Steve on benzworld.org recommended this forum:

Well it's a mystery to me. I have a '87 300D with 302,000 miles and it runs like a top. I recently purchased a '92 300SD as its eventual replacement (180,000).

The 350 diesel has poor acceleration, especially on any kind of hill. So my local mechanic adjusted the ALDA. Lots of smoke and slightly better acceleration. Took it in for a smog test and it failed with a dyno of 55 hp . Mechanic backed off (leaned) the ALDA 1/2 turn. Took it back and it passed the smog test, but the interesting thing was it dyno'd 110 hp. Ran like great.

However, the over the next week performance went back to the old performance level, crummy. Checked the ALDA and the mechanic hadn't tightened the lock nut. So I set about attempting to adjust the ALDA myself. With a couple 'standard' tests, like running up an uphill on ramp and checking the speed, and 100 mile fixed course mileage test.

Now the mystery. At some adjustments I've gotten 64 miles an hour up the ramp (the '87 makes 62) and as high as 29 miles per gallon. A couple days later, touching nothing, she gets to only 54 miles an hour, and 23 miles per gallon.

I'm baffled. I've changed the air filter, and fuel filters, with injector cleaner additive a couple times this year.

One thing I have noticed one thing at speed the MPG (vacuum gauge) won't drop below 10-12 miles per gallon.

Any ideas? Thanks