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 Hot starting problem S4, B6 with 85k miles on it. View next topic
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Brand New, Welcome :)

Car: Audi S4, B6
Power: Standard
Engine code: BBK

Joined: 29 May 2016
Posts: 11

Location: Falkirk

PostPosted: Fri Sep 08, 2017 9:54 am  Reply with quoteBack to top

This is one for the knowledgeable and experienced among you.

Engine starts immediately from cold every time.
Run the engine or drive the car to get all the temperatures up then park and turn engine off.
If you attempt to start the engine within about 45 to 60 mins of stopping, it starts immediately.
If you leave it longer then you have to churn the engine for 20 or 30 seconds or longer to get it to catch and start. The engine runs ok when driving for hours after that.
The strongest clue is that the fuel pump does not run for the usual 1 to 2 seconds at key on when the starting problem is about to occur.

What I have done:
Changed fuel pump relay just in case.
Checked fuel rail pressures when running and during the heat soak period. Pressure does rise in the fuel rail as the underbonnet temperature rises as the fuel tries to vapourise.
Checked water jacket and air intake temperatures when running and during heat soak via VCDS. Also checked fuel rail temperature too using a probe strapped to fuel rail.
All 3 look plausible. there is a rise during the first 30 mins during heat soak and the fuel rail temperature gets to 65 deg C so there is a chance of vapour locks.
Once into the period when the fuel pump does not run at key on, turning on the fuel pump using the output tests in VCDS reduces the fuel rail temperature down to less than 30 degs to remove vapour locks does not allow the car to start immediately.
Checked the fuses S28, S32 and S29 for power to the High side of the fuel pump relay, fuel injectors and ignition modules. Power does get there at key on all the time.
The conclusion so far is that the ECU ends up in a non-ready state based signals from a sensor that gives a wrong signal when it has got hot. The result is the engine needs to be churned to re-sync everything.

The engine speed sensor G28 and Camshaft sensors G40 and G163 are involved in telling the ECU the state the engine stopped in and this is stored ready for the next start.
During a 30 second cranking when trying to get the engine to start, VCDS shows an rpm of 30 to 40. I don't know how to test for TDC detection though.
There are no fault codes from any sensors ever.
When the engine is running the camshaft advance settings seem ok as well.

The forums I have read do discuss similar issues and changes to G28, G40 and the engine temperature sensor are claimed to "fix" this issue. But as is sometimes the case there is no clear description of why.

I don't want to start randomly changing sensors without a good idea of exactly what the ECU checks, and when, at key on, to make sure the engine is ready to start.

Any ideas or pointers to reading material welcome.

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