M50NV Idle drops on deceleration

[Kerrath]

I've searched.

1992 E34 M50.

Recently, I've had an issue with my idle dropping significantly when I start decelerating. It pretty much only happens when moving and doesn't matter whether warm or not. It's bearable, doesn't impair driving, just is mildly annoying. To the best of my knowledge, there are no vacuum leaks, but I don't have a vacuum gauge to test.

Additionally, my idle is about 500 RPM when it is supposed to be more like 750. It's a little bit rough.

Pertinent information that is known:
New O2 sensor (<5,000 miles)
Good ICV
Good MAF
Adjusted/Tested TPS

Any help is appreciated. Thanks.

[SamE30e]

The tach is off by a certain % I forget what it is. But my tach says 500, but it is at like 700ish

[Kerrath]

The tach is off by a certain % I forget what it is. But my tach says 500, but it is at like 700ish

Why do you say that? It makes no sense unless you have a 9k tach or something.

[Wh33lhop]

The tach is off by a certain % I forget what it is. But my tach says 500, but it is at like 700ish

Ehh 200rpm is a pretty big percentage of 700rpm. Does your tach read 5 grand when you're at 7?

My tach has always been absolutely spot-on with the M20, much more so than previous cars. It gets a little bobby at times though.

[screwbiedoo15]

I haven't noticed the idle dropping on my m50NV when decelerating but my engine does idle at 500rpm as well. Also curious if this is normal or problematic?

[Kerrath]

It doesn't really hinder the driveability, it's just a mild annoyance. The idle drop on mine is a major annoyance.

Ehh 200rpm is a pretty big percentage of 700rpm.

That's how I feel. I dont see any way that my tach is wrong there when the revlimit is exactly where it's supposed to be.

[MattM]

mine idles at 500 when its been sitting running for awhile. idles at ~1100 when moving, and ~700 when cold

[Kerrath]

mine idles at 500 when its been sitting running for awhile. idles at ~1100 when moving, and ~700 when cold

Yeah, when I first start it, it hangs at about 750 for a bit, then settles to 500.

K, so I guess we've established that 500 is essentially normal for us, but anyone have advice on the idle drop on deceleration and the sort of roughness when at idle?

[derrith]

Any vacuum leaks? I hastily swapped a NVm50 into my old 528e and had no end to idle issues. I ended up replacing all the vacuum hoses and the intake manifold gaskets and that fixed it. The lines to and from my ICV were especially bad.

[jonmon6691]

You could leave it in gear when you decelerate...

[416stroker]

Yeah, when I first start it, it hangs at about 750 for a bit, then settles to 500.

K, so I guess we've established that 500 is essentially normal for us, but anyone have advice on the idle drop on deceleration and the sort of roughness when at idle?

Mine does it to only with the lights on. but it idles at 500 as well.

How rough? Mines my flywheel what are you running for a flywheel?

[Kerrath]

I can feel it in the car and hear it in the exhaust, even though the idle fluctuates no more than 10rpm. Also sometimes at night I notice it having an effect on the brightness of the dash lights.

I'm running E34 single mass flywheel.

[Wh33lhop]

I just hooked up my tach, it idles around 650-700. I've also got a broken PCV hose and a crappy idle most likely due to that, although it hasn't died on me yet. It's a non-vanos out of a '92 (I believe) E34 using a lightened M20 flywheel. Honestly I think the IM is resting on the brake booster (E32 booster) and I haven't had a chance to shim the motor mounts yet, so that may have to do with it too.

[Kerrath]

I just hooked up my tach, it idles around 650-700. I've also got a broken PCV hose and a crappy idle most likely due to that, although it hasn't died on me yet. It's a non-vanos out of a '92 (I believe) E34 using a lightened M20 flywheel. Honestly I think the IM is resting on the brake booster (E32 booster) and I haven't had a chance to shim the motor mounts yet, so that may have to do with it too.

How do you think the booster is affecting your idle? And I wrapped the parts of my PCV hose that seem to have issues and it had no effect on my idle that I could find.

You could leave it in gear when you decelerate...

I didn't notice this post before.
You could also stall every time you get to a stop light. It's not an option.

[Wh33lhop]

How do you think the booster is affecting your idle? And I wrapped the parts of my PCV hose that seem to have issues and it had no effect on my idle that I could find.

literally the PCV hose won't fit on the nipple. It's torn and too short now.

The IM, being plastic, seems a bit distorted, so it might be boning up the IM gasket-to-head contact. I should also mention that I'm not yet running an O2 sensor..