So yeah, if I'm sitting at a stop and I pump my brakes, my idle will almost instantly rise several hundred RPM's--almost like I'm "pumping" my idle up... As soon as I stop pumping my idle settles back down to normal.

I'm guessing my brake-booster is messed up, since this is the only related component that could have any possible effect on manifold pressure (or whatever).

Anyone ever experienced this, or have any guesses?

 

Pumping the brakes will change the negitive pressure in the intake manifold which could cause your idle to raise or lower. Do you still have the BACV? If so that could raise your idle so your engine doesn't bog down as much.

You can take a vacuum tester to the booster to make sure its not leaking.

 

How is the Turbo running with the N/A Rotors? Strong? Any issues?

 

if you pump the brakes and the idle is affected is very normal it does it in any car with a vacuum booster

 

if you hold the brakes and car dies usually indicates a bad booster because of the trmendouse vacuum leak

 

Oh ok, right on guys. I guess it's normal then. I had no idea.

 

My TII runs great with the N/A rotors. It's got good response/low-end and nice top-end as well. However, I personally didn't build the motor, and I actually would have *preferred* the 8.5:1 turbo rotors.

I don't believe the benefit is worth the risk--but the car is pretty fast.

 

I think as long as extra cooling measures where taken and the car properly tuned it would be ok, but I'm just going off of piston experience.

 

BAC valve is probably a little worn. When the engine load increases it tries to compensate and if it can't move quickly enough you get an idle fluctuation.

 

Surging idle upon braking is in no way shape or form "Normal".... I can't even believe someone would say that is a "normal" symptom. The reason being, as some have said, would be that you probably have a vacuum leak somewhere in connection with your brake booster. Especially because at idle you are in a high state of manifold vacuum. That is why I am guessing your booster has a small leak and when you pump it you are introducing un-metered air into your system, causing idle surge...etc...

hope it helps,

Dave.

 

Could be a few things. Like Aaron said could be something not able to compensate for the change in vacuum fast enough, or it could actually be a vacuum leak or booster diaphragm messed up and leaking. Brakes problems can be tough to pinpoint sometimes

 

every car on the planet will do this.

try it, seriously.

pump the brakes rapidly several times in anything (that isn't diesel)... the idle will rise several hundred RPM as you're doing it, then DROP like a stone afterwards.

Some cars you can even get to stall on the way down.

 

there was no mention of surging idle when pumping the brakes, he mentioned that it raised the idle when they are pumped.

the BAC does not really compensate for booster load, the idle difference when pumping the brakes is in fact normal, try it for yourself...

you guys need to quit giving bad advice, i sure wouldn't want to be looking for a ghost problem.

 

+1

My last 3 mazda pickups did this as well as my dads and my RX-7.

 

i guess it depends on your definition of surging

 

My car does it too. It's not really a big issue just some compensation so the idle doesn't go too low and cut off.

 

Not the booster specifically, but any change in RPM past what the idle is supposed to be "set" to...

No harm in cleaning the BAC either way.

 

the BAC isn't very fast reacting in these cars, it probably would compensate if you kept pumping the pedal for more than a few seconds but the ECU really only tries to compensate for large loads like P/S and A/C and large elec loads and just sets base idle.

 

"you guys need to quit giving bad advice, i sure wouldn't want to be looking for a ghost problem. " Karak


I obviously didn't read that original post correctly. I thought he was saying he had a definate idle surge and that the previous response was that an idle "surge" was a normal condition. In fact, the drop in idle etc, is normal, as long as it is not a surge, sorry for the flamage, I just read it wrong.

Dave.

 

no problem, understandable for people to defend themselves.