Intermittent long crank no start 2015 A6 C7 3l Tdi.

[folkwagen]

My 2015 A6 C7 tdi with CRTE diesel has developed an odd fault. Car has 245K km, it's never failed to start instantly since I bought it in 2018.
On returning from a 600km journey, car would not start the next morning. Cranking no start. At midday it started after 5 long seconds crank. It ran perfectly for 3 days. On the 4th day it failed to start 2 hours after stopping. Engine was still hot after the previous one hour journey. Multiple attempts were made over an hour, no start, and we had to arrange transport home. Next morning it started first turn. No codes stored either time. It has been started multiple times in the next two days. Today it took 4 seconds cranking to restart after 2 hours stopped. Live data scan when normal idling shows high pressure fuel specified value at 250000 hPa (250 bar), rock steady, actual value at 249700, with minor swings plus and minus a few hundred. Immediately after the faulty long crank start, specified and actual were varying widely from these figures, but after a 30 min drive they returned to the steady idle. I suspect the common rail pressure regulator valve, as attempts to settle at a steady idle state intermittently vary widely, and this coincides with the subsequent long crank no start. On the Audi self study guide 840193 pg. 38 it states failure of the Fuel pressure regulator will cause non running. If this was intermittently unresponsive to PCM control voltages, pressure would be erratic during idle, and starting would not be possible unless it closed sufficiently to reach injector pressure. Other devices in the system, fuel temperature sensor, fuel metering valve, will not prevent starting, but will result in limp mode. On some starts it rough idles with considerable misfiring for 4 to 7 seconds, and with the accompanying widely swinging specified/actual pressure.
Any comments on this condition would be welcome in solving this. There appears to be a similar issue with the A6 C8 series which was sorted by a firmware update, but I suspect the C7 issue is one of sensor or actuator which is intermittently losing full connection, or is mechanically intermittent.
https://pics.tdiclub.com/data/517/840193_3_0_TDI.pdf

[johnsimcox]

Might be worth checking the health of the battery, especially if you have not changed it in your ownership it could be the orignial battery. A weak battery can the cause of all sorts of seemingly illogical and unrelated problems on these cars

[folkwagen]

Might be worth checking the health of the battery, especially if you have not changed it in your ownership it could be the orignial battery. A weak battery can the cause of all sorts of seemingly illogical and unrelated problems on these cars

No, brand new battery November'22. I squeezed 8 years almost out of the original, but yes, I would have seen a few post cold start faults, ABS, crankshaft sensor, all due to dropping voltage. This is one of these conditions that has baffled dealers when you read the posts. I thought it was bad fuel, I had got a full fill the night before the first incident. I actually drained thr remainders and replaced it with 25 litres of the premium diesel, but it still stuttered on a start yesterday. At least it started. I'm slightlu overdue an oil change, I'm wondering if the cranking oil pressure is marginallly low. Dpf regenerates fairly frequently as the dpf is on the cusp of full, will need a full remova and clean out. Regen can dilute oil with diesel, thinning it and reducing oil pressure. I'm changing oil today.

[folkwagen]

Fuel rail readings after a cold start this morning with rough idle. High pressure oscillating widely, specified high pressure fixed while cold, and pressure is only adjusted by the pressure regulator valve, not the high pressure pump metering valve. Hers a capture of OBD live data.

A6 C7 CRTE fuel rail.mp4 - Google Drive

[folkwagen]

This sat down altogether and refused to start. No high pressure on cranking. Long process to find the culprit, most advice was injector failure. Took a long time to find some one free. I was suspicious of the fuel metering valve which controls the flow of low pressure fuel into the works if the high pressure pump. It's part of the pump, not listed as a seperate part by audi, but removable, whence you can check if its actuating. VAG materials say this will run in limp mode if it fails, which eliminated it from the first path chosen, injector removal. These were fine, if off calibration after 245k km. I had the serviced and replaced. I ask the tech took at the high pressure fuel pump, which I feared might have lost its chain drive, or some catastrophic failure, though there was no metal in the injectors to indicate the HPFP was in trouble. It turned put the metering solenoid valve was sticking, probably intermittently, and was now fully stuck, blocking fule low pressure flows into the high pressure inlet. This prevents starting, with no fault code, and goes against the vag training manual which states it should limp start. Its very inaccessible, hence the tendency of techs to check the high pressure regulator valve first, then the injectors. A good bit of stripping required, but still a lot less than removing the HPFP itself, which the main dealers will do as they have no part number for the metering valve, so will remove the pump and stiff you fir a new one, 4 figure job. A new solenoid valve sorted it, region of €100, and the injector servicing and calibration was probably worthwhile anyway. Hopefully there are no hidden issues in the HPFP. There should have been some bypass path for fuel to reach the high pressure pump rams, and allow limp mode start. Time will tell, but it's running and idling fine. Problem started the day after I'd done a deal on another car, so messed up my trade in prospects. I'll sell it on anyway, though I'm very attached to it..