Diesel filter change - caution

Greetings fellow VW/Audi lovers,

First of all, I have no intention to mock VCDS or discourage anyone from doing a filter change but i’d like to point out that the VCDS’s fuel pump priming might not work the same on every car.

Yesterday I was doing my major service on my 1.6 CAYC and found that my car wouldn’t start due to air lock. Just in case anyone is wondering, the solution was to bleed the injectors one by one using a spanner.

Perhaps someone can guide me on how to do it in the future so that bleeding wouldn’t be necessary as I believe I’ve done everything as per instructions: 01 Engine>basic settings>test electric fuel pump> 30 seconds intervals.

Thank you

I have not done this myself, and I have certainly not done it on on a 1.6 (because they don't exist on this side of the pond), but I'm wondering...

Air at the injectors is on the high-pressure side of the system. The filter is on the low-pressure side. How is it possible that changing the filter got air into the high-pressure side? I wouldn't expect that to happen at all.

-Uwe-

Air at the injectors is on the high-pressure side of the system. The filter is on the low-pressure side. How is it possible that changing the filter got air into the high-pressure side? I wouldn't expect that to happen at all.

-Uwe-

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It really won't and if it did it would self bleed right at recirculating weeps in first commencement of injection since CR.

i have had this on my 2.7 if air gets into the line at all while changing the fuel
filter i have had to bleed the lines on each cylinder .pain in the .... . there is no prime function
on mine either .next time fill the line to the pump with diesel an fill the filter should be fine then...

Doesn't the old 2.7 support a bleeder screw at pump?

Out of curiocity are you using Output tests or basic settings?? i usually cycle between the 2 if one doesnt work

you shouldnt need to bleed air from a system that runs from 280Bar (start of injection) to 450 Bar (idling) to 2000Bar at full throttle.

we usually crank it for about 20 seconds and they start right up!

to answer Uwes queston the common rail does not maintain Fuel pressure when the engine is switched off. the pressure regulator opens up when de-energized to release the pressure on the high pressure side.

Fill the filter housing with fresh (clean) fuel when changing the filter and it will fire straight up without any bleeding

I've done 100s and never had an issue.

Ive done 4 filter changes(and 4 new injectorrs ) on my CAYC 1.6tdi and always use the tail lift pump and have never had a issue.

Changed the fuel filter insert yesterday (VAG definition being fuel filter=[metal cup + lid] and what you put inside the "fuel filter" is a fuel filter insert ), topped up with diesel, closed the lid and torqued properly. Next, I proceeded with the lift pump priming via VCDS, and since the ECU is UDS I followed the Fuel Pump Basic Settings steps:

If the vehicle uses the latest KWP-7000 (UDS/ODX) engine controller, conventional Basic Setting group numbers do not apply. Instead, choose the Transfer Fuel Pump (FP) test or similar operation from the drop down menu followed by [Go!] to activate the Basic Setting. The EDC should display "Running" and the electric fuel pump will run. After the result of Finished Correctly appears click [Stop].

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NOTE: As announced in the text quoted above, there was no Transfer Fuel Pump (FP) test in the drop-down menu but I found a similar one (can't remember what it was called). The rest was just as described above.

The test ran for about 90 seconds - I wasn't timing it therefore it's just a guess but it was surely way longer than 30 seconds. During the last 30 seconds or so I couldn't hear air bubbles any more so I figured all was good.

Anyway, while trying to start the engine the car began to shake, engine stuttered for about 2 seconds (usually fires up immediately) and I instinctively stepped on the gas pedal for just half a second -> RPMs rose, engine calmed down to its usual state (apart from the higher RPMs while holding the gas pedal, of course) like it had coughed up something. Several subsequent starts were uneventful as usual, like nothing had happened.

Is the UDS version of the fuel pump test set to a fixed duration (I felt it was somewhere around 90 seconds) or is ECU able to tell when there's no more air in the fuel system and sends out the "Finished Correctly" message?
Should I have repeated the test, even though I had a positive response from VCDS and there was no air audible in the system?
Could the shaking and stuttering be from some air bubbles reaching the HPFP? Is so, how much would it hurt the HPFP?

Quintus Rotam said:

Is the UDS version of the fuel pump test set to a fixed duration (I felt it was somewhere around 90 seconds) or is ECU able to tell when there's no more air in the fuel system and sends out the "Finished Correctly" message?

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I believe they are of a fixed duration and some have the option to specify that duration.

I do not think the ECU has any way of knowing when all the air has been purged.

-Uwe-

Sounds about normal if it behaves itself from now.