Lim­ited revs

Car Mechanics (UK) - - Diagnostics Doctor -

My son has just up­graded from a bat­tered Lupo to a smart 2000 Audi A4 Avant 1.9 TDI auto. It starts and runs very well, with no en­gine man­age­ment lights show­ing. How­ever, when he tries to put his foot to the floor, it only revs so far and then seems to lose power, try­ing again re­peat­edly in a very jerky man­ner un­til the throt­tle is re­duced. The car can be coaxed to higher revs and il­le­gal speeds by us­ing softer throt­tle set­tings, but this is a blight on an oth­er­wise ex­cel­lent car.

I have changed the fuel fil­ter and the EGR valve looks new. The EML does not il­lu­mi­nate no mat­ter how many times I ini­ti­ate the prob­lem. Si­mon Sil­vie There is a strong pos­si­bil­ity that the prob­lem is not with the en­gine, but with the ECU con­trol for the gear­box. Even though no EML is il­lu­mi­nated, there may be a code within the gear­box ECU which can be read off and which could pin­point the prob­lem.

If the fault is re­lated to the ECU, then one el­e­ment which will fail with­out leav­ing a code is the mass air flow (MAF) sen­sor. A quick test can be made by driv­ing the ve­hi­cle with the MAF plug dis­con­nected. This will throw up a code, but if the ve­hi­cle drives with­out the prob­lem, this will con­firm the MAF sen­sor is at fault. The plug should not be dis­con­nected for more than a short test-drive.

An­other pos­si­bil­ity is that the throt­tle po­si­tion sen­sor is faulty and not throw­ing up a code. This can be checked us­ing di­ag­nos­tic equip­ment.

The gear­box ECU may be the prob­lem.

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