Pops 300e: Loses Power / poor fuel eff.

I've been driving my 82 y./o Pops 300e ('86, 190k miles) for a few weeks.
The car started and performed great the first couple of weeks.
The car has since developed problem which is becoming a safety issue.
On a recent 2 hour interstate trip, the car began losing power. The issue was instantaneous. The car would take no more gas/acceleration. Also, the MPG/fuel efficiency gauge moved to high side (appox 4:30-5 oclock). After a few minutes, the 'normal' mode would kick back in - car would ride normal and fuel eff would be normal.
The issue happens now everytime I drive the car. It is particulary unsafe when I pull away from a stop and the car will not accelerate and threatens to stall. Last night, in driving rain, this happened as I attempted to cross a large intersection.
Thoughts appreciated.
Chip L
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Chip L wrote:

Chip,
could be alot of things but sounds like a short or that the throttle body portion near the top that houses all the ASR and cruise is faulty. When it senses slippage, or fast turning with lots of acceleration it bogs the motor to limp mode, what it sounds like its doing. It is to protect the driver. First replace all the fuses, who knows that normally cures alot of things. Then if its not that easy check your wiring harness for shorts in the engine compartment. If its not that try that intake/MAS/cruise ASR throttle body. If its not that maybe the ECU. If you get frustrated take it in, Mercedes or advance auto can hook it up to a computer and hopefully pull a code and tell you for sure.
Jason
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This is an electrical problem. There's a relay called the "over voltage protection relay" (it has a fuse on its top) that causes similar faltering when the OVP relay fails. I suggest you do am Internet search on that topic and see if the car's symptoms agree with other owner's experiences. This car is of the right age for this and another relay to fail.
For future reference the other relay to keep in mind is the fuel pump relay. It becomes intermittent - much like the current problem - except there's suddenly no fuel and the motor stops - or doesn't start at this try but will start at the next try etc.
The fuel use is bogus to the problem. The economy gauge is a simple vacuum gauge. When the motor bogs down you instinctively step on the accelerator which opens the throttle plate and so reduces the engine's manifold vacuum. That's all it is showing you.
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