Idle fluctuation

When the motor is worm and I step on the brakes the idle drops and comes back in a 2 second rithm. Up and down, up and down as long as my foot is on the brakes. The brakes work fine. I let off brakes it idles fine. I replaced the brake booster and it helped a little bit. It is possable the rebuilt booster is bad. How do I check that? I checked the one way check valve in the vacuum line to booster it seems to be working ok.

4 cyl Two Wheel Drive Manual 95000 miles
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have the same problem?
Sunday, February 15th, 2009 AT 5:42 PM

1 Reply

Check the engine vacuum

To check engine vacuum, connect a vacuum gauge to the supply hose that runs from the intake manifold to the booster. A low reading (below 16 inches) may indicate a hose leak or obstruction, a blockage in the exhaust system (plugged catalytic converter, crushed pipe, bad muffler, etc.), Or a problem in the engine itself (manifold vacuum leak, bad valve, head gasket, etc.).

The condition of the diaphragm inside the booster is also important. If cracked, ruptured or leaking, it won't hold vacuum and can't provide much power assist. Leaks in the master cylinder can allow brake fluid to be siphoned into the booster, accelerating the demise of the diaphragm. So if there's brake fluid inside the vacuum hose, it's a good indication the master cylinder is leaking and needs to be rebuilt or replaced. Wetness around the back of the master cylinder would be another clue to this kind of problem.

To check the vacuum booster, pump the brake pedal with the engine off until you've bled off all the vacuum from the unit. Then hold the pedal down and start the engine. You should feel the pedal depress slightly as engine vacuum enters the booster and pulls on the diaphragm. No change? Then check the vacuum hose connection and engine vacuum. If okay, the problem is in the booster and the booster needs to be replaced.

Vacuum boosters also have an external one-way check valve at the hose inlet that closes when the engine is either shut off or stalls. This traps vacuum inside the booster so it can still provide one or two power assisted stops until the engine is restarted. The valve also helps maintain vacuum when intake vacuum is low (when the engine is under load or is running at wide open throttle). You can check the valve by removing it and trying to blow through it from both sides. It should pass air from the rear but not from the front.
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Sunday, February 15th, 2009 AT 5:49 PM

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