Odd idling problem while pressing the brake pedal

[raven2345]

1991 Hatch
KA24DE
Automatic

Whenever I'm driving around and have to come to a stop, my idle likes to lope a little bit, then sometimes gets worse into more of a jumping idle. When pressing the brakes, it starts to fluctuate from 700-800. The longer i sit, it'll go from 600-1k. Periodically it will just jump from 300 to 1.1k, and eventually stall itself out. This all happens while I leave the transmission in the 'D' position. If i put it into neutral, it has a very small fluctuation, just moving around in the 700-800 range. The brake pedal feels normal when I'm on it.

Any help is greatly appreciated.

[sx moneypit]

The first thing i would do is clean the throttle body. Check your coolant level (low coolant will drive the coolant sensor nuts) you also may have a problem with your idle air control valve.

[P_856]

check the main vacuum line connected to the brake booster and the back of the plenum intake, also check for vacuum leaks the ones behind the valve cover goes bad quick and whens the last time the car had a tune up

[raven2345]

Throttle body is clean. This might be a stupid question, but why would the coolant sensor mess with my idle? Either way, coolant it full too. IACV will be looked at if I get the chance today.

Both of my PAIR valve lines are plugged because I removed the system. I'll take a look at the other lines. The car has had new plugs, wires, cap and rotor, all in the last six months or less. It's due for an oil change in the next week or so.

This problem started after I had a leaking caliper that I let go for WAY too long. My car (obviously) wouldn't brake too well so I always slowed down early and just put the car in neutral. Right around this time, if I'd put the car in drive, it would start loping at idle. After the caliper was repaired, this problem is here. Maybe the brake booster could be bled (if that's even doable?)... <<<I'll research that for now.

[sx moneypit]

The coolant temperature sensor is critical to your engine operation.It controls several things including fuel mixture and idle speed.If the coolant level is low or there is air in the cooling system the sensor cannot get a correct reading and can cause the car to run rich or have incorrect idle speed. For example , when you start the car first thing in the morning the coolant is cold and the sensor tells the ECU to raise the idle speed and make the fuel mixture richer for better cold engine operation.

[raven2345]

^^^That's legit. Hahah I'll check the sensor itself. Like I said though, the coolant is good to go.

[Klits562]

I had the same EXACT problem, and mine was caused by a vacuum leak on my intake where the maf, fits into the intake piping. The clamp that holds the two together hadn't been tightened right and the maf had come loose from the piping and caused a leak. I tighetened it up and never had the problem since.

I would definetly suggest its a vacuum leak, but it could be in a different spot for you.
the brake booster uses vacuum from your motor to increase the force you apply to the brake pedal, when you hit the brakes the booster will use vacuum from the engine, in cases of extreme vac leak your car will run like crap the whole time, but if your motors holding just enough vacuum to run normal, when you hit the brake your booster will take vacuum that your engine doesnt have.

[raven2345]

The problem showed itself today. I was putting in new motor mounts (the solid mount kit on enjuku is the s*** by the way ) and while jacking up the motor, it must of made the hole in he brake booster vacuum line much bigger. I tried to start the car and it kept stalling soni looked at the line and BAM! Huge freakin crack, and there was one in another spot. They're taped over for now til I got to autozone tmrw for some nice hose.

Thanks for everyone's help!

Go buy some solid mounts, they're AWESOME.