"Blowby" question (what exactly is it)
 

OK, I'm in the market for a 300 series MB diesel, and several of the kind members here have said that a good way to tell if the engine is OK is to remove the oil filler cap and check for "blowby". Being new to diesels, I think I know this is unburned oil getting past the rings, but I'm not 100% sure what's normal and what's not. Exactly what is it that I should be looking and/or smelling for?

Robby

Pull the air cleaner and check for oil at intake manifold. Also remove the oil filler cap while the engine is running - then try to ballance the cap on the opening. If it just dances in place or does not jump too high into the air, then blowby is not an issue.

It is pressure from combustion gases that manage to get between the piston rings and cylinder walls. These gases are then in the crankcase and thus also under the valve cover. All engines have some, and diesels run at much higher compression ratios so the potential is certainly there. A good engine has a little bit - at idle when you remove the oil filler cap it sounds just a little louder and there should even be a little suction (if you put a piece of cardboard over the hole and it sticks that's real good) On the other hand, if when you take off that cap, the sound is quite a bit louder (you hear a pah, pah, pah, pah, pah, pah sound in time with the engine ) AND the loosened cap dances around in place on the valve cover - or even worse, won't stay there, I would say that's too much blowby. Other signs are there, such as oily buildup/drips under air cleaner, dark oily plastic vacuum lines, etc.

I've got to disagree with Dieseldog here, if the blowby knocks the loosened cap off at idle, that's NOT OK.

There is no accepted means to measure blow-by and therefore there can be no hard and fast acceptance criteria for blow-by. In general all internal combustion engines exhibit blow-by as the sliding contact between the cylinder and the piston rings is not a zero leakage interface. Surface defects from wear typically increase blow-by as they interfere with the formation and durability of the oil film seal that forms in the designed in surface finishes of the interface. In a Diesel, higher than "new condition" blow by can typically be tolerated with the engine running mainly because the blow-by products can be routed into the intake and consumed with the Diesel fuel without adversely affecting the performance, until the blow-by is pretty extreme. The first sign of blow-by being a problem is usually cold starting, when the leakage effect is highest because the engine is turning the slowest (more time for pressure to leak past the defective seal) and the heat of compression is vital to achieving ignition. Once started, the engine speed usually makes the threshold for acceptable blow by increase. I have seen some pretty severe cases that, once running stink a bit, but run smoothly and with reasonable power.

So, a good guide might be that if you can start in cold weather, blow-by is likely not a concern for you. This assumes the battery and starter and oil and fuel system and glow plug system are all up to snuff as any one of them in less than good condition can lead to extra stress on the others, and a starting problem.

On my 1982 240D I have a enough blow-by to make the cap, once unscrewed, float off the valve cover and fall to the ground. No geyser of oil mist but not something that is hard to see either. This car starts right up in single digit Fahrenheit weather, so I would guess that amount of blow-by is not compromising performance. I get oily build ups around the cyclone separator connections and some forced oil leakage under the valve cover gasket between valve adjustments (typically 15,000 to 20,000 mile intervals). The engine can look pretty messy, but cleans up nice when I do service (I learned from others on this forum to wash the engine before I do work on it, like valve adjustments and recommend that for any DIY maintenance under the hood).

Hope this helps, Jim

Blow-by - some facts and fallacies,...

This will answer a lot of questions about blow-by.
-Joe

>>>I've got to disagree with Dieseldog here, if the blowby knocks the loosened cap off at idle, that's NOT OK.

....or maybe it's the engine mounts too?

"There is no accepted means to measure blow-by"
UH,,,, actually I think MB has a machine specified in the FSM for this.... ( I think )....

The most accurate method for "measuring" ring blowby in a piston engine is with a cylinder leakdown test. Although, this test has its shortcomings as do other methods. Leakdown tests should be done in conjunction with crancase vacum measurements.

I've heard that piston rings gummed up by lots of city driving can produce blowby/oil consumption symptoms similar to those of a worn-out engine. In other words, some hard driving can sometimes bring a sickly engine back to life. Any opinions on that theorem?

My '82 coupe just turned 200K miles. It's had a blowby problem for the 70K miles I've driven it. On a couple occasions the crankcase pressure has pushed the dipstick far enough out of the tube to permit some serious oil expulsion. It also consumes about a quart of oil every 1200 miles -- 70 mph freeway miles, mostly.

On the other hand, it has very decent acceleration and a clean exhaust. Driveability is very good, and it idles well. I've done the epoxy fix on the valve cover baffle and installed the one-piece vent hose

It's a southern California car, with plenty of rotten rubber replaced. Makes me wonder if it would be of benefit to replace the valve stem seals. Again, any opinions on that?

Thanks for any thoughts.

Russ M.

Russ, It sounds like your " oil separator" on your coupe is stopped up...
it is supposed to catch the oil in the blowby and let the air out...
Something is not right about the ' release mechanism ' for the blowby on your car..... I think....

Maki,

Valve stem seals have nothing to do with blowby, but replacing them may decrease you oil consumption.

P E H

Thanks, guys. I've looked the separator over pretty carefully; it seems ok. I am leaking a bit of oil around the separator drain tube, though.

I was pretty sure the valve stem seals weren't an issue, or I'd see more blue smoke at startup. I really don't see smoke at all except when I've got my foot in it pretty good. Or, at night when the engine is cold and a car is behind me with headlights on.

Russ M.

The poster of this thread had indicated he is looking for an SD or SDL. Based on my experience with the SD over 8 years, I would still advise him to steer clear of a car with significant blowby. Now, I have to admit that I agree with everything that Jim said about his 240D. My 82SD had less blowby than that when I bought it and gradually increased. Eventually, I did have cold starting problems and really wanting to keep the car for a long time, I decided to rebuild the engine. When I parked it, it still ran OK. The turbo engines are I believe more susceptable to increasing blowby because of the increased load on the cylinder walls and pistons. Also, positive pressure in the crankcase restricts oil flow returning from the turbo, which in the long term will have an effect. Since a prospective buyer is in a different position with respect to attachment to any particular car, they can choose according to their needs