Idle Hunting when AC is on

[deloreandmcxii]

Hello! I’ve done some searching to find anyone with this particular symptom – and I didn’t turn up anything that was quite like this. In case it matters, my O2 sensor and Frequency Valve are not operational.

Symptoms:
• My car exhibits the pretty common idle hunt during warm-up. After a minute under load, or a few minutes of holding the throttle slightly open – the car will idle perfectly smoothly.
• Once idling nicely, if I turn on the Air Conditioning the RPMs are pulled down, and it oscillates just like when it is hunting during warm up.

I gather the first suspect should be vacuum leak – I plan on a smoke test (I am researching my options for performing this). A full vacuum line replacement is not too expensive. But I was wondering if the AC-related symptoms would lead someone to suspect a particular vacuum line segment / component.

I can’t find any guidance about how each the vacuum hose segment is replaced. Many are simple and obvious, and I assume the lines that connect directly to the thermotime switch will be a pain (requiring a trip into the VoD), but aside from those – are any vacuum lines particularly challenging? Should I be doing all: engine, and AC / brake related vacuum lines?

[deloreandmcxii]

Edit: Vacuum switch, not thermotime

How could make such a mistake when these things are so named so clearly :)

[David T]

You most likely do have vacuum leaks, most Deloreans do. Vacuum leaks reduce the amount of power the idle control system has so the more leaks the less effect the idle system has to control the idle speed. That said, we don't know if the idle system is even working. When the A/C comes on, not only must the motor power the compressor, the alternator must put out more power to run the fans. At idle it is a BIG drag and the idle system must give the engine more power to hold the idle up at 770 RPM. If the idle system isn't up to the task the idle quality suffers. Verify the idle system is working and fix the vacuum leaks. Even with everything perfect, don't expect a perfect idle when the A/C is running, especially if the Lambda system is not functioning.

[deloreandmcxii]

Thanks David!

I'm still debating how fancy I wanna go with the smoke test.

-Does anyone have any recommendations for a vacuum smoke testing product?
-Where is the best place on the DeLorean to put the smoke in?


In the mean time I tried some easy free tinkering:

I removed and cleaned the idle speed motor. Tested it on the bench and it seemed to operate well. I was able to observe it opening completely and then slowly close - with the key ON engine not running.

I removed, cleaned, and adjusted the idle microswitch and throttle stop. With the throttle fully closed and idle microswitch open, the car idles smoothly at ~1k RPM, and pulls down to ~750 with the headlights and AC on. In the meantime, I may just leave the idle microswitch disconnected - at least then she won't chance stalling due to the hunting when the AC is on at idle.

I attempted the test where you unplug the outside plug on the idle ECU - the car stalled when I did so. I read it was supposed to tach up to ~2k?

I'm missing something I think: on a car with no vacuum leaks, when you close the throttle but do not engage the Idle System, the car tachs high - why is this? Where is the air coming from? Is the default state of the idle motor partially open?

[David T]

Thanks David!

I'm still debating how fancy I wanna go with the smoke test.

-Does anyone have any recommendations for a vacuum smoke testing product?
-Where is the best place on the DeLorean to put the smoke in?


In the mean time I tried some easy free tinkering:

I removed and cleaned the idle speed motor. Tested it on the bench and it seemed to operate well. I was able to observe it opening completely and then slowly close - with the key ON engine not running.

I removed, cleaned, and adjusted the idle microswitch and throttle stop. With the throttle fully closed and idle microswitch open, the car idles smoothly at ~1k RPM, and pulls down to ~750 with the headlights and AC on. In the meantime, I may just leave the idle microswitch disconnected - at least then she won't chance stalling due to the hunting when the AC is on at idle.

I attempted the test where you unplug the outside plug on the idle ECU - the car stalled when I did so. I read it was supposed to tach up to ~2k?

I'm missing something I think: on a car with no vacuum leaks, when you close the throttle but do not engage the Idle System, the car tachs high - why is this? Where is the air coming from? Is the default state of the idle motor partially open?

There is the problem. It should idle at 750 and stay there even with the A/C and headlights on. You have to figure out why it is idling so high.

[deloreandmcxii]

Even with the idle micro switch open? (Disconnected, not completing the circuit even when the throttle is closed)

[deloreandmcxii]

To restate more clearly:

With the AC off and the idle microswitch working properly, the car hunts only during warmup and then settles in at ~750

With the AC off, the car idles at 1k ONLY when the idle microswitch is disconnected (or the striker backed off to not contact the switch).

With the AC on, after warmup and the idle microswitch working properly, the car hunts between 500-1000, and often stalls at the lower end - particularly when taching down sharply such as when clutching and coming to a stop.

[Bitsyncmaster]

With your idle switch disconnected you get ignition advance at idle on a warm engine which is why your idle RPM is higher.

Since your not running the lambda my guess is your idle motor is sticky causing hunting problems. i recommend the Volvo idle motor over the stock unit. It seems to operate much smoother than the stock unit.

[deloreandmcxii]

Thank you so much!

The one I'm seeing online for a b28f has a L shape to the in/out ports - instead of straight thru like DMC. Is this the one you prefer Dave M? Do I need some different rubber hose? Or do you have a link to a source for the one you like?

Any experience with the DeLoreanGo option? https://www.deloreango.com/us/idle-speed-control-valve-regulator-motor.html

[David T]

The idle speed without the idle motor working is not a good indicator of anything. It should be a higher value than when the idle motor is working but there is no spec. I would try to get the Lambda system working now and then the car should behave unless you have a lot of vacuum leaks. Also consider vacuum leaks are cumulative, ie, they all add up meaning a lot of small ones have the same effect as one large one so just because you find one does not mean you don't have others. The most common leaks are the injector seals, the air tube coming out of the bottom of the mixture unit and the seal over the mixture screw. Of course any of the rubber hoses can crack and split making for leaks too.

[deloreandmcxii]

Thanks David T! For better or worse, I won’t be getting lambda operational anytime very soon. My car’s had an old aftermarket exhaust for years with a rusted-in o2 sensor. Until I’m ready to put in a shiny new dpi exhaust, it’s more like lambDON’T for me (I’m here all week).

In the meantime, I just want to get it smooth enough to not stall when using the AC.

[Bitsyncmaster]

Thank you so much!

The one I'm seeing online for a b28f has a L shape to the in/out ports - instead of straight thru like DMC. Is this the one you prefer Dave M? Do I need some different rubber hose? Or do you have a link to a source for the one you like?

Any experience with the DeLoreanGo option? https://www.deloreango.com/us/idle-speed-control-valve-regulator-motor.html

The Volvo unit I use will connect right up to stock hoses and electrical connector but it is a smaller diameter so will not bolt up to the stock bracket. I don't use any bracket and it stays put just fine. I think Dgo sells it new.

[David T]

I would try the old O2 sensor anyway. hey, maybe it will work! If it doesn't it is not that big a deal to remove an old, rusty one. Take it to an exhaust shop and they will use an acetylene torch. It will come right out. You will never get the car running smoothly without it. It will run WAY too rich. All of these engine systems were designed to work together so for the engine to run smoothly they all need to be working correctly.

[deloreandmcxii]

Lambda is on the list! It just doesn't directly effect my enjoyment of the car, so it was lower than some other things.

The car runs smooth as heck... after warmup and with the AC off :) :)

[82DMC12]

Regarding a good smoke machine, I had really great success using this one:

https://www.amazon.com/AutoLine-Pro-Automotive-Detector-Diagnostic/dp/B07JVT1NG2/ref=sr_1_1_sspa?crid=1LAFZL9PSCBB6&keywords=automotive+smoke+machine&qid=1659970113&sprefix=automotive+smoke%2Caps%2C114&sr=8-1-spons&psc=1&smid=A39QUDLVDNA8EY&spLa=ZW5jcnlwdGVkUXVhbGlmaWVyPUExQ0MyQjJFOEJLT1JZJ mVuY3J5cHRlZElkPUEwNjA4MTc5MU5WWjY1UEVWWUxYSiZlbmN yeXB0ZWRBZElkPUEwMTk1Mzk2UlRMWUVEODVHRzc2JndpZGdld E5hbWU9c3BfYXRmJmFjdGlvbj1jbGlja1JlZGlyZWN0JmRvTm9 0TG9nQ2xpY2s9dHJ1ZQ==

Sorry about the long link. Anyway it comes with enough smoke liquid for probably two sessions but you can also use USP mineral oil which they carry at CVS.

I put the smoke into the engine using the hose that connects the oil filler cap to the air cleaner cover. You will also have to seal off the air meter flap because it's not a tight seal and smoke will come out. I did this by cutting a latex balloon and zip tying it around the meter housing. See the picture below. Actually the next time I did it, I removed the backfire guard so that the screws wouldn't push into the balloon and tear it. I found a couple minor leaks this way. The first thing that started to leak was the oil dip stick seal but that's OK because it's not part of the intake system anyway.

68776

[deloreandmcxii]

Perfect! Thanks Andy!

Waiting on a new idle motor to see if that helps. Will investigate vacuum leaks after that.

I am now dreaming of a winter project of a new exhaust and getting kjet fully operational (would be the first time in 20+ years!)...

[82DMC12]

Perfect! Thanks Andy!

Waiting on a new idle motor to see if that helps. Will investigate vacuum leaks after that.

I am now dreaming of a winter project of a new exhaust and getting kjet fully operational (would be the first time in 20+ years!)...


I was also able to rule out vacuum leaks to the front of the car using a MityVac which can draw a vacuum and you can watch the gauge on the tool to see if it falls. If you hook it up to the black hose that goes into the hose barb fitting on the rear of the passenger side of the intake, that is the source for all vacuum to the HVAC and brake booster. You can pull a vacuum from that hose to rule out the whole front of the car. You would have to rotate the mode switch to each position one at a time and re-check the vacuum since it's possible that it only leaks in one position.

[deloreandmcxii]

DeLoreanGo is FAST.

The new idle motor did improve the Hunt-with-AC-On somewhat. It still hunted some, but I think because it is clean and new it responds faster - the result is the min and max of the hunt is reduced. Was ~500-1000 and would often stall, with the new motor it became ~600-900 and it even settles into 750, if you remain at idle for a moment.

At the 600 range the engine still does it's shaky "I'm gonna stall" dance when you snap the throttle closed from high RPMs - Even though it doesn't quite stall, I still didn't love that.

So I cheated and gave the curb idle a tweak. I would have used the (gasp) brass screws, but one of mine is snapped off.

The result is a bit high idle without AC: ~900 cold up to ~1000 warm. But ZERO threat of stalling with AC on: a nice solid 750.
I'll probably move the curb idle back down in autumn when I don't need the AC.

And that is good enough for now! Thanks everyone for chiming in!

[deloreandmcxii]

Someone talk me out of this shady idea for a temporary improvement :)

Wire up a relay to disconnect the idle microswitch when the AC is on.

[Bitsyncmaster]

Someone talk me out of this shady idea for a temporary improvement :)

Wire up a relay to disconnect the idle microswitch when the AC is on.

Just to let you know. The idle micro-switch cuts off the ignition advance. It does not stop the idle ECU from trying to control idle RPM. So are you disconnecting one wire from the idle switch? Removing power to the advance solenoid will turn advance on at idle.

Try the same test pulling the advance hose off the ignition distributor. (plug the hose so no vacuum leak).

[deloreandmcxii]

Thanks for that additional clarity!

Oh jeez, this idea MUST be shady. Look who suggested it years ago :) just found this old thread reading up on mechanical v vacuum advance.

https://dmctalk.org/showthread.php?2340-Ignition-advance/page2

[deloreandmcxii]

Ran your experiment: yup I get it now! Idle motor is always doing it's thing. I misunderstood that.

So back to your 2011 idea (only took me 11 more years to come up with it :) :) ). Why not engage vacuum spark advance at idle, when AC is on? One could accomplish this with a relay wired (in an easily removable manner) to interrupt the idle microswitch circuit.

What AC wire would be a good candidate? is the pink into pig-tail red wire that goes to the compressor only powered when AC is on?

[Bitsyncmaster]

Ran your experiment: yup I get it now! Idle motor is always doing it's thing. I misunderstood that.

So back to your 2011 idea (only took me 11 more years to come up with it :) :) ). Why not engage vacuum spark advance at idle, when AC is on? One could accomplish this with a relay wired (in an easily removable manner) to interrupt the idle microswitch circuit.

What AC wire would be a good candidate? is the pink into pig-tail red wire that goes to the compressor only powered when AC is on?

My car will idle crappy if advance is on during idle. But running my idle ECU, I bump the idle up another 100 RPM when the AC compressor is on (yes the pink wire at the diode). I set my idle at 850 RPM so with AC on it is 950 RPM. It just makes the AC work so much better if stuck in traffic or a long light and the alternator puts out so much more power.

So if you ever get your lambda working you may have idle problems with advance.

[iflights]

What AC wire would be a good candidate? is the pink into pig-tail red wire that goes to the compressor only powered when AC is on?

This is a photo of the location to tap into.68786

Not my photo, but shared previously here or on FB.

[deloreandmcxii]

Thanks!!

This is perfect for driving Dave's Idle ECU inside the car. Is there someplace this circuit surfaces itself in the engine bay, so I can mount the relay close to the idle microswitch?

Ideally, is there a place it *plugs* into something where I make the relay harness plug and play?

I plan on poking around today or tomorrow - but if someone already know the answer...

[Bitsyncmaster]

Thanks!!

This is perfect for driving Dave's Idle ECU inside the car. Is there someplace this circuit surfaces itself in the engine bay, so I can mount the relay close to the idle microswitch?

Ideally, is there a place it *plugs* into something where I make the relay harness plug and play?

I plan on poking around today or tomorrow - but if someone already know the answer...

The wire on the AC compressor would work to drive a relay.