strange idle problem related to brake lights?

[toymdngn]

very wierd problem here...

a friend at work has a clean 85(i think) 4 runner, 22re, 5 speed, 4x4. just recently, whenever she would press on the brake pedal, the engine idle would immediatly drop down, the go back up, kinda like a bouncing idle, only unlike mr2s, instead of the idle raising higher than normal then dropping, this 4runner is dropping down to ~350rpms, then right back up to the normal idle. as soon as she lets off the brake, it idles completely normal.

here is the strange part, someone suggested that she pull the brakelight fuse... all the sudden, the bouncy idle was gone, stayed normal when she pressed on the brake pedal. she put the fuse back in, and there it goes, dipping down like before.

 

[Spider5]

Hard to diagnose but try checking for a problem with the charging system. I would have the alternator check. Maybe when the demand of the brake light, maybe with a short, is on the bad alternator it taxes the engine. Had this happen on my 1966 Mustang many years ago, combination of short in brake light switch and worn out alternator.

 

[mikes4runner]

I agree

 

[toymdngn]

Hard to diagnose but try checking for a problem with the charging system. I would have the alternator check. Maybe when the demand of the brake light, maybe with a short, is on the bad alternator it taxes the engine. Had this happen on my 1966 Mustang many years ago, combination of short in brake light switch and worn out alternator.

i thought of that too, but dont you think it would do it when the headlights/running lights were turned on?

thanks for the input.

 

[fourwd1]

You're on the right track w/the brake light fuse, but not quite.
It's not an alternator issue.

Does all this occur when she is at idle (sitting still, say), or when she is driving/accelerating?

 

[toymdngn]

You're on the right track w/the brake light fuse, but not quite.
It's not an alternator issue.

Does all this occur when she is at idle (sitting still, say), or when she is driving/accelerating?

it only does it once it is about 1/3 the way warmed up. and as far as i can tell, just when she is sitting still. she sat there and let it warm up last night, and as soon as it warmed up enough, she pressed on the brake pedal, and it started dropping the idle real low, then back up to normal, then back down low, again and again until she let off the brake.

another thing i noticed, when she revs up the engine, it slowly revs back down (normal for a heavy flywheeled 22re), but if she revs it up to ~4500rpm, then step on the brake pedal, it drops down to idle in less than half the time it would if she just revved it up and didnt touch the brakes...

 

[Spider5]

i thought of that too, but dont you think it would do it when the headlights/running lights were turned on?

thanks for the input.

Not of there is also a short in the brake light system, like a bare wiring grounding out somewhere or a bad switch taxing a worn alternator more then normal by requesting a surge of power, reducing the idle till it is caught up. Pull each brake light bulb one at a time to see if a short is in one of those segments of wiring. Educated guess because these things are difficult to diagnosis from behind a computer.

 

[Spider5]

You're on the right track w/the brake light fuse, but not quite.
It's not an alternator issue.

Does all this occur when she is at idle (sitting still, say), or when she is driving/accelerating?

It is an idle issue.

 

[fourwd1]

You're on the right track w/the brake light fuse, but not quite.
It's not an alternator issue.

Does all this occur when she is at idle (sitting still, say), or when she is driving/accelerating?

OK, those are the answers I was expecting.

Chances are the idle rpm is just set too high.

There's a injector cut-off circuit that kicks in when you hit the brakes and the rpm's are above idle (you don't need gas when you're braking, right?). So if the idle is too high, the ECM thinks you're accelerating, so the cut-off kicks in when you hit the brakes, even though you're sitting still.
Doesn't do it when it's cold because ECM expects idle to be high for warm up.

Hook up a tach (don't count on dash tach if so equipped) and check idle against spec on hood decal. You may be able to adjust it, or find out what other issue is causing the high idle (some bad sensor or problem).

 

[fourwd1]

It is an idle issue.

You just stated in your post above it was an alternator/charging issue.

 

[toymdngn]

i see. well, one thing i noticed once she got it warmed up, was that the idle didnt drop down much. i will see if i can adjust that down a bit today while at work, see if that helps.

you guys have all been great help, and i will update what is going on when i come home for lunch..

 

[toymdngn]

so its been awhile since i have actually looked at a 22re, isnt there an idle adjustment screw on the throttle body like on my mr2?

 

[Spider5]

You just stated in your post above it was an alternator/charging issue.

You asked if it occured during "driving/accelerating" the effect was low idle, see orginal question. The cause might be electrical/alternator.

 

[fourwd1]

so its been awhile since i have actually looked at a 22re, isnt there an idle adjustment screw on the throttle body like on my mr2?

It should, but I can't be more specific, I have a 22R.

 

[toymdngn]

FIXED! the idle screw on the throttle body was backed all the way out. gave it about 20 turns until the idle started to settle, once i got it down to about where my ears told me it should be, i had her get in a press the brakes, no more bouncing idle.


thanks guys, i would've never guessed it had anything to do with the idle being high.

 

[fourwd1]

You'e welcome

 

[Quakefire]

Power brakes, could be a vac leak