Idle surge/hunt after rebuilding climate control mode switch

After several months of searching, I’m a proud new owner of an 81 Delorean (VIN 1465) with 42,000 miles. Woo Hoo!

It’s 115F now in AZ, so the first thing I HAD to fix was the no air coming from the center and side vents. I used the DMC climate control mode switch rebuild kit and now I have air coming from those vents.

However, the car now has an up and down idle surge (idle hunt).

The odd thing about this is that if I turn the mode switch to either of the 2 AC positions it idles fine. It’s only when the mode switch is in the off and vent positions the idle will surge.

I’m wondering if I did something in the process of replacing the mode switch. I did have to remove the center console because it was in the way.

I haven’t been driving around without AC, so it may have always been like this. I've only owned the car for two weeks. It just seem suspect that the idle evens out when the AC is on.

Is there something under the dash or center console that would impact the idle that I may have jarred loose or would a vacuum leak under the dash cause this?

Thanks in advance for any help.

Assuming the switch is good, there is nothing you could have done wrong to make it surge while in the OFF position.

Could be the compressor and idle system are fooling you... Try unplugging the compressor to eliminate the load and see if it still surges.

My guess is your idle motor is sticky. When the AC is on there is more load on the engine due to the compressor and fan loads so the idle motor runs more open. When the AC is off the idle motor runs more closed which may be sticking in the more closed position.

The idle motor has the most torque at the center (half open). So at the extremes it's more sensitive to a sticky idle motor.

I'd guess mixture is off - air leak or set too rich.

Dave is right, with AC on there is more load on the system and the idle motor opens further, an air leak would have less influence.

In the end - what we have on these cars is an unstable PID controller which leads to oscillation...

Thank you for the responses.

I'll try cleaning the idle motor first. Is something like PB Blaster better to use than carb cleaner or brake cleaner?

Also, what's the recommended replacement part for the idle motor. It looks like DeloreanGo and Delorean Industries has them. Are there others?

https://www.deloreango.com/us/idle-s...tor-motor.html

https://deloreanindustries.com/delor...eed-regulator/

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Originally Posted by Curt_DMC

Thank you for the responses.

I'll try cleaning the idle motor first. Is something like PB Blaster better to use than carb cleaner or brake cleaner?

Also, what's the recommended replacement part for the idle motor. It looks like DeloreanGo and Delorean Industries has them. Are there others?

https://www.deloreango.com/us/idle-s...tor-motor.html

https://deloreanindustries.com/delor...eed-regulator/

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Yes, any of those solvents will work. The electrical part of the motor is sealed from the valve side so not much chance of damage.