LS1 stalling and idle issues
 

hello all, I have a 2002 formula A4 with about 72k miles on it that I recently had a cam put in with typical valve train upgrades and a stall converter. I've been having some stalling issues when coming to a stop occasionally and when starting the car when hot. First off the stalling issue when slowing down to a stop i believe is just in the tune, I had it tuned by Straightline in Joliet IL and he told me I can bring it back to have the tune adjusted for drivability tweaks which is fine. However, BEFORE when the car was stock I would sometimes have a problem when starting the car when it is already warmed up after driving. It would turn over fine and fire up but the idle would be fairly low for a second before idling up to the normal range. Now after having the cam put in and tuned it seems to have the same problem however it' has exacerbated the problem because when i start the car after its already warm it does not want to idle, it will fire up then stall, and it takes a little bit of playing with the throttle to get it to idle properly, mind you the car starts and idles perfect every time when dead cold. So my question is, does this sound like an IAC valve problem considering it would act up before the cam and tune? Or is it simply in the tune? One way or another I'll be taking it back to Straightline and have him adjust the tune, but would like to know if anything else might be the culprit.

If this issue existed before the car got any of the mods you listed, its obviously nothing they upgraded or changed.
It doesn't sound like tune, it sounds like the car is having issues getting gas when warm or it could possibly be electrical.

Yeah, I'm just trying to get some opinions on places to check first, If it were a weak fuel pump or something you'd think it would do it anytime it was warm, however like I said it was very sporadic which kinda led me to belive it may be IAC related since I know that can cause some wacky idle problems. I don't have any idea what electrical issues could cause this, nothing had been messed with prior to the cam (with the exception of headers and exhaust). I never thought to do anything about it since like I said it would only do it occasionally, and it didnt give me any starting problems, it would just start and idle slow for a second and jump back up to normal.

I think your culprit may be the check valve inside the fuel pump not holding fuel pressure after the key is off. Hook up a fuel pressure gauge, key on and see if it holds pressure; if not...new fuel pump.

This would make sense if it had the problem all the time, but if I wait until the engine is cold (sitting for a few hours) it fires up without an issue and idles around 1000 and comes down to it's warm idle of 750-800ish. I've tried "priming" the system by cycling the key a few times before cranking it up when warm and it still does the same thing, so I don't think it's a fuel pressure issue. I'm going to try a new AC delco IAC, if it persists, then i'll check the fuel pressure at the rail.

My car did the same thing for years. Cold starts are perfect and after running for a while, it would hard start. I had to prime fuel pump once, then key off, then prime the pump again while starting the car to get it to start normal. Your car sounds like it idles fine. Warm hard starts are your issue right?

yeah only a problem when its warm. I can start it up cold without issue, drive it for 20 minutes with it running and idling perfect then shut it off at say a gas station for example, get back in 2 minutes later and it doesnt want to idle. It will fire up and stall, then it will crank for a few seconds before firing up again, and if i want to keep it running i have to feather the throttle to get it to idle normally again.

Found the problem to be a stuck and dirty IAC..just ended up replacing it with an AC delco one rather than trying to clean the old one. It fixed the warm start idle issue, we'll see if it still wants to stall occasionally when coming to a stop.

what is the iac?

Idle Air Control. A valve that controls idle on cable throttles