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When press and hold the brake pedal and automatic transmission in 1 gear, the engine jumps up and down 700-950 rpm. Its just happen when car stopped and pressed  the brake pedal.
What is problem, please.?
The car is VW passat 2000 year 1.8
Thank you.  

i wonder if the power brake booster has failed?????

this is a side view of it...




engine off.. pump the brake pedal 4 or 5 times....    hold the pedal down lightly...    start the engine...  does the pedal drop away from your foot as the vacuum builds?????
IT SHOULD drop slightly


with your foot off the brake pedal..  the engine running...   rev the engine a few times... gently..   place your foot on the brake pedal...    hold it steady.. moderate pressure..     shut the engine off...   it should stay for a while... perhaps 20 or 30 seconds...

it should NOT push up instantly under your foot pressure...


why would the engine rev when you depressed the brake.. the brake is vacuum power assisted...   if you  have defective power brake booster.. the additional unexpected air leak/ vacuum leak.. will raise the engine idle speed.. the computer will try to compensate for it by changing the idle air control setting..  that usually takes a second or so..  so your engine could rev..


thats just one of the things...  and more than likely...

you can test depending on the type of vacuum hose is used.. by pinching the vacuum hose to the booster and seeing if the problem goes away..

many volkswagens use rigid vacuum hoses..  so you will have to carefully disconnect it at the booster and plug the opening in the end of the hose to test...