Cold-running problems

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.