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Matt, Mechanical Engineer
Category: Mercedes
Satisfied Customers: 22750
Experience:  BEng hons Mech engineering, in auto industry 22 years
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I took my mercedes ml 300 cdi to a main dealer 'A' service

Customer Question

I took my mercedes ml 300 cdi to a main dealer for an 'A' service and when it came back it had a judder (feels like engine is misfiring). They are saying torque converter needs to be replaced at great cost, but it didn't go into the service with this problem. This happened once before in an earlier service and the fault was corrected when the car went in for some repairs identified by the service, however mercedes have no record of the juddering being registered as an issue. What is the experts opinion as to what could have caused the judder to materialise during the service?
Submitted: 3 years ago.
Category: Mercedes
Expert:  Matt replied 3 years ago.

I'd be suprised if the torque convertor was at fault, it more likely that there's an issue with the fuel supply

Ideally measure the pressure at the inlet to the main engine fuel pump and if this is low also consider changing the fuel filter as if this partially blocked this too will reduce pressure.

If OK then also check the connections to the EGR valve, if either the vacuum pipe of electrical connection are damaged / corroded then the EGR can be on all time which certainly will hurt performance. Its also possible that its a fault with the EGR valve so its worth removing the valve and cleaning it out with brake cleaner, if it looks particularly clogged then replace it.

If this is OK then check the small vacuum lines to the turbo and its connected control solenoid on the bulkhead, any cracks or leaks can give turbo issues and its best to have the boost pressure measured actually measured with a boost gauge to check that the turbo is healthy and that the sensor is reading correctly

If its a variable vane turbo (they’ll be an actuator on the turbo body - but not a wastegate) then check its vacuum pipework as above and check the connections to the diaphragm / solenoid valve

Customer: replied 3 years ago.

In my comments I mentioned that the car did not go into the service with this problem, but came out of the dealership later that day with the problem - does that point to any of the above issues being more likely? or that the dealership could have inadvertently caused the problem? (as stated, the same problem appeared to have happended previously and was somehow solved when the car went in for some other work to be done). I dont see that it could be a cooincidence?

Expert:  Matt replied 3 years ago.

I do think that a fuel supply issue is probable - as the filter should be replaced on the service, whihc is why I say to check its been replaced / fitted correctly, also possible that a vacuum pipe has been dislodged and not replaced correctly
Expert:  Matt replied 3 years ago.

do you still need help with this?
Customer: replied 3 years ago.

I put the car into a different dealership for a 2nd opinion, with your comments, and they tested everything they could think of and even rebalanced the wheels as newish tyres but hasnt made any difference and they say it points towards torque converter.

Expert:  Matt replied 3 years ago.

OK if you put the car into drive and hold it on the brake whilst slowly pressing on the accelerator does the car try and move off ?

can you feel it straining against the brake ?

If you can at what engine rpm does it try and pull away as it sshoudl be about 1500rpm it really start to pull

if its doesn't do this then yes i'd agree the torque convertor may be faulty
Expert:  Matt replied 2 years ago.

do you still need help with this?