Idle Problems? Check your BAC!
09-20-02, 01:33 PM
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Idle Problems? Check your BAC!
Just thought I would relate my experiences on the chance it may help someone else.
1986 GXL - Problem: I could not get the 'hard' idle (initial set jumpered) up to 750 rpm. At best I was getting 680 with the air adjust screw backed all the way out. Car would sometimes stall with the A/C on when I rolled to up to a red light. I spent alot of time checking vac hoses and testing sensors to find the cause, to no avail. I did find, though, while checking the inputs to the ECU associated with idle, that the neutral switch was not grounding as it should. Took apart the connector in the harness adjacent the transmission to find one of the two pins was bent, and not mating into the receptacle. Fixed that, and I'm now satisfied that all the sensors are O.K., but the problem all along was the BAC valve. When I first tested it, as per the FSM, it clicked with 12 volts, and measured at 12.4 ohms resistance. I thought it was O.K., the spec is 10.7 - 12.3 ohms. I later decided to clean it with carb cleaner, but still no improvement. Finally I happened to check it when the motor was hot - the resistance had now increased to 13.9 ohms. Light bulb went off, and I realised that it was the problem. A quick trip to the jukyard, and $10 later I had a BAC measuring 11.7 ohms cold, and a solid idle. Guess those little ohms should be taken seriously.
As best I've been able to determine, when you jumper the initial set connector, the ECU sends a fixed duty cycle signal (you can measure as voltage, mine was 3.15v) to the BAC. Obviously then, at a fixed 'voltage', if the resistance of the valve increases, correspondingly less air will be passed as the valve slows down it's cycling. When you remove the jumper, the ECU will then vary the signal to achieve a given rpm, based on other inputs (such as A/C on, gear position, temp). For instance, after I fixed the neutral switch connector, I found that at an idle of 750, when I put the car in gear, idle will drop to 650 rpm.
Anyway, long winded post, but I also want to say a thanks to Hailers, as my forum searches turned up lots of good info from him on checking ECU pins for signals and stuff.
And if you've read this far, here's a cheap shop helper. Cut a 4 foot piece of hockey stick or dowel. Now when you raise the hood to work, don't use the prop rod, put one end of the stick down next to the battery box, and the other end in one of the puched out holes on the underside of the hood. You'll have a good deal more headroom when messing around the throttle body, etc.
Curtis
'86 GXL autox'r
1986 GXL - Problem: I could not get the 'hard' idle (initial set jumpered) up to 750 rpm. At best I was getting 680 with the air adjust screw backed all the way out. Car would sometimes stall with the A/C on when I rolled to up to a red light. I spent alot of time checking vac hoses and testing sensors to find the cause, to no avail. I did find, though, while checking the inputs to the ECU associated with idle, that the neutral switch was not grounding as it should. Took apart the connector in the harness adjacent the transmission to find one of the two pins was bent, and not mating into the receptacle. Fixed that, and I'm now satisfied that all the sensors are O.K., but the problem all along was the BAC valve. When I first tested it, as per the FSM, it clicked with 12 volts, and measured at 12.4 ohms resistance. I thought it was O.K., the spec is 10.7 - 12.3 ohms. I later decided to clean it with carb cleaner, but still no improvement. Finally I happened to check it when the motor was hot - the resistance had now increased to 13.9 ohms. Light bulb went off, and I realised that it was the problem. A quick trip to the jukyard, and $10 later I had a BAC measuring 11.7 ohms cold, and a solid idle. Guess those little ohms should be taken seriously.
As best I've been able to determine, when you jumper the initial set connector, the ECU sends a fixed duty cycle signal (you can measure as voltage, mine was 3.15v) to the BAC. Obviously then, at a fixed 'voltage', if the resistance of the valve increases, correspondingly less air will be passed as the valve slows down it's cycling. When you remove the jumper, the ECU will then vary the signal to achieve a given rpm, based on other inputs (such as A/C on, gear position, temp). For instance, after I fixed the neutral switch connector, I found that at an idle of 750, when I put the car in gear, idle will drop to 650 rpm.
Anyway, long winded post, but I also want to say a thanks to Hailers, as my forum searches turned up lots of good info from him on checking ECU pins for signals and stuff.
And if you've read this far, here's a cheap shop helper. Cut a 4 foot piece of hockey stick or dowel. Now when you raise the hood to work, don't use the prop rod, put one end of the stick down next to the battery box, and the other end in one of the puched out holes on the underside of the hood. You'll have a good deal more headroom when messing around the throttle body, etc.
Curtis
'86 GXL autox'r
09-20-02, 01:48 PM
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Thanks for your post, slvr7
Glad you solved your problem with some detective work.
so, you had a bad BAC valve after all. That's interesting.
Hugues -
Glad you solved your problem with some detective work.
so, you had a bad BAC valve after all. That's interesting.
Hugues -
Last edited by hugues; 09-20-02 at 01:53 PM.
09-20-02, 02:50 PM
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ya those little ohms can realy do some damage. i had a problem forever tested the coils it was .2 out of spec so i figured i've replaced everything else on the car why not so quick trip to the parts yard and bada bing perfectly running car!
09-20-02, 07:16 PM
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Does anybody have any pics or can tell me where the BAC valve is?? I have a shitty idle where it revs up then drops down and dies, but once its warmed up it doesnt do it at all. Havent had this problem before I ported my intake manifold.
10-07-02, 05:41 PM
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Im so gald i found this post. I had the same problem after i ported my intake. My car idled fine before i ported it but afterwards it wouldn't and I couldn't figure it out, thought it was a vacumn leak...after about a month of running with tension on the accelerator cable so it wouldn't stall in neutral I found this post, checked my BAC valve by disconnecting the plug to it and restarting th car, took tension off the cable and it idled just like before i decided to mess with it. I replaced it with one from the tow yard and now life is ok. Crazy rotary engines...
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trickster
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