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Engine idling better without Master Cylinder?


Eddie 1979

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I've recently made two changes to my vehicle (240Z L28 ET):

 

1. Put the car up onto 17" high cribbing blocks so I can refresh the underbody.

2. Removed the master cylinder for replacement.

 

I start the car up every week just to keep everything fluid (and as an excuse to listen to those L6 vocals).  Uptil now I would have to hold down the peddle for a couple of minutes before if would idle smoothly on its own.  After the two changes above I can pretty much turn the key and it idles fine without any throttle.  Idle RPM is also just over 1k now, previously it was around 800.  I haven't removed the vacuum line from the booster or changed anything engine wise.

 

After the brake and clutch cylinders are replaced I will take the car in to be tuned regardless, but I'm still curious as to what the explanation for this could be. I'm assuming the idle change is not related to either of the above, but I'm a newbie to engine mechanics so I figured I'd ask.

Edited by Eddie 1979
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Don't really have an explanation for your idle difference, just wanted to say if your car is going to be on blocks for a while you are better of just doing the typical 'storage' preparations and NOT starting the engine up periodically. 

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  • 2 weeks later...

So the brake master cylinder is uninstalled completely? With the brake booster sitting on its own with the diaphragm still experiencing vacuum when the engines running, but not interfacing with the completely uninstalled BMC? And you would normally have to press the brake pedal while warming it up, correct?

 

A stock Bosch EFI system from this era will probably try to boost the idle a bit with the brake applied, and probably just uses the brake pedal switch. Is the pedal now sitting against this switch and holding it down at all times, since its no longer resting against the BMC pushrod?

 

Besides that I'd suspect a leaking brake booster diaphragm. I'd experiment with blocking off the vacuum line to the booster as well as those around the idle control valve, and try disconnecting the wire to the idle control valve.

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