MK5 Golf GTI
All Things Mk5 => How to Guides / Troubleshooting => Topic started by: dadawg22 on January 23, 2026, 04:51:31 am
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Hello all, recently I've been battling a vacuum leak issue with my 2007 FSI model MK5 GTI. The engine code thrown up was the System Bank Too Lean that led me to suspect the PCV at first, but after replacing it and the rear breather (huge PITA) with the latest iteration parts I still hear a loud hissing noise when the engine runs, along with very poor idling and near stalling. There's also a heavy smell of unburnt fuel in the cabin, likely from the massive leak.
Was wondering what other common locations might be the cause of the leakage - don't have the equipment for a smoke test but worst case I could round up a couple smoker buddies and let them have a rip.
I've heard that the brake booster is a common suspect, but upon my initial inspection it seemed to be alright (but I'll check it again when the weather gets above -30). Next two suspects are the diverter valve or the intercooler, but those would throw up boost related engine codes which I haven't yet seen. I've also heard that the vacuum pump seal may fail, but there isn't noticeable oil below it. Worst case is a blown rear main seal, but I don't see any engine oil leakage when it's parked, and the engine still performs well when under load with boost noticeably kicking in around what feels like 2.7-3k+ RPMs. There's also some pretty significant rev hang, and the throttle control is rough with the engine sometimes revving itself quite high without any input while the vehicle is still.
Apologies for the huge post, but I want to make this comprehensive for anyone having similar issues in the future. I'd really appreciate any advice on diagnosing this, I love this car and want to bring her back to her best state.