1999 Ford Explorer Sport , Erratic RPMs when idle, and RPMs rev when braking

Hello ,

Hi all, so glad to find this forum.

Here is a YouTube video showing my erratic RPMs while idling in park, then pressing and releasing the brakes and gas to show what happens each time;


I just bought a 99 Explorer Sport. The history I know is that the truck has 200k+ miles on it, with the engine replaced. Replacement engine has 90k on it.

Was told the fuel pump, and serpentine belt (I think, he said “main belt”) were replaced in the last few months.

The car was driving fine on the test drive and for the first few days of owning it. Then a clicking sound started form under the hood while driving.

I first noted the clicking while at a red light, and when I accelerated it stopped. Over the last week it has increased in frequency every day and now is almost constant.

This evening I was coming to a stop with my foot off the gas and when I hit the brakes the RPM went up, and car lurched forward before stopping gently. This kept happening so I drove home and took the above video. Video taken shows the RPMs jumping up when in park and going form idle to idle with brake depressed. when I rev the gas and let go, the RPMs fall, hang for a couple seconds, and then fall back to ~1000. The captions are very accurate, they show exactly when the brake or gas was pressed and when they were let go.

My obvious question would be WHAT IS CAUSING THIS?


finally, SHOULD I AVOID DRIVING IT? I need to drive it a few times through the next few days and I can’t really avoid it unless it will mean serious damage.

You have a vacuum leak. It’s specifically isolated to the brake booster, my guess is a bad check valve. This should be cheap and easy for any qualified shop to diagnose and repair.