Engine Revs under Braking

[Jdizz07]

Hi all,

Finally got my 76' 280Z going, but whenever the brake pedal is pressed the engine revs up to about 3000rpm. Any ideas? Hole in the brake booster? Brakes still work as usual.

Any ideas for a cheap fix?

Thanks in advance

JD

[240260280]

Try turning the screw or bolt under the control valve located under the throttle body.  It is called the boost controlled deceleration device (BCDD).  It is a controlled leak around the throttle plate to reduce high manifold vacuum when the throttle valve snaps shut (usually under braking)

 

The screw turning is trial and error until you are happy with the way the rpms fall when stopping.

[Reptoid Overlords]

Its my understanding that a failing brake booster will result in RPM drop as opposed to RPM rise.

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[Jdizz07]

Thanks alot,

Im guessing this broken wire has something to do with it?

[240260280]

https://www.google.ca/search?q=site:classiczcars.com+bcdd&source=lnms&tbm=isch&sa=X&ved=0ahUKEwjo7LbUlKbaAhVKoVMKHYC-AGIQ_AUIDSgE&biw=2049&bih=1011&dpr=0.67

[Zed Head]

9 hours ago, Jdizz07 said:

Hi all,

Finally got my 76' 280Z going, but whenever the brake pedal is pressed the engine revs up to about 3000rpm. Any ideas? Hole in the brake booster? Brakes still work as usual.

Any ideas for a cheap fix?

Yes, that is the classic sign of a holed brake booster.  When you press the brake pedal you open up a vacuum leak.  People have been known to rebuild them.  Disassembly is actually shown in the FSM.  But most just replace them, with a reman.  They're hard to find, and quality is generally poor.  I don't think that you can get rebuild parts easily either.

Edit - a simple test would be to disconnect and block the vacuum hose to the booster, then press the pedal and see if the problem is there.  Whether RPM drop or rise depends on the size of the hole.

246, you're showing fixes for RPM hanging high while driving.  Pretty sure the OP is just sitting still, pressing the brake pedal.

Edited April 6, 2018 by Zed Head

[heyitsrama]

My 260 had this issue, it was the brake booster that failed. The brake was significantly more difficult to push down when this happened.

I got it repaired at Power Brake Exchange in San Jose, took them a couple days to rebuild. They beadblasted it when I got it back.


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[Jdizz07]

Thanks alot guys.

I have bought a vacuum pump so will test it this weekend. 

Would it have any impact on the engine running rough while driving? Mine seems to be running worse since the rebuild. (was running great last week)