Weeeird Clutch Issue -- RPMs go UP when clutch pedal in

[ATB88]

Bump! Really hoping to figure out what's going on with my clutch. I probably don't have time to drop the bellhousing until December, so I'm hoping to understand what's happening in there and see if I can mitigate further damage by driving carefully until I can repair. Thanks for any ideas!!

The main problem I'm trying to figure out: when I put the clutch pedal in at higher RPMs (4.5k+), when the clutch pedal hits the bottom of its travel I often get strong mechanical feedback through the pedal and a loud rpm-dependent whine, which lets up when I raise the pedal again. This doesn't *always* happen when I shift at higher RPM. I can avoid it by shifting at lower RPM or not completely taking the clutch pedal down to the clutch stop when I shift at higher RPM. It feels/sounds like something in the bellhousing is touching something spinning that it shouldn't be.

Clutch and all associated hardware have 50k trouble free miles on them, all installed when I performed 6MT swap 5 years ago. Used LuK clutch kit, OE BMW throwout bearing, slave cyl and clutch fork, aftermarket upgraded bronze clutch fork pivot pin. Original flywheel (96k miles).

[ATB88]

Just wanted to update and clarify what's going on with this issue! I've been able to reproduce it on shorter drives, not completely reliably, but I can often get it to happen when I'm shifting from in higher RPMS (~4.5k+).

-Sometimes when I shift in these conditions, when the clutch pedal gets to the bottom of its travel I will feel some mechanical rumbling/feedback through the clutch pedal and hear a loud and RPM dependent whine as the revs change. It sounds and feels bad. Based on the feeling in the clutch pedal and the noise, it would seem that there's absolutely something weird/bad happening in the bellhousing.

-When the above happens, sometimes but not all the time, revs will go *up* instead of down when the clutch is fully disengaged (foot at bottom of clutch pedal travel). Since it doesn't always happen, I'll assume at this point that this is a separate issue from the above that just happens to coincide with it sometimes.

I'd first like to focus on the problem in the bellhousing. I realize that if there's something wrong in there (bad throwout bearing, pressure plate, etc) it doesn't really matter what it is the treatment is the same -- I'm dropping the transmission and doing a full clutch job and refresh in there. That said, I would really like to figure out what exactly went wrong here or if I made some error on install last time I was in there. The clutch, throwout bearing, slave cyl, really every part inside the bellhousing aside from flywheel, has 50k miles on them, so whatever this failure is it seems premature.

Any thoughts ideas?

[ATB88]

I actually have no idea, but I'm following closely, I'm very curious.

I thought maybe I had an idea until you said the RPMs keep going up. I'm wondering how high they will go if you just let it go.

Yeah I was curious too, but not curious enough to hold the clutch in that long. The mechanical feedback and sound were enough to convince me that I'm tempting fate every time I do this lol.

In any case, it does seem like there's a problem in the bellhousing that's probably independent to what was making the revs go up, based on my last drive.

[9kracing]

I actually have no idea, but I'm following closely, I'm very curious.

I thought maybe I had an idea until you said the RPMs keep going up. I'm wondering how high they will go if you just let it go.

[ATB88]

!!New Data!!:

Was on a short drive today and tried to reproduce the issue, and found the following: if I'm in higher rpm (~4.5k +) and I floor the clutch pedal, the revs will fall naturally rather than go up, but I'll reproduce the mechanical vibration I feel through the pedal and hear a loud "rev down" noise as the revs drop (pitch drops as revs drop, is loud). This mechanical pedal vibration and noise don't occur if the revs aren't high when I clutch or if the pedal doesn't go all the way to the clutch stop.

So, my inclination is maybe the revs going up is a separate issue, but there's definitely something wrong in my bellhousing. What do we think, shot throwout bearing? Or maybe my clutchstop is too low and I'm pushing the throwout bearing too far in?

[bigjae46]

A guess here but could be running rich and getting excessive fuel through the PCV system? That can cause the engine to rev due to excessive fuel vapor.

In industry, engines will runaway and rev uncontrollably when there's a release of hydrocarbon vapor and the engine sucks it in through the intake. The engine will rev up even if you cut the ignition.

[ATB88]

Vacuum leak that only shows up once everything is nice and hot?

I would smoke test the car when it's at operating temp. Should at least let you know that all the vacuum hoses are good.

Could be! Sounds like intake's coming off for a leak and ITB inspection, and smoke test when I can get my hands on one.

Indulge an unlikely scenario, though: when the revs go up it feels/sounds a little grindy. Is there any scenario where this could be some sort of throwout bearing issue? This issue only happens at the bottom of the clutch pedal travel. Doesn't explain the revs going up, but the grindy sound makes me wonder if it's more than a throttle/air/fuel thing.

[heinzboehmer]

Vacuum leak that only shows up once everything is nice and hot?

I would smoke test the car when it's at operating temp. Should at least let you know that all the vacuum hoses are good.

[jet_dogg]

Don't know bro bro, but it would also be worth having another look at the throttle plates and cable perhaps give them a cleaning.

Back in the birth of the jdm days I was running a custom billet RC throttle body and the throttle plate would always stick. I took it back to Russ (RIP) half a dozen times for him to fiddle with and every time he claimed it was fixed, it would act up again.

When I would slow and come out of gear pressing the clutch in, the revs would rise because the plate was stuck partially open. I had to blip the throttle to get it to snap shut.

I don't think it would hurt to replace the pedal since everything becomes a replacement candidate at the twenty year mark.

[ATB88]

But no on the crank sensor?

Regarding the gas, since the throttle is electronic I don't know why anything else other than an incorrect reading of throttle position would cause the revs to rise unless there is an unrelated-to-TPS issue that affects this functionality as well.

Yes on crank sensor.

One other thing I can say is that when this happens, I think it feels/sounds different than if I was just free revving in neutral, more mechanical noise and vibration through the pedal maybe? But hard to say objectively, I could just be misremembering because I'm honestly pretty freaked out whenever this happens.

But yeah, I agree that if the TPS at the pedal was reading higher than it should in these scenarios, it could cause the throttle to open and the other two TPSs would agree, and no code. Hm. If there aren't any other candidates that come up, throwing a new gas pedal at it is relatively cheap and easy..

[jet_dogg]

But no on the crank sensor?

Regarding the gas, since the throttle is electronic I don't know why anything else other than an incorrect reading of throttle position would cause the revs to rise unless there is an unrelated-to-TPS issue that affects this functionality as well.

[ATB88]

If the throttle input is so mild then perhaps you don't notice it with the car in gear?

For the hard start you did the pump, filter, fp regulator, clean injectors and crank sensor right?

Yeah, I suppose that's possible. And yes on all of those fuel system parts -- threw in new injectors (back when they were still available). Also cam sensors and the fp regulator vacuum line... among other things lol.

[jet_dogg]

If the throttle input is so mild then perhaps you don't notice it with the car in gear?

For the hard start you did the pump, filter, fp regulator, clean injectors and crank sensor right?

[ATB88]

Do you think it could be related to your hard start issue?

Hah, I'd bet they're unrelated -- the hard start's been there going on 4 years at this point. But, again, who knows ¯\_(ツ)_/¯

[ATB88]

I've read some threads on forums where people describe problems where the revs rise just a little bit when clutching, but to be clear, in my case, revs just keep going up until I let the pedal back out again. Haven't tested how high they'll go because obviously something is broken here and I don't want to cause any catastrophic failures...