I've never done a compression test on my 82 240D (135K), but have always adjusted the valves every 15K, strong battery, good glow plugs, etc. However, I've always had a fair amount of blowby. The filler cap really dances as a bunch of air gets pumped out of there. It has consistenly used a quart every 1,000-1,200 miles which most old pros consider to be good. This is a 4-speed and will get up and go. This has never made any sense to me. Can anyone explain this contradiction?

Van,

Blowby is pure and simple leakage past the rings. GP, valves and battery have nothing to do with it.

The rings and piston ring groves are worn so that they can no longer seal the cylinder and they allow some of the hot, pressured gasses in the cylinder to escape into the crankcase. Blowby is a normal component of engine wear.

I have one question: How many of the valves needed adjustment after only 15,000 miles? I have found that at 25,000 miles on my'79 300SD, all the valves were in specification and none needed adjustment.

P E H

The filler cap should be tight enough not to move around. Any blowby should escape the valve cover through the blowby port.
Maybe you need a new filler cap or need to adjust the spring tangs on the underside to grip the valve cover better.

Diesels just seem to have more blowby than the typical gasolene car.

Check to make sure your blowby port and hose are not restricted.

Ken300D

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1982 300D at 351K miles
1984 300SD at 217K miles
1987 300D at 370K miles

P.E. & Ken

A friend of mine always does my valves and usually says that a couple would tend to get tight. Always seems to run a bit better when they're done.
What I meant to say about the cap is that I would loosen it causing it to jump around a lot. If I put my hand over the hole there is a LOT of air pushing out.
It has always been an excellent starter however, even down to 0 F with 15-40 Mystic oil. The glow plugs get a good workout without plugging it in. -35F windchill this morning in NW Iowa. This is what has always puzzled me with with the blowby. I wonder how much blowby one would have to have to drop the compression numbers?
Thanks guys!

Van,

Wind chill doesn't mean much. If it is 0*F with a wind chill factor of -50*F, the engine will never get below 0*F but it will get to 0*F quicker with a lower wind chill factor.

If the engine starts OK in cold weather, the compression is OK and I would't worry about the blowby.

P E H

Worn valve guides, it's time for a valve job. Valves are wearing faster than normal, and a worn exhaust guide will allow exhaust to blow around the valve stem. If it starts at 0F without a block heater, it's not ring leakage, believe me!

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

Van Helden,

Blow by alone is not a good indicator of health for a number of reasons. Some of the most obvious are that by the nature of the piston to cylinder wall seal design there will always be some blow by, especially after combustion, which raises the pressure inside the combustion chamber much higher than the compression ratio due to the piston stroke ever reaches. So, this means a new engine has some "blow-by" and unless this is quantified a later "measurement" that shows you still have some, maybe more blow by is not meaningful for assessing engine health.

If your engine runs strong, but starts hard using the starter, try push starting it (you said it was a manual transmission equipped 240D). If it push starts easily then you have a starter/battery problem in all likelihood.

Push starting by dropping the clutch with the car in 3rd gear when you are moving a bit faster than walking speed with the glow plugs energized should result in an immediate firing of the engine. The main reason for poor starting performance I have seen is slower than required turning of the engine by the starter. Things that contribute to this phenomena are an old, worn out starter, an old worn out battery, both of those, poor performing or not performing glow plugs, and engine oil that is too viscous at low temperatures, any combination of the above, and often all of the above.

When the engine turns too slowly in cold weather the block acts like a near infinite heat sink so as the air is compressed and tries to heat up, the heat is absorbed by the block. Additionally, when the engine is cold the fits of critical parts to the sealing of the combustion chamber are not ideal, and often the pressure being generated leaks by the rings and valves at a rate that reduces the peak pressure, and therefore temperature, below what is needed. If the engine will start in a push start, all these effects are not given the time they need to interfere with starting and the engine fires up.

I have experienced this first hand numerous times. Once while exploring Vermont in the winter in the early '70's with a buddy, we were broke so we slept in the 220D overnight in a parking lot along a highway. I woke up and it was well below zero out. The car was not interested in starting and I hitched a ride to the next town and brought back a huge tow truck from a local garage that I was guaranteed would start the car. Well, it didn't and after about an hour we decided to push start the car even though the parking lot was covered in hard packed snow. It started in less than 10 feet of rolling room. It smoked and clacked something awful at first and then smoothed out and ran all day.

When blow by gets so bad you have to struggle to get the oil filler cap back on the vavle cover, I think you have a problem. To ward this off for a while, follow the board's mantra from Larry Bible - change your oil often and change it hot. In the winter it really pays to use a synthetic in my opinion, and there are many other opinions on the subject so do some research, and I use Mobil Delvac 1.

Also keep your valves adjusted (one of the better features of doing this yourself is you actually get to find out which ones are out of adjustment and by how much - when you pay someone to do it, well, you find out they are "all done" when you pick the car up, but you typically do not get a report on how far off they were and which ones were the bad ones. Keeping this information on file can help trend these kinds of wear patterns and point to problems before they cause big $ issues). It is a good idea to check your battery condition before the Winter sets in and, if you don't recall when the starter was last replaced, figure on a new starter in the future. If you buy a starter, I also highly recommend installing one from FastLane for a 1985 300D/300SD with the "heavy duty" description. This will spin your 240D engine at the right speed for starting for years to come.

Good luck and I hope this helps. Jim

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Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)

Jim

I now have a much better understanding of this topic. Your explanation was excellent and I really appreciate your lengthy response. I hope it helps other as well!!!

DITTO'S Thanks, Treeman

might change your brand of oil. the rings could be sticking.

i have never heard of valve guides causing blow by. but on a turbo motor there would be a lot more pressure in there to blow out and it would come to the oil filler cap.

that is a revelation. i never thought of that before. thanks.

now it makes sense why the turbo motors always seem to have leakage around the valve cover vent tube.

tom w

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.