bad brake booster?

[lethe]

i have a vacume leak somewhere and i think i narrowed it down to the booster itself. i checked the online vacume line going to it and there were no leaks, so that lead me to believe its the booster itself. here's why: this only happens sometimes and when the truck is warmed up. i will come to a stop at a light and my idle will bounce up and down from about 700-1200 rpm, if i let off my brakes it stops. if i so much as touch the pedal the idle will jump around again. sometimes it will go a few days without doing it. am i right to believe its the booster? or is there something else im not thinking of?

 

[84Toyota4x4]

Maybe this will help?

~T.J.

 

[lethe]

Maybe this will help?

~T.J.

where did you find that? ill have to try that stuff either tomorrow night or sunday.

 

[84Toyota4x4]

Factory service manual, hope it helps some. Thats all it has about checking it. I was thinking though, that if you have a leak somewhere in the system, just actuating the brakes could cause a fluctuation in the overall system which could cause the symptoms you're describing. It may have nothing to do with the booster other than the booster uses part of the vacuum system, and as a result, could cause problems if there is a problem elsewhere when it "draws" the vacuum from the system for it to work.

~T.J.

 

[lethe]

do you have a copy of a FSM? i would like to get my hands on one. i know some people have it in pdf form.

 

[ToyZuki]

If I remember right from other posts your in a toyota. Assuming its a toy and its fuel injected I know the problem. The idle is set to high. When the brakes are applied the computer shuts the injecters down if the idle is above 1000 rpms (for decending hills ect. no need to burn fuel for no reason), When the idle becomes to low the injectors are fired back up shooting it back up to the idle setting (which is to high) thus creating a cycle. My truck does the same thing and I researced it with some reliable sources and it makes sence. I still have not figured out how to set the idle (fraid to turn the wrong one) but as long as it only does it when the brake light switch is engauged its electically contoller idle.

Hope this helps. If you figure out which one adjusts the idle with out screwing with air/fuel mix take a pic and post it up for me.

 

[lethe]

If I remember right from other posts your in a toyota. Assuming its a toy and its fuel injected I know the problem. The idle is set to high. When the brakes are applied the computer shuts the injecters down if the idle is above 1000 rpms (for decending hills ect. no need to burn fuel for no reason), When the idle becomes to low the injectors are fired back up shooting it back up to the idle setting (which is to high) thus creating a cycle. My truck does the same thing and I researced it with some reliable sources and it makes sence. I still have not figured out how to set the idle (fraid to turn the wrong one) but as long as it only does it when the brake light switch is engauged its electically contoller idle.

Hope this helps. If you figure out which one adjusts the idle with out screwing with air/fuel mix take a pic and post it up for me.

the idle screw in on top of the throttle body. its too dark for me to take a picture. it started doing this after i swapped a newer engine in it. when i first started it the idle was at 2k with the idle screw turned all the way in, so i adjusted the dashpot to alllow it close more. when i bought my rig back the engine was so fawked i dont know if it was doing this before or not.

 

[84Toyota4x4]

I think I remember reading about that circuit. I can see how that makes sense. By the way, I have the 85 FSM, but its a rather large file. If you look around you can still find links to DL it.

~T.J.