When i brake my RPM's jump up?

It's likely a vacuum leak in your brake booster diaphram. You can often hear them hiss a little, yours sounds bad.
 
I just tried punching the gas and having a buddie lisen... no luck no hiss.. It usually happens when its cold.

I tried just sitting in the truck with it running and steping on the brake it the RPM jumps a few hundred RPM's. I checked the larger brake booster hose and the plastic (L) shaped connection and they both seem in good shape.
 
canadianwheeler said:
I just tried punching the gas and having a buddie lisen... no luck no hiss.. It usually happens when its cold.

I tried just sitting in the truck with it running and steping on the brake it the RPM jumps a few hundred RPM's. I checked the larger brake booster hose and the plastic (L) shaped connection and they both seem in good shape.
Click to expand...
You can increase and decrease throttle using the throttle cable in the engine bay. This will allow you for better control over the RPM's while listening for the noise.
 
Kejtar said:
You can increase and decrease throttle using the throttle cable in the engine bay. This will allow you for better control over the RPM's while listening for the noise.
Click to expand...


Ya tried that... It seems to happen only when the truck has been sitting.
 
Try pulling the vacuum line off the booster and plugging. Then see if hitting the brakes causes your idle to increase.
 
canadianwheeler said:
If it does idle higher or lower what would that mean ?
Click to expand...

If the high idle goes away with the brake booster vacuum hose pugged it's likely the booster is bad. The plastic L in the end of the vacuum line is a check valve. The booster mostly hisses when the brakes are applied and can be heard best from inside, with your ear close to the firewall end of the linkage.
If plugging the vacuum line, doesn't lower the idle when you hit the brakes, I'd look at the pedal/linkage to see if something wasn't hung up in there. Maybe wrapped around the pedal linkage and/or the gas pedal. I've rerouted wiring in that area before or tied it up out of the way with cable ties.
If you have a cruise control, it's possible the vacuum release switch (by the brakes) is allowing vacuum to escape or is venting a broken vacuum line to the cruise control. But unlikely, the vacuum lines to the switch are fairly small, you need a good bit of vacuum to have it idle up to 2500, like a quarter inch hole or maybe larger.
It takes a fairly serious leak to make it idle up to 2500, unhooking the vacuum canister hose from the intake and leaving it open will do it. But all the time, not just when you use the brakes.
 
canadianwheeler said:
When i hit my brakes on my xj my rpm's jump to 2500 then settle back down.
Click to expand...
Does it go up when you press the brake, or when you release it ?
 
canadianwheeler said:
When i press it
Click to expand...

Actually i tried it today and it does jump maybe 100 - 200 rpm when i RELEASE the pedal sitting at dead stop (xj in park). It didnt do it today yet but when i am cruising at a decent speed and press down on the brake to stop for a read light or stop sign thats when it jumps much higher. Usually when i press it a little harder then usual.
 
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