sluggish 400e under load

[darbykidd]

Hello, I'm brand new to Mercedes and thinking of purchasing a 1992 400e with 130,000 miles. The current owner recently replaced the battery, fuel pump and fuel relay to get the engine running. I test drove the car and under load it doesn't seem like it has near the power it should. (My 250k '88 325i accelerates better). During the test drive I punched it and when reaching ~4500-5000 rpms it wouldn't shift, but instead would just stay between those rpms, back and forth.. under normal driving, it seemed to shift fine, albeit a little firm between first and second I think. When in park however, the engine seems to rev nicely. However after reving it to ~3000 rpm and letting off the accelerator the rpms drop down low enough for the car to die. The owner claims that since it had been sitting for awhile it probably needs new plugs...I'm suspicious that it is that simple of a fix and fear it may be more significant. The oil, coolant levels and quality seem good. I can't seem to find any information on my own so I'm hoping some of you experienced owners can help out with some possible causes. Thanks.

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[Jim's500E]

Sounds like the car is in 'Limp' mode....are there any lights on in the dash?

The car will put itself in this condition as a safety measure. The power and driveablility are cut until you can get it home or to a shop for a fix. You are going to have to see if pulling the codes from the CAN leads you to anything to fix or replace. Either a dealer or indy with a MB STAR diagnostic computer can pulls the codes...or you can make or buy a cheap LED blinker to retreive codes.

Usually its an elcetrical proble that causes LH mode. Bad wiring harness (es), bad throttle actuator, dying alternator, or as simple as a brake light switch or nuetral lock-out switch can all cause LH mode. Check the wiring harnesess first, for this car from the factory they were designed as bio-degradable; and seem to start degraded upon delivery ! But really, the heat from these motors often cooked things like the underhood electronics and the plastic coverings on all the wires....turning them brittle and causing wire on wire electrical shorts and power spikes.

Search here and on the web on 'W124 V8 Limp mode' and read up...it can be a pretty common problem on this vintage car. Hope it is a simple switch as a fix for ya'...new wiring harnesses and a throttle body add up to a couple grand alone.