B6 1.8T BFB Fuel Pressure

Biffo_the_bear

New Member
Evening all !
Our BFB 1.8T has developed a reluctance to start. The engine cranks fast and fine, but it will not initially fire. Then it catches a bit, runs a bit rough, then clears and runs fine. If I turn it off, then immediately fire it up again it starts just fine. But if I leave it for a few minutes or more it struggles again. I have run VCDS on it (proper Ross-Tech version paid for by myself !) and it is not throwing ANY engine codes and only a couple around the HVAC (no refrigerant) and the radio (aerial amplifier).

Here's what I have done so far:
1) Replaced the fuel filter (Bosch)
2) Replaced the spark plugs (NGK irridium)
3) Replaced the crank sensor (Bosch)

I have also replaced the camshaft sensor (which was a newish looking but unknown pattern part) with a believed good second hand OEM one.

I have removed the injectors and cranked the engine with them pointing up in the air - all firing and all patterns seem fine, and no dribbling when shut off.
I have checked the compression on all cylinders - all around 135-140, so plenty good enough.
I have checked I'm getting sparks at the plugs - all good.

I've had a few old fuel injected Triumph cars in the past, and what this reminds me of is when I used to get air trapped in the mechanical injectors. Crank and crank with no start, eventually a reluctant start, runs rough until air is gone and then runs fine. With the A4 its like the pump is having to build pressure before it will work properly. But the injectors all seems to spray fine when out of the car.

How can I measure the pressure at the fuel rail to see what its doing when it first cranks and what happens when it sits after running ? Does the pump have a non return valve to maintain rail pressure or is it supposed to pressurise instantaneously when cranked ?

As an aside, I was surprised that VCDS does not have fuel rail pressure as an option for this car, and I note there is no fuel pressure regulator on the rail, just a schraeder type valve at the firewall end.

Or should I try a new camshaft sensor ? Are there any specifications I can test mine against ? Same with the coils, and specs to test them against ? I've measured resistance between all terminals, and the coils are all the same, but I suppose they might all be bad ?

Help !
 

lowlife89

Registered User
The fuel pressure regulator is inside the fuel filter. That's why you need a good one. It could be finnicky because of the Bosch instead of an original Audi filter, but not to the point where it won't start.
Not sure if these would be the symptoms, but fuel pumps tend to go bad and sometimes injector seats leak. Tried hooking up a pressure gauge to the fuel line?
 
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