1G Idle surge and rev hanging

[AndrewC]

I just picked up a 91 Talon TSi from a tow yard. Just needed a key and started right up. Compression is 130 across, I know it's kinda low but I'm just gonna be dailying it. I started the car and it rang out to 3500-4k rpm and held there. It turned out to be a too tight of a throttle cable and I loosened it up. Then the idle surge. It's the common version, cold start, revs up and holds rpm fine. Then drops after running for about a minute and bounces from 1200-1500 rpm. The BISS screw is maxed out right now to keep it as low as I can. I know this is bad for the ISC. I unplug the ISC, and nothing changing (frankly I don't know if anything is supposed to or not) I unplugged the closed throttle switch and jumps rpm to 2k and holds. I unplug TPS and no change at all. I want to do a boost leak test as my next step but want to know some other things to look out for. This is a BONE STOCK 4G63T, besides a turbo manifold, and a vented BOV. Again I know, bad. I'm gonna be putting a stock one back on it soon. But if it helps while it's surging the BOV is closed and not open. I think I hear a vacuum leak from around the TB area but it's so loud it's hard to tell. My next issue could possibly be revolved around it but I'm not sure. Basically when I'm driving, it drives perfectly fine. No issue, no bogging, the car doesn't threaten to die, nothing. But when I'm accelerating and let off the gas. There's about a half second delay from when I let off the gas to when the engine actually cuts fuel and starts to engine brake. It does have cruise control if that matter. I've already took the box apart and lubed everything and everything is smooth.

 

[steve]

If the BISS is turned all the way IN then air is reaching the cylinders via another path. Pulling the IPS connector confirms that the engine is/wants to idle @ 2k, does it drop as the coolant warms up?

The BOV will let unmetered air in throwing off the AFR but it still have to get past the throttle butterfly so it's not causing the issue. Pressure testing the intake would be a good start so see where the air is coming from. You can start at the TB elbow rather than at the turbo until you have found all the leaks that far.

 

[AndrewC]

Finally got around to doing some testing on it since I blew the transmission. The idle will stay at constant 2k at operating temp. But when I was looking around the throttle body, I did notice that one of the studs that has a nut on it holding the throttle together, was a little wet, and had bubbles coming out of it. So tomorrow when I'm at work I'm gonna get new throttle body gaskets and replace those. I also need to invest in a pressure tester.