Chassis: WDB 202128-2f-771563 6/Engine:605960-22-027802.
Problem: Since having a B Service by a Merc Agent back in Sept the engine sometimes refuses, to rev over 4200/kick down a gear. To begin with itseemed to only occur within a mile of a morning cold start. Now it occasionally does it when at normal running temperature - unpredictable and therefore dangerous when overtaking - feels like "reverse thrust" has been applied!
The Agent (his senior specialist has been with me when it happened so knows it does happen) has never found a software fault on his Merc Lap-top diagnosis tool. To-date it has been very much repair by replacement ie: New inlet manifold sensor and vacuum hose, new fuel pipes in engine compartment (done twice within two months as an air-lock was suspected as a half-inch or so bubble of air was visible in pipe from fuel thermostat to the filter assembly - air STILL appears, bleeding back from the filter end and lodging between the fuel thermostat and the filter, when the engine is switched off. The Agent having checked other cars, including the CDI, seems to think this is normal - I cant understand how it can be as it is a closed-loop system, or so I'm told).
The fuel thermostat was renewed. The Injector bleed-off hoses have been changed. The inlet manifold seals have been renewed. I THINK that the inlet air-mass sensor has been changed but I'm not sure (checking). A BOSCH Agent, Panda Fuel Injector Systems of Fareham, Hampshire, had the car for a week and reported that it was nothing to do with the fuel pump - the Merc agent suggested that the pump might be the problem! A new EGR assembly has also been fitted.
One morning when it happened a "chatter" similar to a relay rapidly switching on and off, was audible in the cabin - but this has not happened since. The agent said that the battery was at the end of its life - changed! No difference!
Any ideas? The Mercedes agent seems at a loss and I've asked that Mercedes (UK) get involved, now that it occasionally happens when the engine is running at operating temperature I'm concerned it could cause an accident.
AWP
19 Feb 06
Finally, I've tried using a branded diesel additive to no avail.
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You've checked out the Car-by-Car Breakdown, I imagine, which does refer to "Kick down problems on automatics (which is most C Class) can be caused by a failing mass airflow sensor".
This may be completely irrelevant, but . . . similar symptoms on the W126 300 SE I acquired last April were caused simply by a bent throttle linkage rod that was impeding the full travel of the throttle valve (experts excuse my inexpert vocabulary, please). It had been caused by a spanner monkey trying to undo the oil filter, which is in an awkward place if you don't have the correct tool. That little distortion was also preventing the cruise control working at all. My mechanic (not the spanner monkey) just straightened the rod and all is fine. When it happened, my first thoughts were of the "Oh my God -- autobox fault?" variety.
Your car (a C250DT, presumably) is so different that this may not help at all, but who knows?
One point: is it worth revving a diesel beyond 4200 rpm, even when you can? I accept that it may need to go that high during kick-down.
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Thanks for taking the time to respond. The throttle on this one is electronic, via a potentiometer (I think)and as the car runs ok most of the time (the fault is very occasional) I doubt this is the problem. Yes I have visited the Car-by-Car Breakdown section and noted the MAS, which I think the agent changed but I'm going to check (he changed almost everything else in the engine compartment!!)
Once again,
Thanks
AWP
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I suspect the physical fuel system is fine and you have an electronic problem - most likely a sensor or connection thereto.
My suspicions would fall firstly on the MAS and then possibly on the pressure sensor in the fuel rail. Don't overlook that it could be an electronic throttle actuator fault (i.e. actuator is stalling before WOT).
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Many thanks but this model is not Common Rail, its one of the last to use the old-fashioned pump fuel delivery system.
AWP
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