I recommend that you perform, or have a qualified mechanic perform a compression test. If the numbers turn out low, he should add some oil to the cylinders to make determination if the problem is rings or valves.

If the numbers turn out OK, then you have something to work with.........the thoughts of a slow turning starter might be accurate.

Ether can be devistating to a diesel engine, even though some people have gotten away with using it. I myself will NOT try it with a diesel. Always better to be safe than sorry.

Check grounds well..also I am asking this becasue I did not notice if it was mentioned...were you using the same battery on both cars you compared to each other as far as cranking speed?

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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

1984 300d Turbo
Remove Glow Plugs And Use A Reamer To Remove Carbon, Also Install A Rebuilt Starter It Will Turn Over Faster And A Good Battery.

Has anyone seen carbon restrict the air and fuel mixture so much from keeping a car from starting, I remember a van I had this AND was the problem for my hard starts and rough idle,the carbon was so bad the mechanic was chiseling globs of carbon off the internal combustion area.

Common problem on VW TDI's. Air flow is greatly restricted due to the crud building up in the intake plenum and manifested by power being way down. Should not have an effect on starting unless almost totally obstructed.