S6 hard start - any suggestions?

[mithril]

Hey all,

Hoping someone can give me a new direction to look here as this is getting quite frustrating. In the morning when the car has sat for a long period it fires up without any issue whatsoever, once running it's perfectly smooth. However if I shut the car off an come back in a short(ish) period of time it will just crank and crank and crank. Occasionally it will catch, sputter and die. This behaviour is, of course, intermittent so sometimes it starts fine. No codes are being recorded other than an occasional SAI insufficient flow warning.

I've taken to the habit of putting the key to accessory while I buckle my belt, let the steering wheel come out to the stored position, etc as this seems to help. This problem is now getting progressively worse so I'm at a bit of a loss. So far I have:

1) replaced the N80 purge valve as I had been getting a CEL from secondary air injection codes.
2) replaced the ECT sensor in case this was temp related (and because it's cheap)
3) replaced the intake air filter (never expected this to solve the problem but it was due)
4) replaced the crank position sensor (I've experienced #4 before in my old 4.2 A6 and the symptoms were very similar; long hard crank with no motion on the tach)

None of these have resolved the issue. The car cranks strongly and reliably so I know it's not the starter or ignition switch. I know the 4.2 does not use ICM's, any suggestions on where to look next? Will post a video later hopefully.

[rollerton]

Well..two things come to mind; Cam position sensors and Fuel pump. Cam sensors wouldn't have any variable related to time idle or engine hot / cold or anything, but my I had a weird intermittent hard start problem and I would occasionally get a Cam Position sensor code. Replaced (both i think) and didn't happen again.

Then there's fuel pump. Maybe? Definitely would be variable based on temp or time at rest (cooled off). The pump is an electrical motor and they do not work as well with age or temperature. So, maybe it's wearing out, getting hotter than it can tolerate and not working as well?
Sitting overnight and cooling off might be helping it.
Just first things that come to mind.

[mithril]

I was thinking fuel pump too, so a pressure test would be the next step likely. The car is at my transmission shop (of all places right now) as my previous indie had just closed his doors and this caught me out, and they don't really want to get into troubleshooting this. Can't blame them, they were just doing me a solid by replacing the CPS after I fought it for several hours.

The weird thing is that the car runs fine once it's going? I'd expect to see more issues if the fuel pump was dying? Similarly the cam position sensor(s) should be throwing codes if they're dead or dying? The lack of log info is the frustrating part about this one, it feels too much like the old days of troubleshooting my G60 Corrado :/

[4rings2turbos]

Also calling fuel pump. They'll start hard and randomly (or not start randomly) for a while before I cuts out completely on you. Possibly while you're driving.

[mithril]

Reasonably certain it's the fuel pump, I don't even hear it prime now.... guess who only has 13km on a new tank of gas :(

[4rings2turbos]

Ahhhhhhh, same thing happened when my allroad's went out :(

[mithril]

When the CPS died on my old A6 it was the same symptom. I think the ECU is programmed to cut both fuel and spark in that scenario.

[mithril]

Yup, fuel pump = winner winner chicken dinner :( After several attempt I managed to get the car started, just as the tow truck arrived of course, to get the car over to the new indie shop. I've never smelt exhaust so rich in my life, a TON of unburnt fuel was being dumped through the system. Fuel economy has been getting gradually worse as well so probably all related.

[A Stupid Turtle]

That's the smell of high performance. Efficiency is not important! More fuel!