Its a really random problem to have, searched and didn't find anything like it?

So I have a 1990 GTU chassis with a S4 keg/turbo/driveline swap, using an S5 intake setup. All ECU, pressure sensor, and AFM are N370, full T2 swap.

I have a idle problem, it starts in the morning cold and idles just fine, but as it warms up, it either holds steady at 1500, or surges between 1200 and 1600, if my electric fan is on, I can downshift when slowing down and press in the clutch and it'll idle at my set 750-800 just fine, if I give it gas with the clutch in though, it surges again. But wait, it gets worse, I can kill my idle back down to 750-800 steady if I use the clutch in gear with the brake on to slow my idle. Then it holds at 750-800 after the clutch is pressed back in.

I have had this problem for months, and its really bugging me. I replaced the TPS (brand new OEM, in spec, voltage and ohm checked and set properly), I just replaced the coolant temperature sensor to ECU (NOT the gauge one), I replaced my air intake temperature sensor, the BAC is hooked up (coolant lines through BAC gone), and it has NO vacuum leaks, already checked with spray can and pressure tested..

But whats stalling me is I can kill it with my clutch and it'll stay, it won't bounce back up until I go again. The car runs extremely strong, it has NO problems driving, its like new. Engine rebuilt 22k ago, new transmission, NOTHING should be wrong with it.

I do think its in my throttle body, but I don't see how me slowing it down with my clutch will make it stay steady, also having an electrical load helps steady it as well. I think either my thermowax, or the stop for it is not working properly, or my BAC valve is having an effect on it.

For the hard throttle air mixture and idle adjustments, does anyone have the screw turn specs from the factory?

I just want my car to idle properly again, this is the LAST thing wrong with it.

Also does anyone have the measurement and fitting specs to make a PVC jump pipe to replace the intercooler for tuning, at worst I'll just go measure.

 

While on the topic of idle issues... I have a problem with my 93 rex its fairly standard apart from stragiht through exhaust and 2 air filters... When I start it up its grand.. But every1 now and then wen i rev it.. It seems to splutter and back fire or it mite rev then you could lift off and rev again and it would splutter.. Can someone help me please??

 

On an S4, the coolant line going to your bac leads to the "thermowax." Is it different for an S5?

 

*bump* because I have EXACTLY this problem. Halp...

 

I have this same issue, and it also ran lean, decided to check for vacuum leaks. I figured out once this problem started my oil injectors would not hold vacuum. When I did the turbo swap the oil injectors held vacuum, and the setup idled clean, so it made me think a vacuum leak would give a bad idle and cause it to run lean. I have not put the car back together after replacing the oil injectors, so I guess I will see.

 

My oil injectors lines are broken. It's possible that this is creating a vacuum.

 

Oil injectors lines broke will cause a vacuum leak because they go directly into the lower intake than the keg

I had the same problem as OP,except mine is a s4 n/a, and after i reset my idle per the FSM, and readjusted my TPS, it idles smooth as KY @ 750rpm idle

 

Damn. Even if it's just the line going to the front cover?

 

And on an S5?