Audi A6 C5 2.5 TDI – Hot starting problem and intermittent 00550 injection timing fault – VCDS log attached

[seanious]

Hi everyone,

I'm hoping someone with experience of the 2.5 TDI V6/VP44 injection system can help me diagnose a problem with my A6.

The car is a 2004 Audi A6 C5 (4B5), 2.5 TDI, and I've owned from new. The engine is the AKE/BDG-family 2.5 TDI with the Bosch VP44 injection pump.

The car has developed a hot-starting problem. When cold it starts normally, but once warm/hot it can crank without starting. During one of the failed warm starts I noticed some grey smoke from the exhaust, which made me think that fuel was being injected but that the injection timing wasn't correct.

I've been investigating this with VCDS.

Initial findings​

I originally noticed that the timing in VCDS Group 004 appeared too advanced/retarded depending on operating condition, so I started looking at the injection timing more closely.

At around 20°C coolant temperature, the engine started normally and I recorded approximately:

Group 004

• Specified start: 6.2° BTDC
• Actual start: 6.2° BTDC
• Cold start valve: 100.2%

Group 005

• Starting quantity: 65.6 mg/str
• Actual start: 6.2° BTDC

As the engine warmed, the timing changed as expected, but I eventually started getting fault code:

00550 – Start of Injection Regulation

• 08-10 – Control Limit Surpassed – Intermittent
• 17-10 – Control Difference – Intermittent

The fault generally appears once the engine is properly hot. On one occasion it appeared after only about a couple miles of driving.

Basic Settings Group 004​

With the engine fully hot, I checked Basic Settings Group 004.

Initially I had:

• Late: approximately 0.4° BTDC
• Early: approximately 14.4° BTDC

I subsequently made a small mechanical timing adjustment because the workshop information I have specifies approximately 2° BTDC for the basic timing.

After adjustment I managed to obtain:

• Late: approximately 1.8–2.0° BTDC
• Early: approximately 14.8° BTDC

The car sounded noticeably better and responded well on the road.

However, after driving approximately five miles, the 00550 fault returned, and when I checked Basic Settings again with the engine hot, Late had returned to approximately 0.4° BTDC.

I therefore stopped making further mechanical timing adjustments because I don't want to chase the problem or move the pump outside its correct setting.

Hot readings after the fault appeared​

With the engine around 90–92°C, I have seen readings such as:

Group 001

• 742 rpm
• Injection quantity approximately 5–6.6 mg/str
• Coolant approximately 90–92°C

Group 004

• Specified start: around 2.4° ATDC
• Actual start: readings have varied, sometimes around 0–1.6° ATDC
• Cold-start valve around 75–100%

Group 005

• Starting quantity around 14–16 mg/str
• Actual start broadly follows the Group 004 actual value

The particularly interesting part is that the actual timing can appear reasonably close to the specified timing immediately after the fault has occurred, despite the ECU having stored 00550.

VCDS log​

I've made a several-minute VCDS CSV log containing Groups 001, 004 and 005, starting with no fault present and continuing until the 00550 fault occurs.

I've attached the CSV for anyone who wants to examine the timing behaviour in detail.

What I'm particularly interested in is whether the log shows anything characteristic immediately before 00550 is triggered — especially any divergence between specified and actual start of injection.

My questions​

I'd really appreciate opinions from anyone who has experience with these engines and VP44 pumps.

1. Does this behaviour look like a VP44 internal timing/advance mechanism problem, particularly one that becomes apparent when the pump gets hot?
2. Could the timing advance solenoid/advance piston inside the VP44 be sticking when hot?
3. Could the needle lift sensor / injector feedback signal produce this sort of 00550 fault?
4. Is it possible that I'm misunderstanding what the Basic Settings Group 004 Late/Early values are actually telling me, and that the apparent movement from ~2° BTDC back to ~0.4° isn't evidence of the mechanical timing moving?
5. Are there any other VCDS measuring blocks or tests that would help distinguish between:
   • incorrect mechanical timing,
   • VP44 timing regulation,
   • needle-lift feedback,
   • fuel supply/air ingress,
   • or another electrical problem?

I'm deliberately not changing the starting quantity adaptation at this stage, as the VCDS data doesn't appear to point towards a simple lack of starting fuel.

The car otherwise drives well, and the problem appears to be strongly related to engine temperature.

Any advice from someone who has dealt with this particular fault on a 2.5 TDI would be greatly appreciated.

CSV VCDS log attached: Groups 001, 004 and 005, from cold/no fault through to 00550 fault.

I'm hoping its not the VP44 pump as I've already fitted a reconditioned one from Bosch (£650 ish) about 4 years ago.

Thanks in advance.