Blow by

[jwduffey]

I have a 1980 240D with about 200,000 miles on it. It runs great and has a lot of pep for a 240. I recently switched to Amsoil synthetic diesel oil. The car had no leaks before the switch, but I've noticed increased oil consumption, so my first thought was I've sprung a leak. I noticed the block had some oil where the manifold connects to the it, but not enough to leak on the ground. I removed the air cleaner housing to get a better look and it appears blow by is so excessive that oil is accumulating in the exhaust manifold and leaking from the seal where it connects to the block. Before using synthetic oil the whole engine was dry as a bone. What would cause blow by in the first place? Does that mean my motor is shot? How do I fix it, a complete rebuild??? The engine now uses about a quart every 400 miles! Before synthetic, maybe a quart every 1,200 miles. I guess I will switch back to Rotella T, but I sure have enjoyed the smoothness and quietness of the engine since using synthetic. The cars runs so well it's hard to imagine I have a serious problem. Any suggestions on what I should do would be greatly appreciated.

I can only theorize about what has happened. I'm not sure about this brand of Synthetic oil, but I know that Mobil One does a ridiculously good job of eating away ANY varnish and deposits that may be present in an aged engine. Much of the glaze on a cylinder wall, at least the part that fills in the grooves and voids in the cylinder walls is actually a deposit from combustion. Possibly the synthetic oil removed this glaze allowing blow by.

Regardless of the reason for the blow by, I expect that the only way to fix it is sleeving the block if the pistons check out okay, or boring and using oversized pistons. If you have good oil pressure, I would expect that the crankshaft will not need turning, but you should plan on a valve job with new valve guides.

These engines are expensive to rebuild even if you can do the work yourself. I have my daughters 300D apart right now, and expect to have about $1,800 in the rebuild, and that's with me supplying the labor.

Best of luck,

[DieselHead]

I've been thinking about switching to the Amsoil or Mobil 1 synthetic oil. As far as I know, I have a perfectly healthy engine with only 125,000 miles on it. If I do decide to make the oil switch, and I get the blowby (so far I have virtually none) that JWDuffy got, could I switch back to Rotella and assume that the blowby will go away again? Thanks.

I've wanted to try synthetic for quite a while but am always so hesistant at the store because of the risks.

Alex

DieselHead,

If it were me, I wouldn't take that risk on a bet! And I feel quite sure that switching back to Dino oil will not correct this problem.

That's my opinion. Stick with the Rotella T. If it ain't broke, don't fix it.

Have a great day,

[psfred]

jw:

Get a compression check and leakdown test done -- this will tell you if the cylinder walls or rings are leaking. The other source of blowby is worn valve guides, something I'd not be too shocked to see at 200,000 miles.

Do you get a puff of blue smoke when you start up after an overnight? If so, you also have bad valve stem seals.

The one drawback to synthetic oil in an old engine is the superior cleaning action -- removes ALL the varnish, etc, and if that is what is keeping the valve stems seals working, guess what!

New valve stem seals may fix a considerable amount of the oil consumption problem -- vast quantities of oil can get suspended above the exhaust valves and hence blown into the intake if they are bad. If the guides are worn, though, this will only help, not fix. Same goes for the amount sucked down the valve stem on the intake.

A valve job isn't too horrible -- just make sure the shop uses MB guides and doesn't ream them out to give "chevy" clearance -- the problem will be the same.

"Normal" oil consumption is less than a quart in 500 miles, so you aren't in huge trouble yet, but I really personally don't like that much.

Peter

[DieselHead]

Larry and Peter,

The reason I think my engine is in good condition is because it pulls extremely strongly, and does not consume a drop of oil. I just did an oil change with Rotella yesterday and the oil level was at the same level it was 4000 miles ago. I overfilled it last time I did the oil change (in my haste) about a half quart, and yesterday, when I checked the oil before the change, it was still a half quart high. The car has been garaged all it's life and I was thinking about the synthetic switch because I figured if I were going to do it, I should do it while the engine is still "young". Any thoughts? Thanks. Right now I get very very minimal blowby. Misting if that.

Alex

[psfred]

Alex:

My brother put Mobil 1 (15W50) in his 87 300SDL -- wasn't using any oil before, still doesn't. I think something is wrong with his boost controller now (high pitched whistling, sounds like escaping air) and poor power, but then he still has the trap oxidizer on there, too. No oil consupmtion.

I put Mobil 1 in the 220D -- didn't help with the oil consumption much, but it seems to have better compression, starts better, and has more power. Uses a quart of oil ever 100-200 miles, has horrendous blowby, needs a rebuild, but the body is shot, so......!

Just put Mobil 1 in the 87 300D in an attempt to free up the rings -- low compression in general and at wear limit on #4, so #4 doesn't always fire. Don't care if it causes blowby or leaks since I will probably have to take it apart anyway.

You can expect slightly higher oil consumption with most synthetics -- more works up the rings (a good think for engine life!) and it can shrink old, hard o-rings and seals enough to slip out more -- the cure is to replace the seals, of course. If you engine is indeed tight, you won't have any trouble, but I'm not going to recommend anything!

Peter

[jwduffey]

Thanks to all that have responded. psfred, there is no blue smoke on start up. Actually, there is very little smoke at all. Last night as I was driving around I would accelerate quickly from stoplights (has a 4speed stick) then look in my side view mirror to see if there was any smoke. The lights of the cars behind allowed me to see, but there wasn't much smoking at all. I just checked my records and I had a valve job done less than 3,000 miles ago. I'm serious, this car runs great- has great oil pressure, always starts on the first turn of the key and runs almost as smooth as my gasoline powered cars. Tomorrow I'm gonna call my mechanic to schedule a compression check and leak down test. Whatever the problem I hope it's not too expensive to fix.

[psfred]

jw:

Check to make sure the oil trap tube in the valve cover is still intact, and that the oil trap in the air cleaner isn't leaking, and that the foot valve on the bottom of the drain tube from the air cleaner is still there. Synthetic can cause old hard rubber to break. Also check for oil leakage at the upper oil pump bearing (where the distributer would go if it were a gas engine, up fron below #1 injector). I have had quite a leak there, requires a new o-ring under the large aluminmun crunch washer.

You can check for excessive blow-by by slightly pulling up the dipstick while the engine is running -- distinct chuffs of vapor indicate something blowing. I don't recommend taking off the oil filler cap -- you will get oil everwhere as it flies off the camshaft.

Otherwise, you probably need valve seals. If you don't have smoke at full throttle, you have good compression.

Peter