Greetings all,

Here is a problem that has stumped me almost since day one (about 3 years ago) of owning a 1980 300Sd Turbodiesel, brought out from the midwest to California.



Description of what goes on: 95% of the time, with the engine running, the tachometer needle swings wildly about, all over the place, evenif the engine is running steadily at constant rpm (Lets call this usual performance).5% of the time (usually when the car has warmed up, but warmup is not a guarantee) the tach stops swinging wildly, the tach needle pegs at the lowest tach reading, and simultaneously there is a tremendous engine power surge, at least 20% better power, better and smoother acceleration, the car becomes like a rocket (I call it steroid performance)compared to its usual performance when the tach needle is swinging around.

Usual performance is as follows: max speed on flat road is 80mph,very poor acceleration,very rough shifting in the automatic transmission(you have to let up on the accelerator to get it to shift into the next higher gear), with power somewhat unsteady (the car is nicknamed "Lurch" by my family). Tach needle swings wildly.

Steroid Performance: Tach needle buried at lowest guage reading, top speed easily 100mph (its scary), shifts super smooth, great acceleration.

When I experience the rare and thrilling steroid performance, the car can revert back to usual lurch performance without any warning.

Some background: the car is a midwest rust bucket, probably under 200K miles, engine seems to have very good compression, it starts immediately without using the glow lugs. Tranni was rebuilt by a reputable shopwhen the car arrived here 3 years ago. I replaced many of the vacuum tubes in the engine compartment when the car came out of the tranni shop.

I suspect the power surge relates to an as yet undiscovered vacuum problem (any suggestions very welcome) but I am totally baffled by the connection between engine performance and what the tach is doing. Any Ideas?