joel91

Ok...here is what is happening. My 91n/a( 30k on new motor) has a perfect 750 idle without any electrical loads on it, but as soon as a load is placed on the engine the idle goes way down and starts to hunt....i've tried everything ,tps, three bac valves that all check good according to manual, no vacuum leaks, new alt, new bat, tried every position of the air screw on top of TB with check connector grounded, installed new grounds, but nothing will keep the idle right. I know about the bac resistor in the ECU. Does anyone know, that has replaced the resistor, how to put it in; and if u can buy it at your local radio shack?? This is the last thing that i can think of that could be causing this. Can anyone help me out??

NZConvertible

The info you need is in this article:

http://www.teamfc3s.org/info/articles/idle.html

WAYNE88N/A

Good read- except for the part about the air bypass valve doing duty as the 3K warmup- which is 95%+ controlled by the BAC itself on my N/A...

Weird...

joel91

man that sounds complicated to replace....where could i find a good remanufactured ecu at???

HAILERS

Do you still have those other bac valves? If you do, then try this. Remove the plug from the bac on the engine and put it on one of the spare ones. Turn the key to ON. Now look at the spare bac. Is it vibrating? Touch the piston in the hole and feel for a vibration.

You might even idle the car with the spare on the end of the bac electrical plug and see if it vibrates. I figure if the thing vibrates, then the transistor in the ECU isn't busted and the problem is another thing.

If you can't feel any vibration, then make sure both contacts in the electrical plug are not pushed back and that both wires have continuity back to the ECU.

I used that site to fix a bad ECU. The fix works. Part was available at the local RadioShack for a buck fifty. That site was for a series four. I'm not sure about the series five and part number for a series five and location in the series five ECU.

HAILERS

Or there's another way to tell if the ECU is good or not. And you have a spare bac. Connect the spare to the bac harness. Then reach under the car and pull the small wire off the starter. Then place the bac where you can see the diaphram from inside the car. Turn the key to START. The bac should go full open when you do that when the key is held to START. As you know that is a function of the bac upon startup.....to feed large amounts of air during startup. That's the only time it will go full open and STAY full open. If it does not respond......check the wiring then the ECU.

joel91

ok...ill check that too, but i ohmed the two wires that go to the ecu from the bac and i got 7.5 ohms...so does that mean that the circuit in the ecu is ok?? I did what u said to i got one of the spare bacs and hooked it up to the conector and turned on the key, YES it is vibrating...but still doesn't the ecu control the duty cycle??? or if it vibrates at all does that mean that the whole bac system is ok???

joel91

oh...i used a 10k ohm resistor accross the bac terminals that go to the ecu to get the 7.5

hugues

check the DC voltage the BAC sees ...
The more electrical load, the lower the voltage ...

That way you can tell me if the ECU is reacting to the loads

Hugues-

HAILERS

If it is vibrating, then the ECU is good and the bac is good. You won't see much of an opening as it vibrates with the car at idle but if it vibrates it is working.

To properly ohm out the wiring you need to disconnect the plug AT the ECU and leave the plug on the bac. Then put your meter on the two wires at the ECU plug and read the ohms. This should not be required since your getting a vibration from the bac's piston.

I'd stop trying to ohm the wiring from the bac plug thru the ECU. NOT a good idea. Gonna smoke that transitor if you keep that up.

You might ohm the bac itself. Just a meter lead on one pin of the bac and the other lead to the other pin. I forget what it should ohm to. If it reads off the figures just a touch don't worry.

I don't think there's a thing wrong with you bac or its circuit.

Yes the ECU determines the duty cycle. Again I don't think theres a thing wrong with your ECU or the bac. While your there, you might spray some carb cleaner into the spare bac and let the vibrating help clean it a bit. Dump it and do it again.

Don't force a bacs piston or damage the piston with sharp objects like screwdrivers.

LAST: Here's the deal. You car when fully hot, should idle at approx 750 with the bac disconnected altogether. IF it does not, then you need to set your idle speed/set the timing/set the tps/etc until it does idle at approx 750 without the bac being plugged in.

I read your tps thread. It seems the tps has an open in it if your ohming it out and moving the plunger and the meter goes OL then as you keep moving the pluger it shows ohms again. I said it seems that way. I ain't thar to see it for myself.

Try this. Get the car fully HOT. Then at IDLE, put your meters neg lead on a proven good ground and the positive lead in the back of the green/red wire at the tps connector WITH THE CONNECTOR FULLY CONNECTED UP. The meter should read apporx 1vdc. If not, then turn the tps screw til it read approx 1vdc.

Now turn the engine off....then turn the key back to ON. Watch the meter as you slowly, slowly open the throttle so the plunger goes full open and then full close. The voltage should rise and fall and not go OL on you while doing that. Sorry, never done that myself, but it should do that. DONE.

joel91

ok..ill try that hailers and let you know....Ill try setting the tps to what you described and before that i'll set idle at exactly 750....one question i've tried every position on the idle set screw already so i doubt if it will change anything...but the tach inside where is exaclty 750 without using a tach on the coils...or do you think it would be more precise to just go and buy one...???

joel91

and yes i've grounded the test connector before turning the idle screw with no load on the car if your wondering, man im starting to hate my car....lol last question im trying to think from every angle....i've sprayed and sprayed my engine to no avail of vacuum leaks....if i did i wouldn't have a steady idle anytime right?? well ill try these couple of things first....man im starting to hate my car...lol

HAILERS

The tach in the car will work just fine. You may not be able to get exactly 750, but you should be able to get close.

No vacuum leaks. Ok. That's good.

The tps really should not effect the idle unless it's really far off. Personally I set mine with a hot engine and at idle. Sort of like what is written in the fsm. Your tps might be bad if when you were checking it out with it disconnected from the harness and the ohms reading was going OL while you were depressing the plunger and then showed a resistance after you pushed it a touch more.

Have you ever turned your variable resistor while trying to get the idle speed up? If you have not then just note where the screw is before turning it so you can return to that place later if needed. It only turns about 180*. R is rich, L is lean. Only for idle fuel adjustments. Effects nothing else.

If worse comes to worse.......there's on the front of the throttle body is a stop screw for the secondary plates. It could be turned so that it keeps the secondary throttle plate open a touch to raise the idle.

Wish I had not written that. Actually you need a fsm and the fuel section out of it. There's a fast idle adjustment screw that can be *played* with per fsm. But on a series five it's a touch harder to get at vs a truboii. Its on the aft side of the throttle body and nearly impossible to work with since the screw head is facing downwards. Of course the throttle body could be taken off to screw with it. Sorry, I hardly ever touch my 86 n/a anymore. Out of practice.

joel91

ok....here what i did i reset the tps while the engine was nice and hot i set the tps the last time using two leds but i hooked a voltmeter up this time and it read .92volts so i turned the adjustment screw till i got 1.00 volts...the idle seems more stable now but i still get a little drop to a 500rpm steady idle under load, but no more hunting idle when placed under a load....i have approx. 4 complete turns on the idle screw from completly screwed in. Maybe i need to turn idle screw a little more...ill try that and see what happens....let you know soon.

joel91

does my car even have that variable resistor??? i've looked for it before but couldn't find it isn't it supposed to be on the passenger side strut tower???

NZConvertible

Only S4's have the variable resistor.

joel91

PLEASE READ!!!!! Man i hope this has to do with something,well lets just say my idle is still dropping but what i did notice is i tapped on the pressure sensor on the right strut tower and the idle moved every time i touch it??? I unplugged the stupid thing of course i got a check engine light but does this input from the pressure sensor, could this be my whole problem?? man i hope you hurry up and read this and let me know what u think, man i'm getting sick of working on this thing....thanks again.