I've got an insanely weird issue happening with the clutch hydraulics that is driving both me and a few mechanic friends of mine crazy

2018 GT350, 43000 Miles.

Context:

The issue started about 6 months ago after a spirited drive in the fall. During the drive, leaves had almost completely blocked all of the vents in the front, which meant my intake temps were much higher than usual for about an hour.

On the way home, I went for a redline shift from first into second and the car wouldn't let me into gear. The clutch pedal was also stuck on the ground, and I had to pump it a few times to get pressure again. I drove the car home gently with no issues and put it away for the winter. Bringing the car out for spring again has revealed it's now a consistent issue.

The Problem:

Driving the car around under 6K RPMS is completely normal. It never misbehaves.

Upshifting at redline always results in one or more of the following:
- Unable to get into the next gate, and clutch Pedal stuck on the floor and needs pumping(only one or two times) to feel normal again
- Successful shift, but clutch Pedal stuck on the floor and needs pumping(only one or two times) to feel normal again
- Successful shift, but clutch Pedal stuck halfway and needs pumping(only one or two times) to feel normal again

The hydraulic system itself is losing pressure when the issue occurs, as it always requires one or two pumps to feel normal again.

Troubleshooting:

Complete clutch bleed was always done following factory procedure (vacuum > pump pedal and repeat) after each step. Brake fluid is OEM Motorcraft and new.

- Replace Slave/throwout bearing w/OEM unit. This was CONFIRMED to be leaking a little bit as there was a bit of fluid in the bell housing.
- Replace Clutch master cylinder w/OEM unit.
- Inspect all clutch components while swapping the slave, which all look extremely healthy

After these repairs, the problem behavior is the EXACTLY same. There is NO brake fluid loss anywhere.

Thoughts and Next Steps?

- New braided metal cable? If the rubber cable is deteriorated it could be ballooning during high RPM shifts, but that shouldn't make the entire system lose hydraulic pressure.

- Still air trapped in the system? Behavior points to this, but we've bled it so many times.

- Overall, the key here is still that the system is losing hydraulic pressure when this happens and needs to be pumped one or two times to feel normal again. The diagnoses would need to align with this fact. For example, we're thinking it's not mechanical or clutch related, as that wouldn't cause the whole system to lose pressure.

Replace the clutch. My 2011 behaved exactly the same after bumping the rev limiter up to 7500. The clutch looked fine. A new clutch and slave cylinder fixed everything.

EF1 said:

Replace the clutch. My 2011 behaved exactly the same after bumping the rev limiter up to 7500. The clutch looked fine. A new clutch and slave cylinder fixed everything.

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If it's a mechanical/clutch issue, were you able to determine how that root cause was causing the clutch hydraulic system itself to lose pressure and require pumping?

In my case there was no leak. It worked fine below 7200 rpm. Over that it stuck to the floor. It was not a hydraulic issue. The clutch was not strong enough to return it at higher rpms.

EF1 said:

In my case there was no leak. It worked fine below 7200 rpm. Over that it stuck to the floor. It was not a hydraulic issue. The clutch was not strong enough to return it at higher rpms.

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Gotcha - so the symptoms are different in our cases yeah?

I feel like a core part of my issue is that the hydraulic system loses pressure when this happens and needs a pump or two to feel normal again, whereas in your case, the clutch stuck, but the hydraulic pressure was OK once it came back up.

In my case when it stuck I had to shut the car off and restart it to restore function. Fun on track. Could not find any leaks and fluid level was normal. Drove fine on the road. Stuck again on track at high rpm. Went through this. Cycle 3-4 times and put it on the trailer for the weekend. I did not want to damage the trans. It was a pressure plate issue. It just could not handle the additional power/rpms.

If you think it is a hydraulic issue pull a vacuum at the reservoir to make sure you don’t have a leak somewhere in the system.

That's the puzzling part. The system was bled 5-7 times after each part was replaced and most recently has been bled over 10 times following ford's vacuum/pump procedure.

On your car is the clutch circuit separate from the brakes?

EF1 said:

On your car is the clutch circuit separate from the brakes?

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Yes it is. Same reservoir but different circuit. Brakes are 100% this whole time as well.

I like EF1's solution. Pressure plate issues seem to be well documented with Ford clutches (at least with S197s anyway). However, sometimes people can have a tough time getting a vacuum bleed to work right. You could try one of these: https://www.americanmuscle.com/mcleod-mustang-quick-connector-clutch-bleeder-assembly-139250.html

If it isn’t leaking and is losing hydraulic pressure it would have to be the clutch master cylinder. If the slave was losing pressure it almost has to be a leak. I am not convinced it is losing pressure. Clutch master cylinders can go bad but you don’t hear about it very often on these cars.

EF1 said:

If it isn’t leaking and is losing hydraulic pressure it would have to be the clutch master cylinder. If the slave was losing pressure it almost has to be a leak. I am not convinced it is losing pressure. Clutch master cylinders can go bad but you don’t hear about it very often on these cars.

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Check out OP, that was replaced with an OEM part as it was out initial thought as well.

Current theory is that maybe the clutch hose is degraded and ballooning and we should try a new one.

I had the same issue of not getting in gear only happened at the track and after the car was run hard, changed the clutch line to braided line and properly vacuum bled the clutch, it has never happened again in 2 years so far and the car sees 8K rpm on track regularly. The plastic line enlarges enough that the volume of fluid when the clutch is pushed just isn't enough.
Good luck.

rgator said:

I had the same issue of not getting in gear only happened at the track and after the car was run hard, changed the clutch line to braided line and properly vacuum bled the clutch, it has never happened again in 2 years so far and the car sees 8K rpm on track regularly. The plastic line enlarges enough that the volume of fluid when the clutch is pushed just isn't enough.
Good luck.

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This is my current theory as well. I have a new steel one coming.

Can you describe your symptoms in more detail when it happened? What was the behavior of the clutch pedal? Did it stay on the ground/halfway? Did you need to pump it a few times to get it to feel normal again?

Go with the easy fix first. Replace the clutch line and rebleed it. If it fixes it you are done if not on to the next component.

Replace your clutch line, definitely. In my case, even with a new clutch line, mine continued to get worse and more symptomatic over time (pretty rapidly) so I ended up replacing all of the clutch components (slave, pilot bearing, clutch, flywheel & pressure plate) w/McLeod RST and OEM bearing + slave.

Talk about a fantastic upgrade. I'd recommend going this route if you even think of dropping the trans again.

Update:

We've replaced the entire clutch hydraulic system and the issue is still there exactly as it was at the start.

There's also a new symptom I experienced where when going for a high RPM 2->3, the clutch was extremely stiff and would not let me out of the gate.

RushB said:

Update:

We've replaced the entire clutch hydraulic system and the issue is still there exactly as it was at the start.

There's also a new symptom I experienced where when going for a high RPM 2->3, the clutch was extremely stiff and would not let me out of the gate.

Click to expand...

I would have pointed out the obvious boost check valve between brakes booster and Intake Manifold (the little plastic thing) this have a tendency to fall but you said your brakes were fine. Usually when the pedal start dropping to the floor or is hard to press is a TOB but you replaced that as well. Which leave us with the clutch so maybe inspect pressure plate for issues.

I had this issue with my 2017 GT350 and the hose that comes of the reservoir going to the clutch had been known to not seal properly. I saw a fix for it online and it was put a small hose clamp on the hose on the barb or a zip tie. I put a zip tie on it and never had another issue for the next three years until I sold the car. I was at the drag strip when this happened to me.