ok this is my second post, hopefully i can make it more accurate.

car: 1991 turbo, 3k on rotary ressurection rebuild
mods: fmic, boost controller, walbro 255, racing beat dp back

problem:

for a while, i've had to play with the throttle when starting the car to keep it from dying, then it would idle ~900 rpm, nice and smooth.

recently, the idle is crap. the car shakes, it idles at about 400 rpms, but doesnt 'hunt' (by hunt i mean bounce up and down a lot, it moves maybe 5 rpms in any direction)

when i get it idling, it continues to shake and rattle but it does idle.

today i tried the idle screw on the BAC to raise it a bit. i can unscrew the bac screw almost all the way out, with no change at all. the only time it changes is when i screw it all the way in.

so, i tested the BAC. i removed, cleaned, and applied 12v to it. it clicks nicely. i reinstalled, same problem. i tried putting the A/C on to check the rpms, and when i put the A/C on, the rpms raise about 700 rpms and drop back down to garbage idle.

i checked and made sure the factory grounds were ok. i went over every vacuum line to make sure they are plugged in (The ones i know about and could find). i checked (did not set because im missing a socket extension) the timing, and leading 1 shows the pin between the 2 marks.

what could this problem be? intercooler couplers are all solid, i checked them all. i replaced plugs and wires, with no luck.

when i am DRIVING the car, its a dream. its fast as hell, and nice and smooth. the second i put the clutch in, or put it in neutral, the rpms drop to about 400 and its crap again.

i have a feeling im running rich, because i get some popping occasionally.

has anyone experienced this and fixed it? i have searched and searched and i always read about the same problem, but theres never a conclusion. nobody posts what they did to solve it.

thanks

 

The best way to check vaccum/intake leaks is buy a can of starting fluid (DO THIS WITH A COLD ENGINE) I am not responsible for fire, spray it at any and all vaccum hoses and intake connections if the idle surges you have a leak if not you can truly eliminate that possibility

 

i had no starting fluid or carb cleaner on hand, so i sprayed around with ZEP (penetrant like pb blaster, some people remember zep )

i got no surging at all, UNTIL i sprayed to the RIGHT of the turbo. waaay deep inside. what is back there to cause a vac. leak?

also, im curious as to why my BAC screw adjustments do NOTHING... but the bac is obviously working

 

only thing back there I can think of is LIM. You may need to remove and regasket the manifold. Or it could be just a loose manifold bolt.

Dont know the answer to the BAC question.

 

This may seem silly, but it happened to me one time, so why not. I was having a very similar problem to you. My connector going from the harness to the BAC was broken. The BAC was working when tested, but just the connector had to be replaced.

 

The vaccum problem in the manifold connection seems to me to make sense in your circumstance because your car runs great later when a small leak would result only in a small loss of boost pressure but at idle it is more problematic.

 

the last thing i wanted to do was wiring harness hacking. i hate it. is there some way to test? before i go in there and end up having to trace the bac wire all the way to the ecu?

 

you should be getting 12v at the connector when it should be in use right? That will tell you if the BAC is working properly since you already know the unit is good. You could go to RETED's website and he may have something on testing the BAC wiring.

 

make sure the TPS is readin at 1 ohm.

 

Check the TID for cracks especially right where it meets the compressor housing - you said it surged when you sprayed it near the turbo. Also check vac lines that go to the oil injectors (they are hard to see) - one of mine popped off after a hard run and caused the exact problem.

 

from your details, u did not say u grounded or jumpered the green connector near the leading coil pack, if u do any adjustment on the BAC, that must be jumpered or grounded (S4 and S5 respectively) first.

but then, if u sprayed near the turbo, and it jumped, most likely is a vac leak.... so that needs to be dealt with first...

 

If you have a large enough vacuum leak, which it sounds like you do, the BAC will not introduce enough air to change the idle. The BAC is basically a controlled vacuum leak allowing the idle to be adjusted.

With such a huge vacuum leak the car will run rich, all that air is not going through the AFM so it is un-metered, the O2 sensor will send a lean signal to the ECU telling it to correct the A:F ratio, this is why it would be overly rich.

 

Was there ever a solution confirmed on this one? I recently had the same issue. I have a ruff idle like it is missing, but still pulls hard.... My water pump just recently died right when it happened.... Any ideas?

 

just for the Heck of it,try Pulling one Plug wire at a Time.when the car is Running.each wire should throw a spark.Please do this with an INSULATED pair of Pliers(ZZZZT!)..if you find one coil not firing to the Wire,then it is a bad Coil.I found that out(I had a bad trailing coil On one side),after a long time,hunting a bad Running engine.It would have power but Buck every once in awhile.Idle would be smooth with the odd sort of Bump.it sounded a little off at the rear of the car too.

 

that didn't help much. Thanks though. Any more ideas? I am going to do a compression test in a few minutes.

 

turns out my compression on the front rotor was 70 70 30 .. I think that means I blew a side seal.

 

Yup, time for a rebuild.