A6 3.0TDi BMK often won't start when hot. No fault codes, what to investigate?

[bentwenty]

Good morning all,
Over the last few months my 2005 A6 C6 3.0 TDi has developed a hot start issue.
When it has been on a good run and is nice and warm, it will refuse to fire. It is now happening more than once a week!
It will always crank perfectly, for exactly the same amount of time (about 3-4 seconds), and seemingly fast enough as well. But it wont fire at all. Being a diesel, the only thing stopping it from running can be fuel.
So far, every time it has done this, I've left it for about 10 minutes, and then its started perfectly.
I've scanned it with VCDS a few times and seen no codes (both after the fact, and while I was trying to get it to start).
I expect the ECU will be making a number of checks while the engine is cranking, Presumably fuel pressure, certain pressure switches and other parameters in the engine, maybe also the immobilizer as well? But I don't know what the conditions are that need to be met, or how to check them.
The last time it happened (Last night in a services in north wales!) I had my laptop with me, so tried to log a couple of things in VCDS, but I wasn't really sure what to log. I logged fuel pressure while cranking, the first time I tried it, it got to 96Bar, which seems a little low, but the second time it got to just below 300Bar and still didn't start. I then went and popped the fuel return off and put it into a mug while my GF turned it over, there was indeed fuel coming out, and it started! So we had to stop it and put the return on, and then drove the rest of the way home.
When it first happened a few months ago, I knew I also had the normal injector issues, so I put it down to excessive injector leakage preventing fuel pressure buildup. But they have now all been replaced with Rev AH injectors which aren't leaking (no smoke now and correction factors have settled at sensible values), so I've ruled that out in my head.
Does anyone else have any ideas? I can keep my tools and laptop in the boot so I can investigate it next time it happens. I really need to get to the bottom of this!
Cheers all!

 

[desertstorm]

Check the fuel temperature sensor is putting out sensible readings as well as the coolant temperature sensors.
96 bar is as you say low should be something more like 250 bar at idle / starting.
If you have VCDS check the in tank pump is working correctly and priming the system.
When was the last time the fuel filter changed ?.

 

[bentwenty]

Check the fuel temperature sensor is putting out sensible readings as well as the coolant temperature sensors.
96 bar is as you say low should be something more like 250 bar at idle / starting.
If you have VCDS check the in tank pump is working correctly and priming the system.
When was the last time the fuel filter changed ?.

Cheers Karl, I'll have a look at those when I'm home tonight. Presumably it will refuse to start if either the fuel or coolant temps are too high? If it happens again, I'll log the fuel pressure again and see how reliable it is under cranking.
I'll check the in tank pump later, does it run all the time? Or just during starting?
Fuel filter was last changed about 2 weeks ago. I've been servicing the car on 10k intervals with all OEM filters and quantum longlife 3, so it's had 2 services in the last 8 months.

 

[sliced]

Hi mate if you google hot start fix, you'll see many pages on it, however I'm not sure if it applies in this scenario. Worth reading though.

 

[bentwenty]

Hi mate if you google hot start fix, you'll see many pages on it, however I'm not sure if it applies in this scenario. Worth reading though.

I'll have a read on my lunch break, cheers mate. I know it was a big issue with the early 2.0tdi, but it could be mapped out. I had the hot start fix done on mine when it was mapped. I've not seen anything to do with a known hot start issue on the 3.0 though, but I'll see what I can dig up.

 

[sliced]

I'll have a read on my lunch break, cheers mate. I know it was a big issue with the early 2.0tdi, but it could be mapped out. I had the hot start fix done on mine when it was mapped. I've not seen anything to do with a known hot start issue on the 3.0 though, but I'll see what I can dig up.

Yeah had that on my 2.0tdi too but probably doesn't apply then on the 3.0. If vcds isn't showing any faults then it's gonna need the good old fashioned way of finding the fault :-(

 

[bentwenty]

Check the fuel temperature sensor is putting out sensible readings as well as the coolant temperature sensors.
96 bar is as you say low should be something more like 250 bar at idle / starting.
If you have VCDS check the in tank pump is working correctly and priming the system.
When was the last time the fuel filter changed ?.

Morning all, this problem is now happening with more frequency, so I have had a chance to investigate more with VCDS.
I've checked the coolant and fuel temp sensors, I've never seen fuel temps above 78deg and coolant temps above 84deg even when it's not starting.
I've checked the rail pressure while it's experiencing the fault, it seems to stay below 100 bar (often at about 99 bar) under cranking. I have yet to log the pressure during a normal start.
I have made a very interesting discovery which may hold the answer; it seems to start on the button if the brake peddle is pressed during cranking! Twice now it has refused to start, doing the same routine of cranking but not firing, but as soon as I press the brake peddle it starts straight away. I will do it the next time it happens to confirm.
I'm still not sure what the fault is though! It's a tip-tronic auto, and I don't thing the brake pedal normally has to be pressed to start, there certainly isn't a prompt for it on the screen.
Does anyone have any ideas?
Ben

 

[Telvis]

Hi bentwenty, I no this is a really old post but did you ever get to the bottom of this issue as I’m experiencing very similar problems with my 3.0 tdi Q7! Thanks

 

[bentwenty]

Hi bentwenty, I no this is a really old post but did you ever get to the bottom of this issue as I’m experiencing very similar problems with my 3.0 tdi Q7! Thanks

Afternoon mate, yes I did, I should have updated the thread at the time! In my case it was the crank speed/position sensor. From memory it’s located near the bellhousing on the back of the block, possibly at about the 9 ‘o’ clock position on the bellhousing when viewed from the back.
It was a very cheap part, around £15, but pretty fiddly to replace as it could be only be accessed from the bottom, through the exhaust tunnel, and the downpipe/cats get in the way a little.
Take that with a pinch of salt though as it’s been 2.5 years since I had a C6, and 3 years since I did the job!
Good luck

 

[Telvis]

Thanks for getting back to me, appreciate it! Fingers crossed as that’s what Ive just ordered!

 

[Bjørn Roger Larsen]

Heile Telvis.

Did this fix your problem?
I'm having similar problems with my q7.
Will stop when hot as if out of fuel.
Starts when cold.
Have changed the hpfp and filter.
No air bubles in fuel line in front of HP pump.

 

[D7rel]

Hello,
Did you manage to get your car fixed? My 3.0 tdi Q7 is doing the same. Occasionally cuts out and will not start when hot.
Thanks.