Rough idle when brake and belts

#1

I have a 96 Mazda Protege DX. 1.5L engine, now with 161,000 miles. 5 speed manual transmission. I had it for 7 years. Originally bought it for $1900 with 121,000 miles.

This has been a problem for about 5 years. When I am at a red light or idling and I step on the brake, my idle gets rough and my engine feels like it is about to turn off. The idle does not bounce up and down. It just goes down to 400-500rpm from the normal 800-950rpm range.

Again, this only happens when I step on the brake while the car is not moving. It does not seem to happen when I coast in neutral and hit the brake to slow down. I raised the idle a bit, but it still does it. I could not find any vacuum leak around the brake booster and I know the brake booster works because I pumped my brakes while the car was off, and when I turn the car on, the brakes soften up.

Also, my brakes feel weak. I usually hit the brakes nearly to the floor board to stop the car. New pads, rotors are good. Brake fluid is at right level. Mechanics say nothing is wrong with my brakes.

Secondly, my belts screech at every start up. Lasts anywhere from 3-20 seconds. This has been happening for the entire 7 years I have owned the car. Mechanics replaced the serpentine belts twice in that time. Replaced a pulley and A/C compressor. I tried using WD40 but it doesn’t effect it. Mechanics tell me that the belt tension is correct and they do not know why it still screeches.

#2

Don’t spray WD-40 on the belts or pulleys. It will shorten the life of the belt and cause it to slip. The rough idle and brake problem are likely connected. Set the parking brake, start the engine and let it idle, get your ear as near as possible to the brake pedal and press the pedal down. You might hear a vacuum leak below the pedal indicating that the brake booster is failing.

#3

That brake thing could be related to the engine not running right. Maybe you have a vacuum leak in the area or the mastery cylinder. Check the hoses, making sure they aren’t cracked.

#4

Try as a diagnostic step disconnecting and plugging the vacuum line to the brake booster and see if the idle problem disappears. Be aware when you try this that your brakes may require a lot more effort to stop and may feel hard. Don;t go driving around a playground in a residential neighborhood to do this test.

Post back with the results. The test will tell you whether or not you have a vacuum leak in your brake booster.

#5

Great idea, MB. That’s a very good way of telling what’s wrong.

…Also don’t drive in the back of any 1977 Ford Pinto.
They tend to turn into a flaming inferno when hit in the back.

#6

Thanks for the compliment and the chuckle. Ah, sweet memories!

#7

I bet the guys are on the right track about the brake booster.

#8

Please were you able to resolve this problem? I am having the same issue on my infiniti fx35. It jerks like crazy when i step on the brake but smoothens up when i step on the gas. Please did u resolve this?

#9

Nelson, this thread is about a completely different vehicle than yours and the person has been gone for 6 years. Please start your own thread with info such as vehicle year, mileage , engine and what you have tried to solve this problem.