I have an 88 turbo 2, runs fine when your driving but when at idle, it will slowly rise to about 1700 and them drop to about 1000. If u hold throttle to about 3-4k it will fluctuate from what u have it at and down about 500 rmp. I have a complete bac delete for information for any help. Anyone's input or ideas are helpful, also if anyone knows of or is within a couple hours of clarksville tn would be willing to meet up to look at it. Thanks

 

LOL, read my threads, and you will find the exact same problems lol.

I have my BAC back for that reason, didn't help squat. But I'd check ALL vacuum hoses, the injector grommets can go hard and let air through as well.

Other than that I am fiddling with my ignition now, I can get it to smooth out from the exact same range as you to 700-750 no fluctuation, but the timing is advanced so far that it runs like crap, I can get full boost in neutral lol

 

Ya the bac isn't needed as long as u have it cooked off correctly and sealed, with the injector grommets going hard, spraying the carb cleaner or starting fluid down there would do the same thing as a vacuum leak. I unplug my tps and start the car, it started fine and ran fine but just wouldn't hold an idle. When I held the throttle it actually held and didn't fluctuate. So the fluctuation, in my eyes has got to do something with the tps

 

I have tried three TPS, one of them had a hole, 3k ohm jumped to 9k then back, the other two are perfect all the way through, set correctly and all.

So, that wasn't my problem at least.

Try ohm yours, 1-5k or there abouts, check for holes in the range.

 

You are getting more air than expected at idle. Either through the throttle body or a vacuum leak.
This causes the engine speed to increase as it's running the "idle map" since tps~ 1v and rpm<1500.
Once it goes over 1500, it switches to decel fuel cut and the rpm's drop.
Some combination of vacuum leak/throttle stop setting is causing it. Of course having the BAC makes this adjustment a whole lot easier (if not unnecessary), but what's done is done.

 

Not if you have a BAC without idle adjustment ofc. Like JDM or S4 NA.

But on the other side, can you read my thread and see what you can get out of that as well? Similar problem sourced to ignition (timing) probably.

 

Well his 88 Turbo did. You're right though, it is the oddball.
My point was that you just need to get the settings in the ballpark and the BAC will do the rest. Now it's completely on the OP to get it just right.

S5/jdm idle control is a bit different and I'm much less familiar with it.

 

S5 in general should be the same, except 2 TPS's, BUT, the JDM S5's does not have idle adjustment on BAC, US models should have.

But it seems this problem is the same as mine almost, which is ignition. Exact same issues. TPS's are adjusted correctly, the butterflies are as shut as they can, BAC is the only thing keeping my car running on idle atm.

 

... Running the s4 only 1 tps lol

 

Hence my comment about S5's having two...

 

Ok we'll I've been busy with work and life, but my alternator went out got a new one she is back up and running. I let her warm up and plugged the tps just to see, and what do ya know she drops to 900rpm and holds it??? Is that just telling me that the tps isn't adjusted correctly or that the sensor may possibly be bad?