Mercedes-Benz: smoking 1985 300 turbo diesel

Smoke is unburned fuel. So why is the fuel not burning as it should.
Could be as simple as a dirty air filter or as expensive as the low
compression of a worn engine. That said, most of these old diesels will
smoke upon hard acceleration because they always have. Low sulfur fuel
cuts the smoke and low sulfur fuel is being distributed more widely as
time passes.

A diesel is a compression engine, that means the engine's 21:1
compression ratio sufficiently heats the air inside the cylinder to
ignite the fuel at the moment of its injection. Worn engines no longer
have 319-348 psi of compression, they're down around 220-240 psi - below
that they won't start. You begin to see the equation: compression makes
heat and heat ignites the fuel. Now lower the ambient temperature to 10
degrees F. and the marginal engines don't start.

Blowby is simply the lost compression - every engine, gas or diesel, has
some blowby, there's no measurement of blowby so forget blowby.

If you want to buy one of these diesels have an independent M-B shop do
a compression test for that's the ONLY way you will know the internal
condition of a diesel engine. It will cost you $100 or $150 and is $$
well spent and without regret to KNOW the car you're buying is
reasonable or to avoid some run out dog that needs an overhaul before
the cold weather hits.

A reasonably maintained 617 M-B diesel should be good for 250K to 300K
miles, longer if very well maintained. So if the car looks great and the
owner has complete maintenance records that show engine oil and filter
changes every 5K miles it's probably a good car, otherwise be a skeptic.