Cold-run­ning prob­lems

Car Mechanics (UK) - - Help! -

I have a 2007 Mercedes-benz C220 CDI au­to­matic with the 150bhp diesel en­gine. It’s been ser­viced and runs fine when hot. How­ever, when first switched on when cold, it starts and runs fine, but af­ter a quar­ter-mile it hunts badly for 200 me­tres or so, caus­ing a very jerky drive. It then runs fine un­til the next cold start. If you start the en­gine from cold and leave it to idle un­til warm, there is no hunt­ing, so it only hap­pens while the en­gine is driv­ing the car.

The EGR has been cleaned and I have re­moved the cover and put paint marks on it to see if it op­er­ates while the en­gine is run­ning and it does. The EGR plunger is smooth and easy to op­er­ate. Martin Hob­son

As you have checked the op­er­a­tion of the EGR valve and the plunger ap­pears to op­er­ate smoothly, this can hope­fully be dis­counted as the source of the fault. The prob­lem you are ex­pe­ri­enc­ing is pos­si­bly down to a fault in the diesel in­jec­tion sys­tem. This may be due to one of the injectors not per­form­ing cor­rectly when cold or else a fuel rail pres­sure prob­lem. Al­though you do not men­tion if an EML is il­lu­mi­nated, it would be worth read­ing the ECU for any stored fault codes.

I would also carry out a leak-off test on the injectors. To do this, re­move all the leak-off pipes from the injectors. Us­ing a set of suit­able length hoses, con­nect one end of each hose to the in­jec­tor leak-off and place the other end into a re­cep­ta­cle (one for each in­jec­tor), prefer­ably a clear con­tainer to make the mea­sur­ing of the fluid eas­ier. With the in­jec­tor elec­tri­cal plugs re­moved, crank the en­gine for around 30 sec­onds. The fluid level in each re­cep­ta­cle should be iden­ti­cal. If one of the lev­els is higher, this in­di­cates wear in that in­jec­tor. As your fault is when the en­gine is cold, I would carry this test out with a cold en­gine.

Af­ter the test you will pos­si­bly need to clear the fault codes from the sys­tem caused by dis­con­nect­ing the injectors’ elec­tri­cal con­nec­tions.

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