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

[ATB88]

EDIT NEW INFORMATION: After some extra time testing to reproduce the issue, I've seemingly been able to determine that there's definitely an issue with the clutch or some part in the bellhousing that may be independent of the rev up behaviour

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 likely the same -- I'm probably 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, was refreshed with my 6MT swap and has 50k miles on them, so whatever this failure is it seems premature. I did the work myself.

Any thoughts ideas?

Original Post below:

I've done a few long (5-10 hour) drives/roadtrips in the last couple weeks and have come across a very strange intermittent issue: sometimes, after I've been driving for a while, when I push the clutch pedal all the way down, revs will go UP until I let the clutch pedal back out. Yes, my other foot is completely off of the throttle when this occurs. It doesn't happen when the car is cold, or on short trips. Only after I've been driving the car for an hour or two, and/or I've been doing some higher RPM runs. When my car gets into this "operating condition", it will happen reliably when I push the clutch pedal down low enough, but I can avoid it if I don't push the pedal all the way down to the clutch stop.

Anyone ever experience something like this?

The only possible thing that makes sense to me is that there is somehow something wrong with the throttle/fuel system, as I can't imagine how any sort of failure in the clutch system could cause revs to go up when the clutch is disengaged and I'm off throttle. Am I wrong? But the car drives completely normally otherwise, and I would think if the throttle was getting stuck open or something, that would be pretty noticeable in normal driving, no?

No error/fault codes.

Really scratching my head here. Clutch has 50k miles on it. Fuel system (including injectors and pump) less than 3 years and 30k miles old. Last time I had the airbox off the ITBs looked fine. Any ideas?

[jet_dogg]

Sup atb, I've never heard of this but since it's throttle related have you changed all throttle position sensors?

Perhaps your gas pedal is going bad which also houses one if I recall correctly.

[ATB88]

Sup atb, I've never heard of this but since it's throttle related have you changed all throttle position sensors?

Perhaps your gas pedal is going bad which also houses one if I recall correctly.

Yeah, both engine-bay TPSs have been replaced once or twice each in the last few years. Never the pedal, though. While I'm open to all suggestions at this point, I would really think that if the pedal was keeping the throttle open even when my foot's not on the pedal, I would notice this when the clutch isn't disengaged? Also would expect a SES light when the TPSs don't agree, but who knows!

[jet_dogg]

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

[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...

[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)_/¯

[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]

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]

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]

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]

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.

[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.

[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.

[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]

!!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?