I don't know if this is a stupid question or not, but I've never heard of this... when i press the brake in my '88 305 TBI camaro, my rpm increases about approx. 200-300 rpm for about 2-3 seconds. I was just wondering if anyone knew what would cause this?? The only aftermarket mods i have are edelbrock tes headers, a flowmaster american thunder exhaust, and an edelbrock open element air filter.

 

I've seen on the early 90's Corvettes some have faulty power brake boosters and it can cause the RPM's to drop and flutter a bit but I havent heard of the RPM's increasing from holding the brake.

You're sure you have your foot off the gas pedal?

Have you ever tested it to see how high the RPM's will actually go when sitting with your foot on the brake?

 

Yes, I could totally understand it if my rpm was dropping, that would make some sort of sense, but it's increasing, and it only increases for about approx 2-3 seconds before dropping back to normal even if I hold the brake, but I can continuously press and release the brake and hold the rpm 200-300 higher. and yes, im sure my foot is off the gas pedal

 

Is it auto or manual?

 

My guess would be something like your tranny shifting into a lower gear when you are pressing the brake for some reason. Are you having any transmission problems?

 

It is an automatic, and my tranny seems fine, absolutely no complaints with it while driving. Keep in mind this is an idle problem, as in idling in park. I actually haven't tried this in drive, but I'll give it a whirl tonight and see what happens.

 

Do you have a vacuum leak in your booster? My 94 350TBI truck had a gasket go bad under the throttle body, and that resulted in an increased idle speed. Methinks the booster in your ride may have a small leak whilst mashing the pedal, causing the idle speed to increase. Try unhooking and plugging the vacuum line from manifold to booster, then drive it and apply the brakes, see if your idle speed increases. Caveat, do it where you have plenty of room to stop, the pedal will be VERY hard to mash to the floor.

Good hunting,
Tim sends