UncleSam

Hello. I have an '11 model 2.0tfsi in a Q5. I've had an engine warning light for some time now, for a too lean code (long term adaption too high). It's been as high as 55% extra fuel, but I've checked later and it's been at 39%. I'm pretty sure I've had an underboost code also, a couple of times when I've scanned it so I'm pretty sure it's a vacuum leak and not bad injectors or whatever.

How do I find the leak? It's not the PCV on top of the engine, and it does not react at all to start gas in the engine bay. I've used a whole bottle, on several occasions, as good as everywhere, under the intake, around the turbo and so on. So what parts are most prone to fail on these engines that cause a vacuum leak and how do I find my culprit?

Any advice is greatly appreciated.

ThomasWShea

troubleshooting tips:

SMac770

P0299 underboost has nothing to do with the fuel trim. If it's high positive, it's probably injectors. If it was the PCV path, it's easy to confirm if that's the case or not. Just remove the PCV hose from the intake manifold and plug the port on the IM. Do your trims return to something normal? If it's an intake path leak, then pressure test the intake path (from start of hose that would go on the turbo outlet to end of hose that connects to the throttle valve).

UncleSam

Okay, I misunderstood the part about the underboost then. I think it's the injectors, but they're like $150 a piece so I need to be positive one of them are bad before I change them. How do I diagnose that? I had the coil on cylinder one go out twice since I started having the too lean code. Could that be related to a bad injector? I have access to VCDS but from what I can read there's not much values to tell if an injector is actually bad or not, as the readings will only show the signals to the injector not what it actually delivers. I'm getting real tired of this car not running right and I'd greatly appreciate any help here.

SMac770

https://www.genuineaudiparts.com/oem...tor-6h906036ae
If you buy Audi OE, they are going to be expensive. I've never bought Audi OE fuel injectors, I get the Bosch OEM ones, which are the same units.
https://www.boschautoparts.com/p/gdi...jection-62855-
https://www.fcpeuro.com/Audi-parts/A...&b=6&d=1082&v=

That is odd that you've had the coil go bad twice on a particular cylinder. You replaced the plug also when you replaced the coil? Might have wiring issues to that coil, or could even be a problem with the circuitry for that coil inside the ECM.

Yeah, welcome to the annoyance of the internal combustion engine. There's very little means to validate if a part is working or not other than to replace it and see if things get better.

UncleSam

After a lot of searching for the source of the leak, I built a smoke machine from a YT vid, connected to the hose from the air filter housing and filled the engine with smoke, it took several tries and a lot of time before I could see a very small stream of smoke coming from the back of the engine. I knew it was the rear main seal between crank and flywheel but I even put a inspector cam back there while filling with smoke to be 100% sure it was coming from the hole by the flywheel, and it was. It's a bit weird as there isn't that much of an oil leak back there, but it's still obviously failed catastrophically as I could plug the air intake hose with my hand and the car would still run exactly the same at idle lmao. So, I have to pull either engine or gearbox to repair it. I don't think box is very doable DIY even with a car lift as it's 150kg. I also have a failed engine mount so I will remove the engine at some point to swap the seal.

But the other problem, the cylinder one misfire issues, I can't wrap my head around. I've changed the coils and the plugs twice, and also the injector on cylinder one, and was hoping that had fixed it but after several months of driving with no misfires, it suddenly ran on three again. This time I can't seem to get it to ignite cylinder one again either, but I haven't tried that hard. It smells a lot of gas, so it's obviously getting fuel. Compression is good on all cylinders. Could this problem be connected to the RMS failure? I'd think that's unlikely. What could be the cause? It would suck to use what, four days to pull out and reinstall the engine if the car will still fail to ignite cylinder one at random times...

SMac770

RMS separating at the top so you have an air leak while not really having an oil leak is not unheard of.

Could be an issue with the wiring from the ECM to the coil, or even the ECM circuit driving that pin itself? RMS interaction would seem unlikely.