Hi fellas, so I've had this problem for a few months and no shops seem to know how to fix this and its driving me nuts.

Idle drops in neutral when brakes are pressed, car doesn't die.

A/C on brakes are pressed at a stop engine dies.

BAC is working properly, TPS is set, Idle is adjusted to a decent 850 or so rpm.

Anybody have any ideas? Power issue? Some sort of a switch? ECU?

Any help is much appreciated.

Thanks in advance.

BTW I have a 90 GTU n/a with a stock set up.

 

Is the check valve on your brake booster vacuum line working properly?

 

Yeah, that was one of the things I replaced, but it didn't fix the problem unfortunately.

 

Another bit of info, I manually pressed the brake pedal down and played with the brake light switch a little, and it seems like when I let go of the switch the idle drops by a lot, that tells me that its somehow electrical or load related.

 

The BAC may be working, but could be dirty; try cleaning it. Also, a good ground cleaning never hurt anyone.

 

That sounds like a pretty good idea, I'll try cleaning the BAC and grounds and post up the results. Thanks for the help so far guys!!

 

Does pressing/depressing your clutch change anything (while in neutral during idle)?

Check your neutral switch on the transmission?

Take it and have the Alternator tested (free at most local parts stores).

 

Thats a good question, no, when I press the clutch nothing changes.....Clutch switch is probably bad. I've heard the spring inside usually breaks.

The neutral switch was my next guess......Does that really do anything like raise the idle and so on? Because my idle drops instead of raise up like it does when I shift into gear.

Checking the alternater is a good idea as well, it can't hurt to get it tested.

Thanks for the tips!!

 

Does your car have cruise control?

 

No, no cruise control.

 

I might be wrong but clutch switches are found only on those equipped with cruise control. Again, I might be wrong.

Also, your problem might be related to a voltage drop when you hit the brakes fully at a full stop. I believe the BAC is supposed to sense a voltage drop and help to assist the idle when this happens but I might be wrong on this. My car has the same problem but it is limited to the first mile of driving upon cold start up and then it disappears completely thereafter. Always found that strange.

Additionally, there are two hoses connected to the brake booster, one before and one after.

 

My car seems to have both switches attached to the clutch pedal, one in front one in back.

Both hoses have been replaced so I don't think they were the problem.

As far as the voltage drop you might be right because I feel like the idle only drops or shuts off when there is excessive load on the engine, AC, Brake lights and so on. To fix this temporarily I raised the idle from 850 to about 1000. The idle still drops down but not down far enough to shut off. I'de like to fix it the right way instead of putting a temporary band aid on it like this.

I think for now I'm going to go through this checklist.
1)Clean BAC.
2)Clean Grounds.
3)Check voltage, battery and alternator.
4)Check neutral and clutch switch.

Thanks for all the tips guys!! I'll post up results when I perform all these checks!!

 

Might not amount to much but it is an easy and quick test. Perhaps you could use a jumper wire on your brake light switch which will effectively turn your brake lights on and then with the car running put the car into neutral and apply the brakes fully and see if you get the close to stall scenario. You would have to adjust your idle down to normal range though to do this.

 

Thats exactly what I did only without the jumper wire. What I did was press the brake switch with my hand, then depressed the pedal all the way down. Nothing happenned by pressing the pedal, but I would get that close to stall scenario every time I pressed the switch......What do you think?

 

I guess if you disconnected the brake light switch altogether and then did the neutral thing followed by brake fully depressed and the car didn't want to bog down then you would think it is a voltage issue. If this were the case then perhaps the alternator isn't providing enough juice or maybe even a ground wire is to blame. On my car it can't be a sticking BAC because I've disconnected its plug and the car behaved just the same. From what I know, the ECU does not know directly that the brake pedal is being fully depressed but does know in an indirect manner from the drop in voltage. Also, my car will not cause the problem to occur in that first mile or so if I am able to crawl when at a stop light. That is when the brake is somewhat depressed but not fully depressed. I've never checked to see how far the brake light switch has to be depressed for it to activate the lights and create a drop in voltage. This would be fairly easy to do by checking the voltage on the proper wire as the switch is slowly depressed.


EDIT: How far did the switch have to be depressed to cause the idle drop? Part of the way or fully depressed?

 

As soon as the brake lights come on, I notice the drop, I'm not sure if its part way or all the way though.

 

Alright fellas I fixed the problem. It seems like a combination of cleaning the BAC, grounds, TB and raising the idle just a little did the trick, no more idle drop or engine stall.

Thanks for all the help and ideas!!!