Ok guys and gals last year i replaced the engine in my 1983 300D with one fro ma 1981 300SD that had 123K miles on it.
My old engine i could top out at 110mph 115 going downhill

The old engine had 235K when it quit (timing chain tensioner broke)

The new engine preforms and sounds just like my old one......the turbo was used off my old engine and performs good.
But i cant get my car to make it over 90mph RPM's are about 3850.....yet it reaches 4400 in the other 3 gears. Why is this? My mechanic said use some fuel cleaners and im trying that now. He also said that it might just not have enough steam in it. Please help!! i want to go over 100 again!!!!!!!

you might want to check things like chain stretch etc.....maybe that new motor wasn't quite as good as you hoped, or has far more miles than you think on it. And are the fuel filters new?

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Proud owner of ....
1971 280SE W108
1979 300SD W116
1983 300D W123
1975 Ironhead Sportster chopper
1987 GMC 3/4 ton 4X4 Diesel
1989 Honda Civic (Heavily modified)
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Section 609 MVAC Certified
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"He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you." - Friedrich Nietzsche

I read somewhere that the top speed of a 1983 300D-T is 102.5mph, now either that figure is wrong or your speedometer is out of whack, I'm guessing the former since 102.5 seems quite slow for a top speed, at 100mph, there seems to be some space left before you hit redline in 4th gear in these cars.

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'81 MB 300SD, '82 MB 300D Turbo (sold/RIP), '04 Lincoln Town Car Ultimate

Sooner or later every car falls apart, ours does it later!
-German Narrator in a MB Promotion Film about the then brand new W123.

Got any data to confirm "performs good"?

Or is this just the butt dyno?

What's a boost gauge read at 3850 rpm?

Is the ALDA getting full boost?