slickNSX

New member, bit the bullet and got the S8, impressive car, just got it today and got check engine light and a little rough idle. It actually cut off on me while idling and was hard to start, I thought it would need to be towed, car has 63k miles and carfax shows just had 55k mile service was done in march 2017. I'm coming from the AMG camp (E55). Took the engine cover off and I was like oh boy, this is not what I'm used to. I would like to clean the mass airflow can anyone direct me to where that is? The car seems to run better with check engine light on then off, Im puzzled. I did search for this issue but didn't see anything on a D4, Please help!

the_duke

There is no mass air sensor. Your best bet is to get a VCDS cable and scan the car. Generic readers are not very useful.

https://store.ross-tech.com/shop/cat/VCDS

slickNSX

I got an Autel 906S scanner which is very detailed, after turning car off and restarting it in about an hour it is hard to start and stumbles and cranks long you can smell fuel under the hood and after long cranking finally starts, I'll be taking it in to Audi this afternoon, lets see what they come up with, my shop so far diagnosed it to be a leaking injectors that collect fuel and floods the motor. The idle is rock solid and no hesitation in acceleration no black smoke the codes are now historic and the check engine light turned off on its own this weekend.

Eyvind Mondragon

This is a good read:
P0172 and P0175 - OBD-II Trouble Codes

Possibly, bad O2 sensor. That shouldn't be too bad.
Get a VCDS. Or just go to the dealer (the VCDS would probably pay itself in one visit, and it's more fun).

Cheers,

slickNSX

Thanks for the help guys, the dealer as usual was no help, wanted to sell me a 65k service for over $300, which was basically an oil change, they did have an ECM update for air/fuel mixture (I had them do the update), and they said the car didn't have hard starting while they had it. There explanation is that car hasn't been driven a lot and the fuel has mixed into the oil and that's what they think could be causing the rich condition and hard starting. So tomorrow I'll do the oil change at my shop using Mobil 1 0w-40. I doubt that will do anything, but they said if I wanted they can dig in deeper and charge 3hrs of labor to test fuel injectors. I'll update tomorrow keeping my fingers crossed.

slickNSX

also Audi had the car overnight and when I went to fire it, it fired right up. Once I got to my business it sat for 1hr and then when I went to start it, it stumbled and was hard to start, long cranking and then finally started. So this issue only occurs when the car sits after being driven for an hour or 2, but seems like overnight and the next day it starts right up on the first try.

LexusLSguy

Change the oil and run some good fuel system cleaner. Like the Duralube system cleaner in the big bottle. It sounds like a fuel delivery and / or quality issue. If it did not get regular oil changes maybe an engine flush would not hurt.

Return For Deposit

An injector(s) that leaks down after shut off can certainly cause the symptoms you describe. Get a Vag Com cable from Ross Tech and download the software. It's money well spent. It will log your misfires and can tell you which cylinder(s) are causing the issues. Techron Fuel Injector cleaner has solved deposit issues on both my Corvette and Ford Diesel Fuel Injectors in the past.

At your mileage, carbon deposits on your valves can also cause cold start issues.

slickNSX

no cold start issues at all, I got techron in there right now. I've found other audi threads that are plagued with these same issues, some are leaning towards low coolant, coolant temp sensor and others are going deeper into plugs and injectors, going to spend some more time researching those, oil change will be done today, the car shows oil change in 8500 miles or 274 days but I'll get it out of the way today just to eliminate that. I'm only have issues in restarting after it runs and sits for about 15min-2hrs, after that it seems to start right up. But thanks for the suggestions.

LexusLSguy

Check out this thread. For him it was one of the fuel pumps.

https://www.audiworld.com/forums/a8-...2915178/page2/