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Engine racing and reduced braking assist (long)

541 views 3 replies 2 participants last post by  tardis24 
#1 ·
A strange thing happened last night.

I was on the freeway interchange and about to enter a bend, when I noticed that the brake pedal was harder than normal and had little effect. When I let go of the brake I immediately noticed that the car was moving with no input, almost as if the cruise-control were on.

While finding a place to pull over and trying to slow down, I noticed that the revs would jump sky high (> 5000rpm) if I disengaged the clutch. Ended up killing the engine and rolling to a stop at the side of the road.

Unfortunately I have not been able to find a root cause. I am willing to accept theories such as the throttle being stuck open, although the throttle linkage seemed to be in the right spot, and the cable did not appear to be caught anywhere when I checked it.

Was able to start it up drive home normally right after.

I think my biggest concern right now was the momentary lack of brake assist, which had never happened before. I am trying to understand whether it was because of a possible stuck-open throttle or because of a vacuum leak.

The wiring harness to the cruise control was disconnected. Is it recommended to remove its connection to the throttle as well? I can't seem to get a good view of how it's attached.

Thoughts? Car is a '95 CSE 5-Speed (no TCS).

Thanks, in advance.
 
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#2 ·
The brake servo is a standard vacuum assisted booster. If it leaks air then there would be a sizable air leak into the manifold. Because this engine uses an absolute barometric pressure sensor for fuel mixture (and not the older AMM systems) any air leak that bypasses the throttle plate will give overrevving symptoms. Because this is a turbo quite modest air leaks can produce significant power and certainly a high "idle" speed would be likely.

Also, a failing brake servo will give a very hard brake pedal. An air leak into the brake booster would do this as would a vacuum leak between the brake booster and the intake.

There is a diaphragm inside the brake booster which could crack and a non return air bleed valve which allows the booster to release vacuum when your foot comes off the brake pedal. My bet is an intermittent fault in that air bleed valve. Another possibility is an intermittent leak in the vacuum line to the brake booster. i would suspect the fitting at the intake plenum rather than a split in the hose because the problem is intermittent.

New parts should solve the problem once you track it down.
 
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