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Idle Wanders After Master Cylinder Replacement


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We have a 1992 Honda Accord LX that my two 17 year-old twins drive. This afternoon I replaced the master cylinder and bled the system. When I started the engine to drive the car to make sure the brakes were working correctly, I noticed that the idle was cycling back and forth between about 1,100 RPMs and 1,600 RPMs. It would stay on each for about two seconds. I figured that while my hands were in there loosening the nuts on the master cylinder mounting bolts, I might have knocked a hose off or something. Everything looked okay. I drove the car for about five minutes, checking the brakes. When I returned to the house, the idle was still cycling back and forth. The kids say the idle was fine this morning and this afternoon.


Any ideas?


Thanks in advance for your help.



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First sheck to see that the brake booster is working fine. The pedal should pump firm with the car off and with the car running the pedal should be softer. If it is hard with the engine running then their is an issue with the brake booster and this can cause idle problems. This is a quick easy check. The brake booster would likely need to be replaced.


Second, when you removed the master cylinder was their moisture between the cylinder and the booster? If so that needs to be dried out and the seal repaired or replaced.


Third, check the grounds in the area where you were working. Clean and re-tighten or repair.


Fourth, check for cracks on the vacuum hoses first. You can do this visually or by lightly spraying each hose with WD-40 to see if the engine speed spikes or sometimes you can find the issue by pinching each hose one at a time with a pair of pliers or by focusing your hearing along each hose and listen for a lite "sipping" sound. Beware, finding a vacuum leak, if that is what it is, can be very difficult, so often times it is best to simply get new vacuum hose and just replace them.


Other then that it sould not have anything to do with the replacement of the master. If you had ABS on the car then it could be something to do with that, but that should not be the case.


Lets us know how you make out and we can recommend additional information after you check these things out.

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