M42 High idle, weak with A/C on

[lolcantturn]

Hey guys. My M42 has the mess under the intake done, car runs nice and strong. New ICV, no obvious vacuum leaks, working great AFM, new TPS. My idle is rather high, even with various ICV's tried. The mechanical screw idle setting on the throttle body has not been messed with. It idles at about 900-1000. Though it does idle fairly smooth once warmed up. It has room for improvement. When it's cold its fairly shaky. I'm not throwing any CEL's either. I'm trying to figure out what would make my idle so high? When I put the fan on without A/C, the idle goes down to normal, about 600rpm, and is still really smooth. Once I turn the fan off, idle goes back to 900-1000rpm.

Additionally, my idle is very weak with the AC on. If I'm in stop and go traffic, going from gear to neutral with the AC on will kill the engine and it'll stop. It happens virtually every time. It tries to hold idle and immediately dies.

[lolcantturn]

Nothing ?

[oldmann]

No obvious vac. leaks may be your issue. Valve cover, dipstick (tube), Throttle shafts.

Throttle body plates closing all the way? Idle screw may not be messed with but linkage between the primary and secondary plate may not be correct. Or crud built up on the edge of the butterflys may be holding plates open.

[lolcantturn]

No obvious vac. leaks may be your issue. Valve cover, dipstick (tube), Throttle shafts.

Throttle body plates closing all the way? Idle screw may not be messed with but linkage between the primary and secondary plate may not be correct. Or crud built up on the edge of the butterflys may be holding plates open.

Drenched the throttle shafts in brake cleaner, no up revs. my vac lines look to be in good shape, though a smoke test might bring out imperfections.

Throttle plates are definitely closing all the way, checked.

One thing I noticed, my car runs exactly the same with the O2 disconnected or connected. Is this normal?

[bmwm42]

Harmonic balancer

[oldmann]

When my alternator was going out I had an issue with the idle, when I had the heater, rear window defroster, headlights on. Early morning, 10 degrees out. Idle would drop to 600 650 surge up and down 100 rpm. Alt. and battery wouldn't keep up with demand.

After replacing alt. idle was back to normal, didnt connect the two (low idle, bad alt.) for a couple of weeks. Nice weather.

Maybe bad regulator, or ground strap syndrome.

[lolcantturn]

When my alternator was going out I had an issue with the idle, when I had the heater, rear window defroster, headlights on. Early morning, 10 degrees out. Idle would drop to 600 650 surge up and down 100 rpm. Alt. and battery wouldn't keep up with demand.

After replacing alt. idle was back to normal, didnt connect the two (low idle, bad alt.) for a couple of weeks. Nice weather.

Maybe bad regulator, or ground strap syndrome.

Well, my problem isn't low idle, its high idle!


Like I said before, when I disconnect the O2 sensor the car idles the same..normal?

[Threehz]

Yeah, but when your AC is on it has a low-->dead idle, although I don't think the alt would cause that.. AC is more of a physical strain on the engine. But, it's interesting that the idle drops when you throw just the fan on. Getting the alternator checked is easy and free at most auto stores, might be worth a try. You could even just try turning on a bunch of the electronics and see what effect it has.

As a note isn't the ideal idle for an M42 750 - 850 RPM?

Regarding the O2 sensor question, do you have the infamous firewall plug connected or no?

[lolcantturn]

Yeah, but when your AC is on it has a low-->dead idle, although I don't think the alt would cause that.. AC is more of a physical strain on the engine. But, it's interesting that the idle drops when you throw just the fan on. Getting the alternator checked is easy and free at most auto stores, might be worth a try. You could even just try turning on a bunch of the electronics and see what effect it has.

As a note isn't the ideal idle for an M42 750 - 850 RPM?

Regarding the O2 sensor question, do you have the infamous firewall plug connected or no?

I guess I can try and max out my alternator and see what happens, I'll update accordingly.

The infamous firewall plug is disconnected thank god, it caused a lot of trouble for me when I first had the car haha

[lolcantturn]

bizzump.

[freeride53]

lol i honestly think it's the power surge that comes from the a/c.
my alternator is a reman'd unit that was installed a few years ago.
get the same issue as you.

cheers
~s

[fourmotioneer]

Interested to see what people have to say about the O2 sensor. I feel like I have the same issue.