limp mode

[delinbrook]

My 2000 320cdi has developed a problem which I believe could be a "limp home mode" situation. My local Mercedes Benz main dealership is proving to be less able to identify the problem than I would have expected so I find myself trying to sort it myself.
The common feature seems to be an increase in engine temperature and when driving in heavy traffic. The result is a lack of engine power, failure to kick-down and general sluggishness. I am normally able to restore the situation by switching the engine off and restarting.
My best guess (and of course I am ready to be proved wrong) is that the computer is recognising that the engine temperature has increased and that the cooling fan has not activated and has instructed the computer to reduce engine power to prevent the engine from overheating.
What do you guys think is the problem and, what is the cure?
It might be worth mentioning that the Mercedes workshop manager didn't know what temperature the fan activated at, or what temperature the stat opened at, or what could be causing this problem.

 

[Xtractorfan]

Two things come to mind, one is the MAF..air flow meter, this is probably the biggest single cause of engine going into limp mode. The other is the thermostat, a blocked thermostat will cause the temp rise.
The MB dealer should have put your car on the star diagnostic system and then sacked the Workshop manager...

 

[delinbrook]

Hi Xtractorfan, many thanks for your response, the MAF air flow meter is something I hadn't considered probably because of the implied cost!!! I am still in favour of either the thermostat or the temperature sensor/sender but until I have the opportunity to run the temperature up high enough to see if the fan comes on I am still a little in the dark. As for the workshop manager at MB I cannot disagree with you.
Many thanks
delinbrook

 

[HERBIEMERCMAN]

hi delin, the cooling fan cuts in circa 95 deg. +or- 5 deg. this should bring the temp. down to 80 deg. the fan stat is usually a bi-metal clixon switch and they rarely go wrong. on some cars the fan motor sticks, many times due to lack of operation.

i had MAF problems with my e300 td. disconnecting the plug on the air intake just after the filter element box will give you a clue, on mine the dissconnection was obvious that i had a duff MAF. the engine ran much better, no power, but smoother, another limp home example.new MAF and power and smootheness returned.

"limp home" is the fail safe for many faults,i have had the autobox oil in wiring loom, MAF and turbo boost pipe all inducing the limp home mode, and as you suggest it may be protecting your engine. with the massive damage that overheating can cause, head gasket etc. ( £600+) and the fact that all limp home codes should be cleared from your ems ( engine management system), then i would sus out a good "inde" ( independant merc specialist), to resolve your problems. i do not reccomend mb main dealers, they are expensive and many are only familiar with new cars.

best of luck. herbiemercman.