iWon

iTrader: (13)

I've been dealing with a high idle on my 383 build... the best I have been able to get it is around 950-1000rpm. Today I went out and I just noticed something.....


When the car was parked and idling, I pumped the brakes pretty quick/hard and the idle shot up about 50-100rpm. It would do it when I pressed the peddle down quick, and it would also go up if I took my foot off quickly. What does this mean?

lncboytre_d

Sounds like the brake power booster check valve.

jaycenk

iTrader: (1)

Your taking vacume away from the engine when you pump them repeatedly. IAC responds VIA the PCM see's the MAP sensor change. Also stock tacks are known to be off by 200 to even 300 rpm some times. Check what your ear hears and the pcm see's with a live data scanner.

iWon

iTrader: (13)

Yeah the data scanner was showing 950 rpm... so is the brake thing normal or should I look into it?

myltwon

iTrader: (1)

unless it's surging pretty bad I wouldn't worry about it. better to have the idle raised when vacuum is used on the brakes than for the idle to drop.

SS RRR

iTrader: (2)

You need to study up on the fundamentals and functionality of a brake booster and vacuum. No vacuum is being taken away from the engine when the brake pedal is depressed.

iWon

iTrader: (13)

It's not surging bad at all, but enough to notice it...

quik95lt1

iTrader: (7)

dude thats totally normal lol...........when u press the brake it uses the stored vacuum to assist you in pedal travel......when you release the pedal the diaphram fills back up with vacuum in the booster........now think about soemthing what is vacuum? lower pressure and the lack of air right? so when you are "filling" the booster with vacuum you are essiantly "removing" the air from the booster.....and that extra air goes into the intake manifold and causes the idle to bump up slightly for a second due to the extra air.......its like pulling off a vacuum line on the intake and then plugging it back in......the idle goes up...........totally normal any car with a vacuum booster will do that........

iWon

iTrader: (13)

Alright cool, thanks for the explanation. I've never noticed it in any other car, it's just way more prominent in this car because the cam is so choppy and the exhaust is dumped.

quik95lt1

iTrader: (7)

yea my Heads and cam 355 used to do it pretty noticeably.....i assume my 386 would too but i cant run power brakes with 5inhg of vacuum at idle hahaha

ElkySS

iTrader: (6)

my 396 does not do this. replace the booster check valve. they are cheaper that a gallon of premium.

SS RRR

iTrader: (2)

Yep. Neither does mine. Unless the engine is not generating any sort of vacuum to begin with, the booster is constantly under vacuum and there is no way in the world pushing on the brake pedal should influence vacuum to the point of altering engine idle. I would venture to say the check valve or the diaphragm in the booster is leaking.

iWon

iTrader: (13)

How do I check the check valve/diaphragm?

Badazz 97 TA

iTrader: (18)

my car does it, both my parents cars do the same thing when you pump the brakes repeatedly in park.

jaycenk

iTrader: (1)

He said Pumping the brakes. That is fundamentals because the vacume stored has been used and now needs to be stored up again.

jaycenk

iTrader: (1)

Easy way to check it is to run the car and then shut it off. Now pull the valve out of the booster. If you hear air rush in then it's fine. It should hold neg pressure for a while so you can run it and pull the valve out like an hour later and it should still be neg charged. or just hit the brakes and listen .

quik95lt1

iTrader: (7)

i still think whats hes experienceing is normal but whatever lol

SS RRR

iTrader: (2)

I reckon it's possible for an engine producing less-than-normal vacuum at idle.

Oh, and OP, you better get a custom cam instead of that 306 because "it's good for nothing more than peak numbers..."

Kidding!

iWon

iTrader: (13)

I like my 306 so far Gonna ride her till the wheels fall off!