Dif­fi­cult start­ing

Car Mechanics (UK) - - Diagnostics Doctor -

My car is a 2008 Audi A3 2.0 TDI (en­gine code BKD) show­ing 100,000 miles on the clock. It is ser­viced and gets new oil, fuel and air fil­ters reg­u­larly. Over the past year, it has de­vel­oped a strange fault. It starts up first thing when hot or cold, re­gard­less of the out­side tem­per­a­ture, for a day or two, but af­ter 3-4 days it be­haves as if there isn’t diesel com­ing through the sys­tem. It will even­tu­ally start on the se­cond or third try.

Apart from this, all sys­tems are OK and there are no warn­ing lights. Audi has tested it for hid­den faults, too. Aside from this, the car runs like new, cold or hot. Ge­orge Pate­laros The en­gine in your Audi is the Pumpe Düse unit. The in­jec­tion pres­sure is de­liv­ered by the in­jec­tor it­self, us­ing the cam lobe to op­er­ate the in­jec­tor and pro­vide the pres­sure. The in­jec­tion tim­ing is still elec­tronic and a wiring loom runs within the cam car­rier hous­ing to trig­ger the in­jec­tors.

One prob­lem that is very com­mon on this en­gine is for the loom that runs within the cam car­rier hous­ing to frac­ture. This will give an in­ter­mit­tent con­nec­tion and can cause a prob­lem such as you are ex­pe­ri­enc­ing. A re­place­ment loom costs around £60, but when this prob­lem oc­curs it does nor­mally trig­ger a fault code.

Other com­mon faults in­clude the fail­ure of the in­jec­tor rub­ber seals and prob­lems with the camshaft op­er­at­ing sys­tem. If either of these are present I would ex­pect to see smoke when the ve­hi­cle started and/or a slight mis­fire. As you re­port nei­ther of these prob­lems, we can hope­fully dis­card these pos­si­bil­i­ties.

This leaves the fuel pres­sure sys­tem. One pos­si­bil­ity is that the fuel pump at the end of the camshaft (tan­dem pump) is drain­ing when left for a pe­riod of time, mean­ing no fuel is ini­tially sup­plied to the in­jec­tors.

With this in mind, I would ex­am­ine the fuel pipes and con­nec­tions on the fuel fil­ter. Also in­spect the fuel pipes be­tween the fuel fil­ter and pump. There is a non-re­turn valve built into this pump, but it can fail over time. A cheaper al­ter­na­tive to re­plac­ing the tan­dem pump, which also sup­plies the vacuum to the brak­ing sys­tem, is to fit a sep­a­rate non-re­turn valve in the sys­tem. VAG manufacture such a valve (part num­ber 165201542B) and it costs around £18. Fit­ting one of these in the re­turn feed from the tan­dem pump to the fuel fil­ter should rem­edy the prob­lem.

The one-way non-re­turn valve.

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