Redwood

88 13B is the engine. I just started it up after about two months (it's in something else now). It idles at about 2600 rpm and holds that pretty well.
Yesterday, I let it warm up completely and adjusted the TPS (without jumping the coupler). This did nothing to the idle. Today, in my ignorance, I started out by messing with the idle adjust screw on the intake. This also did nothing. Then I read a little in the FSM and a little in my searches.
Now that I know about the initial set coupler, I tried jumping that to try adjusting stuff again. As soon as I jump it, the car sputters down to nothing and dies.
I know that this is something that happens because I read a glimmer of info in one of the threads my searches turned up, but I could not figure out how to fix this.
Thanks in advance.

HAILERS

Hey. Do this. Instead of jumpering the initial set coupler, at idle go to your bac plug and take it off. Did you get the same results as when you jumpered the initial set coupler. Write back your results asap. Shouldn't take you ten minutes to check it out.

Redwood

Okay, did it. It did almost the exact same thing. Sputtered down same as before, but just barely held on this time. What do you think?
EDIT: It didn't "hold on" well enough to count as idling. IT was at that point where it is just barely managing to turn over once more. So I would say that it did do the same thing.

HAILERS

2600 really isn't a idle in my opinion. But, I think you have one of the only cases of a possible stuck wide open bac. BUT before we go toooooooo far with that idea, does the car still do the 2600 rpm after two or three minutes of running???????

Here's why I say its a stuck bac. On a normal car, during cold start, the rpms will go to approx 3000 rpm for 17 seconds. Then it will go down to approx 1100 rpm and as the car warms up, finally go to 750 rpm. Now I discovered a few months ago, that the 3000 rpm is created by TWO items. One is the bac and the other is the airbypass solenoid. I found one day I had broken the air bypass solenoid harness. I fully expected that I would have no 3000 rpm at start up the next morning(at that time I thought the air bypass solenoid was the only player). Well the next morning the car went to 2400 rpm much to my surprise. Either 2400 or 2600, I forget.

So as soon as I saw the 2600 figure I suspected the bac was stuck wide open. Try taking the bac off and jumpering a ground and a 12v source to it and see if it clicks or opens and shuts. Might spray some good lubricant in the holes and exercise it. See what happens.

Another thing though. With the bac plug off or the initial set coupler installed, the car should idle. Thats how you set the real idle. Install the coupler and turn the adjustment screws until you get 750 rpm. THEN you take the initial set coupler out, which restores the bac funciton. Bottom line is THE BAC IS NOT SUPPOSED TO BE SETTING YOUR IDLE SPEED. THE IDLE IS SET WITH THE BAC DISABLED I.E. THE INITIAL SET COUPLER INSTALLED(NO FEEDBACK FOR THE BAC).

Anyway, there is a chance the bac is stuck wide open. Maybe yes, maybe no. We'll see.

Redwood

I did it. It clicks.

HAILERS

Has to be the bac or the bac circuit. Think about it. All you did was take the bac plug off and the idle fell like a rock. Thats what the initial set coupler does also. You install it and it kills the feedback to the bac so it won't interfere with the setting of the idle. Then again it could be the ECU sending a full open signal to the bac for unknown reasons. I think there is a relay involved. I'll search for it. I've got things to do now. It'll be tomorrow sometime before I give a answer.

You might take the bac plug off and see if theres a dead short on the wire that goes to the ECU. I'd unplug the ECU middle plug, and put the meter on ohms and touch the wire thats not black/white with the positive lead and touch the other lead to the battery negative lead. Should show wide open. If it shows any resistance, theres a short in the wire. Gotta go take care of business now. Tomorrow.

HAILERS

Well, one more thing to clear things up. The bac has a duty cycle. It is not a on/off solenoid. It works at approx 125hz. If you held it in your hand while the engine was running(another would have to be installed to fill the void), you'd feel it vibrate at 125hz or there abouts. Just wanted to clear that up. NOT A ON/ OFF SOLENOID

HAILERS

Could you answer me this since I might have read you wrong. If you let the car warm up, is the idle still 2600rpm??? Thats the impression I got earlier.

The relay I was thinking about was the air bypass relay located in front of the afm. I've come to the conclusion that isn't the problem.

If the problem is as I originaly thought i.e. the 2600rpm is there after warmup and never drops, then in my mind the bac is causing the problem because just by removing the plug on it caused the idle to drop down to zip.

If thats the case I'd leave the plug off the bac and adjust the idle per the fsm with the initial set coupler in place and adjusting the screw on top of the dynamic chamber. Also set the timing while your at it. And the tps while your at it.

After that I'd try to find out the problem with the bac circuit. Its got 12v going to it on the black/white wire all the time and is controled by the blue/green wire by the ECU. I'd be interested in what the voltage is on the blue/green wire at this 2600rpm idle. You could pick the voltage off the plug at the ECU. Pin 2Q. Thats the middle plug on the ECU. It would be on the top row, far left pin. Everything would have to be connected up and the car *idling*. When I get home I'll put a meter on that wire and startup one of the cold cars and see what the voltage is for the first 17 seconds of startup.

I've never had to deal with a car that idled at 2600rpm and could be brought to its knees by taking the bac plug off. The above is the only cheap way I know to work the problem.

Redwood

Yeah, you got it right. The car stays at 2600 even after warming up. I checked for a dead short on the BAC plug and came up with zero resistance. I will check the voltage at the ECU a little later today.

I tried unplugging the BAC to set the timing and whatnot. It actually didn't die this time, so that's a plus. It dropped to about 1400 and then would drop to about 900 every 5 seconds or so. I actually didn't set the timing yet or adjust anything, I'll be doing that a little later today. I will get back to you with the results.

hugues

If the car was not run for 2 months, it will probably run shitty for some time. Then, things may get back to normal by themselves. How was it running before ??

Do you have FSM 88 from www.fc3s.org under 'manuals'. Get the 'fuel and emissions' chapter, there is a section about troubleshooting the BAC, air bypass solenoid valve and relay. That should help you quite a bit.

As Hailers said,
do you get 2600 rpms even after car is fully warmed up (say, 5 minutes of running) ?

Let's assume it always run at 2600 even when hot.

Being able to run at 2600 rpms with just the BAC is very surprising. I think the other valve is stuck open (the air bypass 1)

Is it possible the air bypass relay is stuck in the closed position causing the air bypass solenoid valve to always be in the "on" position.

Good luck,
Hugues -

PS: above assumes it s a 88 NA. If 89+ or TII, quite different.

HAILERS

HUGUES.....On my turboii I broke one of the wire to the air bypass solenoid. I fully expected the high idle for 17 seconds would be disabled the next day. Not so. I got the high idle for 17 seconds, but instead of the usual 3000rpm I got only 2600rpm. Thats what caught my eye when I first read this post and then asked him to pull the bac plug off while it was idling. He said it fell like a rock, similar to putting the initial set coupler in to disable the feedback to the back so you can set your timing, idle etc.



Looking at page F1-77 of the 89 manual it shows the output devices and what they do at various times. The bac is shown to have *A LARGE QUANTITY OF AIR* during cranking. This explains why when AKAGLEAS fixed his bac, his hot start problems went away also i.e. not enough air during startup.

So you can see its really not unusual for the car idling at 2600 just using the bac. Happens everytime I start my car in the morning for 17 seconds(unless I tap my throttle, as I learned on this forum).

hugues

ok, HAILERS, thanks for clarifying the function of the Air Bypass Solenoid Valve. It just adds a tad more air on top of the BAC. It makes sense.

I still think the BAC is probably ok. The inputs to the ECU that control the BAC operation must be off. The first 1 to come to mind is the TPS (others are shown on attached picture). The ECU itself could also be bad.

From what you have just posted, your idle is now coming back down but hunting, so TPS could be off.

It is always a good idea to look at FSM for the later years 89+ from www.fc3s.org since it has a lot more explaination even though it is not exactly what you have on your car.

I am sure HAILERS will figure out what's wrong with your car.

Hugues -

HAILERS

Not sure for sure. Wish I had it to play with. One thing that turns the ECU on to give you that 3000 rpm at start up cold, is, on a 86-88, the water temp switch on the bottom left of the radiator. If you pull one or both bullet connectors off, the 3000rpm will not happen even if its 20 degrees. You might try pulling one or both leads off the water temp sw and see what happens. Only takes a moment. It'll either work or not work. I doubt it but its a outside chance thing. I dismissed this idea earlier because there should be a 17 second shutoff point.

Redwood

HAILERS - Since I have the engine/ecu in an 84 and aftermarket gauges to monitor everything, I've actually ditched the water temp sensor and pretty much everything like that. I should have mentioned this earlier as it could be playing quite a serious part in this. Does the ECU need the signal from that in order to allow the BAC to function? What signals do I need to be sending to my ECU? It's possible that I have accidently cut/lost some of the connections in there and that's affecting it. Thanks for helping me out hailers. I'll be away until tomorrow, but if you give me some more ideas I'll try em and let you know the results.

Yeah, I wish you were here to play with my engine too. It would sure speed things up a bunch.

HAILERS

Yeah, I keep forgetting you put it in a 84. Sorry about that. So forget the water temp sw on the radiator. Only effects a cold start and then for 17 seconds. If I disconnect mine, there are no more anomalies. So thats not it then.

Its as if the ECU never realizes the car has started and is running. It seems to think the car is still cranking even after the car is running, so its keeping the bac wide open all the time. Thats my guess. I'll finally get around to see what the voltage is at my bac when I'm cranking, then maybe we can compare it with yours.

I don't think I'm stuck in a rut with thinking its the bac or the signal going to it making your idle 2600 rpm. You take the bac plug off and the idle drops like a rock. Has to be the bac or the ECU stuck thinking the cars still cranking.

What I'd do while waiting to figure out what went wrong with the bac, is take its plug off. I know that it won't idle that way right now, but thats because the idle has not been set. Try to use the initial set coupler and set the timing, tps and if the sucker won't idle by turning the screw on top of the dymanic chamber, then go to that throttle stop screw on the front of the throttle body (a very tiny screw held with a 8mm wrench size jamnut) and turn that throttle stop screw until the car will idle without help. Try to maintain around 750. If you set the idle above 1000 or so you can't set your timing right because the ECU will advance it and mess you up.

Darn . I've got a 82 that I don't drive much and a spare turboii motor, so if I..............
I can't think of any sensor that would make it act like it is. Your using the stock coil assy I'm pretty sure and the cas. I figure the cas must tell the ECU whether or not the engine is up to speed. Duh.

Redwood

What exactly does the BAC do when the car is not idling (meaning when it's driving)? I will set the idle and all that without the BAC attached. I actually tried it a bit yesterday and it stayed alive for quite a long time. Long enough that I could start adjusting stuff and actually see results. That was really cool.
The funniest thing about all this is that yesterday when I had it idling in my driveway and was sitting in the driver's seat monitoring the temps and all that, the freaking gas light came on. With all the wiring problems and splicing and complications I couldn't believe it. Anyway, out to the garage. Thanks a bunch for all the help Hailers, if you think of anything else, let me know.

Yeah, this is almost the exact statement I made before starting. It's been a hell of a lot of fun and I've learned more than I thought possible. This is my first engine swap and I don't think there's anything out there that could have taught me more about rotaries, wiring, and all the bs that needs to be done to make something fit.

HAILERS

According to the manual on page F1-77, the bac when you are cranking *LARGE QUANTITY OF AIR*. When idling *FEEDBACK DUTY*, When under medium load, acceleration , heavy load and deceleration *FIXED DUTY*. When the ignition is on and not running *FIXED DUTY*.

Most of the time its there to take up the slack when you turn the a/c on, the lights on, power steer the car. There is some more load put on the engine when you do those things so the bac ups the ante and adjusts the idle to stay around 750 instead of the engine dying or dropping down to some godawful low rpm.

Until you fix your bac circuit expect the engine to drop down a bit in rpm when you put a load on the engine. On the whole its not detremental to run without a bac. If I think of something I'll let you know.

Redwood

Thanks actually very reassuring. I have very few things that could put a load on (no a/c, no airpump, no p/s, no heater, no stereo, etc.). I guess I'll just get it working without that for now until I figure it out. Thanks again, I'd be screwed without your help, like always.