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  #1  
Old 02-17-2008, 02:37 PM
exhebetche's Avatar
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Blow-by on OM617

What's normal, and what should be worried about?

It's hard to find info on this, but from what I have read so far the 310,000 mile 300D I have is in for a world of hurt. Found a youtube vid suggesting that my engine is on it's last legs: http://www.youtube.com/watch?v=vbg6Qn3-IOA&feature=related

A test outlined there suggests taking the rebreather hose off, and plugging the valve cover hole with a finger. Seconds are then counted until the engine stalls, and how long it takes to stall is the indication of the engine's condition. A new engine is supposably 30 seconds, with a good engine being over ten seconds. Mine was 6 seconds warm .

Any opinions offered would be appreciated.
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  #2  
Old 02-17-2008, 02:41 PM
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Here's my opinion: Don't use that test. It's not a very reliable test and puts pressure on all the seals in the engine and could cause a seal to blow out.
If there are no other problems with the engine, don't worry about the blowby.

The YouTube engine has virtually no blowby. I've seen 617's puff like steam locomotives yet still run fine.
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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
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  #3  
Old 02-17-2008, 03:05 PM
300SDog's Avatar
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Yeah man, nobody's ever heard of diesel engine dying from excessive blowby. Unless it's got other symptoms yer 310k mile 300D oughta see 410+ one day with regular oil changes. Dont be duped by "high mileage syndrome" that claims ALL engines are near death at/near merely 299k miles.

Meanwhile if you like weird mystical blowby tests, try the equally scientific "poorman's compression test" based on cranking the engine without glowing the plugs - say after engine's been warmed enough to barely move the temp guage needle like 1/100th inch where if it fires up immediately with zero glow then it's got decent compression.
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  #4  
Old 02-17-2008, 03:31 PM
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* I agree with Kerry. If you're really curious about how well your combustion chambers are sealing, use the primary test that's in every Mercedes shop manual, the cylinder leakage test. Testing a cold engine with a cylinder leak-down tester and, while you're at it, also with a compression tester (w/ fuel delivery shut off) will give you a "worst-case" set of numbers. These tests will require some basic test equipment and several hours, but the readings will be a useful baseline.
* Then just drive the car. Blowby carries atomized oil out to the oil separator in the air filter housing. More blowby carries out more oil, so be sure to check the air filter and housing drain tube a little more often (as well as the engine oil level). Diesels run better with a little oil in the combustion chamber, so driving won't be as much of a problem as cold weather morning starting. But then there's lighter weight oil, block heaters, and garages.
* If you think power is down, be sure your transmission is shifting correctly to make the best use of the power you've got. There's one more condition that I've seen a lot. Brake calipers dragging. Nothing will make a slug out of a diesel more than brakes that don't release all the way. Flush the brake hydraulics so the pistons move freely. Clean the pad lands so that the pads move freely. Instant "tune-up" for your diesel. Oh, and air the tires up. Zoom-zoom.
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  #5  
Old 02-17-2008, 07:46 PM
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Thanks everybody, nice to know that the beige brick wasn't a 500$ mistake!
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  #6  
Old 02-17-2008, 10:47 PM
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Ok, what would you all consider to be excessive blow by?

My SD puffs like the mentioned steam locomotive, through either the vent hole, or even through the much larger oil fill cap, if it's open.
It now has enough pressure that it blows oil out under the valve cover gasket, and even out around the dipstick, lately!

I really need to find another MB diesel owner to compare notes with, and let look at this. I've worked on other cars for a good while, but I still don't know what is normal, tolerable, or "fix it now!" on the MB's. Some things that I just don't pick up from reading commentary here in the forums.
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  #7  
Old 02-17-2008, 11:14 PM
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Quote:
Originally Posted by hey_allen View Post
Ok, what would you all consider to be excessive blow by?

My SD puffs like the mentioned steam locomotive, through either the vent hole, or even through the much larger oil fill cap, if it's open.
It now has enough pressure that it blows oil out under the valve cover gasket, and even out around the dipstick, lately!

I really need to find another MB diesel owner to compare notes with, and let look at this. I've worked on other cars for a good while, but I still don't know what is normal, tolerable, or "fix it now!" on the MB's. Some things that I just don't pick up from reading commentary here in the forums.
Have you done a compression check?
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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13
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  #8  
Old 02-17-2008, 11:19 PM
ForcedInduction
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Blow by is an arbitrary guesstimation of an engine health. There is no "excessive" amount.

What matters is compression, how well the engine cold starts and oil consumption.
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  #9  
Old 02-18-2008, 12:42 AM
hey_allen's Avatar
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Quote:
Originally Posted by kerry View Post
Have you done a compression check?
Yes, but it was when I bought the car, 2 years ago. I am waiting on a replacement set of heat shields, and will be running the compression checks again, hopefully this week.

When I originally did the check one cylinder was a little out compared to the others, by about 40psi, but that put it around 240-250, if I remember correctly.
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'82 300SD 303k miles "Das Projekt" (replaced steering bits and newly re-engined.)
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  #10  
Old 02-18-2008, 01:02 AM
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Some of the blow by could be coming from worn exhaust valve stems/guides which my have little or no effect on the valves sealing compressing in the cylinder.
If your compression was marginal besides the hard starting I would think that you would be having some abnormal smoke issues until your engine warms up. Is this happening?

My Father had an old 68 Plymoth Valiant w/144 cu in slant six station wagon. The valve guides were worn enought that if you left the valve cover breather on oil would start to come out from under the pan and valve cover gaskets. The car was driven without the breather and the blow by bringing tears to your eyes. I barrowed this car that otherwise ran great and took it for a 4 hour trip (with the windows open) on the Freeway. I recorded my milage and was shocked to find that I averaged 26 miles per gallon on the the trip. What was going on in the cylinder must have been OK.
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  #11  
Old 02-18-2008, 01:06 AM
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Valve stems/guides are not going to have an affect on blow by at idle as there is no measurable positive or negative pressure to go past them.
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Old 02-18-2008, 01:06 AM
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