I have a NA S4 GXL, no major emissions removed, just vacuum pump, and a couple of lines capped off. This issue just started the other day, has been running super awesome, and I've had barely any issues, nor did I change anything engine wise on the car recently, this just started out of the blue.

The car runs fine 90% of the time, cold starts up to 3k rpm, drops down after a minute, idles fine. Drives fine, when hot it might very rarely misfire, but is very smooth and steady most of the time. Issue only happens if the car is turned off, and restarted while hot, then it surge from 1100 rpm, to almost nothing, and shakes quite badly when it tries to recover. After the car has been ran for a bit, it will return idling to normal.

Only other issue is that when I let off throttle and it comes down to idle, it will dip below 700 rpm before returning, this can be an issue when turning into my parking stall in my buildings underground garage. Seems like it takes a second for the ECU to catch up and when the P/S pump is engaged with a high load it can almost die, but I am not sure how related this issue is, as it has always done this.

What I have done:
-Checked for vacuum leaks with propane, none that I have found (short of removing the UIM and checking out the rats nest)
-Replaced a vacuum line from my LIM to the solid line that goes to the booster (not sure if it was leaking, but close proximity to hot exhaust made it brittle)
-Removed/cleaned/tested BAC valve & connector
-Adjusted TPS to 1k ohms while hot


It is very hard to trouble shoot because after a minute it idles fine again, so I have to constantly turn it off and restart it to try to identify the culprit.

I would very much appreciate any help, I have a drift event this Saturday, and I don't want to beat on the car when its not running correctly.


YT Video:
https://www.youtube.com/watch?v=_bci...ature=youtu.be

 

Anyone please? Today I tried cleaning out my throttle body and a few connectors including the MAF and Mixture adjustment. Still no solution in sight. I am not sure where to go from here.

 

Have you personally replaced all vacuum hoses ? Have you verified the bac valve solenoid is in spec and the diaphragm isnt torn ? Have you metered the tps to check for any variance across its range ? Did you check for diagnostic errors being blinked by the ecu ? Compression test? Checked that the afm is within spec across its range ? Good fuel pressure regulator ? Replaced the fuel injector grommets?

Lots of diagnostic work ahead of you :-)

 

I second a vacuum leak. Check gaskets between LIM and Block, as well as UIM and LIM

 

I just wanted to update, after many hours of troubleshooting, replacing intake gaskets, vacuum lines, fuel filter, replacing sensors, I finally corrected the issue. Turns out I had a cold weather thermostat installed, and was causing the car to run hot on hot days, making the coolant thermo sensor lean it out, causing the BAC to over correct causing this weird issue I was having. I replaced the thermostat with the correct one, and temps reduced, so far I've never seen the temp gauge go past 1/3rd the way up, when previously it would run around half. It now purrs like a kitten!

 

Thanks for following up.

 

you just replaced a thermostat!? how long was the warm up? im having surging issues. somebody said to re-check the tps readings with the motor warmed up.