens more on the cold engine. On a hot engine the idle motor may be closed as far as it can go so the leaking deceleration springs would now increase the idle RPM because the idle motor can not close anymore.

My car with the bad springs would idle fine at a solid 775 RPM until the engine was warmed up. Then it would idle at 1000 RPM even with the curb idle screw backed out fully.

Crazy question....If you remove electrical connector from Vacuum advance solenoid, do your rpms go down...and does that close off vacuum to the advance on distributor? (I am far from a Delorean Guru)

[Rokkinlen]

The deceleration springs would have a constant leak cold or hot but a cold engine requires more throttle to keep it at the set 775 RPM. So the idle motor opens more on the cold engine. On a hot engine the idle motor may be closed as far as it can go so the leaking deceleration springs would now increase the idle RPM because the idle motor can not close anymore.

My car with the bad springs would idle fine at a solid 775 RPM until the engine was warmed up. Then it would idle at 1000 RPM even with the curb idle screw backed out fully.

Okay, so it sounds like it could be my springs. Do you think I should inspect them first or are there easier things I can troubleshoot beforehand?

[Rokkinlen]

Crazy question....If you remove electrical connector from Vacuum advance solenoid, do your rpms go down...and does that close off vacuum to the advance on distributor? (I am far from a Delorean Guru)

When I removed the electrical connector from the vacuum advance solenoid, the rpm’s went down from ~1500 to ~800. But that was when the car was warm. I don’t know if it closed off vacuum to the advance on the distributor, but I do know that it was the hose in the center of the solenoid that did it, and I could feel vacuum when I touched my thumb to it.

[Ron]

The interesting thing was that after it was warm and idling at ~1500rpm, I pulled the hose (the one in the middle between the electrical connection and the other hose) on the vacuum solenoid, and the rpm went down to spec. I put the hose back on, and it kept idling beautifully. Gave it a little gas, and settled back at 1500rpm.

While the engine is warm, pull "the hose in the middle" off and see if it has vacuum on it. It should (RPM high or low). If not, check that the other hose has vacuum and swap them. (This allows the vacuum to vent when the solenoid disconnects the vacuum from the distributor. Otherwise, once you rev it, it can have vacuum to the distributor until you shut the engine off...)

I pulled the electrical connector off the vacuum solenoid while it was running and it sparks a little, so it's getting power. I didn't notice any change in the warm engine idle when the solenoid was connected vs disconnected electrically.

Crazy question....If you remove electrical connector from Vacuum advance solenoid, do your rpms go down...and does that close off vacuum to the advance on distributor? (I am far from a Delorean Guru)

That is the way it should act whenever the throttle plates are shut (idling). (The wire to the solenoid is always hot -- The switch grounds the solenoid to close the valve and vent.)
When the throttle plates are closed, the switch should turn the idle system on and the vacuum to the distributor advance off. When the plates are open, the idle system is deactivated and, if the engine is warm, vacuum is applied to the solenoid -> distributor. (If cold the Thermal Control Valve routs vacuum to the Warm Up Regulator instead of the solenoid -> distributor). Rather than pull "the hose in the middle", pull the other hose and check that there is no vacuum on its solenoid port when the plates are closed (idling, warm).

Although you replaced the mode switch recently, there is something wrong there. Moving the mode switch to cooling modes should not affect the engine RPM except for the AC compressor adding a load (the idle system should basically compensate for that). No way would a good AC system pull the RPM down 500-700 RPM. Double check ALL of your vacuum routing.

You may have bad springs, but that wouldn't cause the mode switch to affect the RPMs.

Again, you can plug the main vacuum tap to eliminate a lot...