My brother has a 1984 300SD. We were talking about blow-by and he brought over his SD and removed the oil filler cap while it was running. There was quite a bit of smoke coming out.

How worried should he be and how serious is this?

The vehicle has around 275k and loses less than a quart of oil between changes.

Thanks for your help.

-David

__________________
My Fleet

1987 Mercedes 560SEL 178k [COLOR="Red"]SOLD
1996 Chevrolet Suburban 140k
1977 Cadillac DeVille 98k
1996 Cadillac Fleetwood SS
1985 Mercedes 300SD 303k SOLD
2000 Corvette 27k SOLD

I wouldn't worry. You can try just loosening the cap and leaving it there. Raise the RPM a bit if necessary to cancel out vibrations and look for any motion. If the engine starts and idles good when it's cold, it has good compression, so even this test may be unnecessary.

__________________
2004 VW Jetta TDI (manual)

Past MB's: '96 E300D, '83 240D, '82 300D, '87 300D, '87 420SEL

Dont worry & just drive the car.

Im with dieseladdict

Don't worry, be happy! There's a tune there somewhere!

If it starts well in cold weather and consumes minimal oil, I can't see what he would need to worry about. The more I see of these old cars, I am becoming more and more convinced that it's all about condition and maintenance.

Has anyone not seen smoke when removing the cap on a hot diesel engine? I'm not positive why it does that, maybe it's due to the oil squirting against the bottom of the pistons for cooling. Anyone have a good theory on this?

__________________
Sam

84 300SD 350K+ miles ( Blue Belle )

On my 240D I can loosen the cap and it dances like a pressure cooker cap. Good or not, it still starts fine and runs ssslllllooooowwww like it should.

a little is to be expected.

a good one it will just drift out a bit.

a bad one will smoke like a steam locomotive. but still might run and start fine. it is all a matter of degree. if it is really good you can pay more. if it smokes a lot use it as a bargaining tool to get the price down.

tom w

__________________
[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.

What is the fix for excessive blowby? Mine seems to be getting more difficult to start when cold, and I am just curious what all needs to be done to cure it?

__________________
87 300D - Running on Veggie oil; 260,000 plus miles; Original #14 head

There's no "fix" for blowby, other than an engine rebuild. But before you start panicking, hard starting doesn't have to be due to blowby. Check your valve clearances, fuel filters, air filter, battery and starter condition, and glow plugs. Also be sure you're letting your glow plugs work well past the light before starting the engine cold.

__________________
2004 VW Jetta TDI (manual)

Past MB's: '96 E300D, '83 240D, '82 300D, '87 300D, '87 420SEL

my 79 300D had been holding the driveway in place for a month. the oilcap had been doing a smoky little dance before she stopped running and I was worried I was in for big service, possibly even a new engine...
as it turns out, the gp relay was fried and kept the gloplugs on, thus burning out the gloplugs. so there wasn't enough heat to start and not enough juice in the battery to continue cranking fast enough to overcome the lack of heat.
I replaced the gp relay (big thanks to hawthorne90250) and upgraded to the parallel pencil gloplugs, also adjusted the valve lash to be sure (most were out of adjustment). now she starts up on the first try and runs like a top!
I still have smoky, dancing oilcap but only at idle. just a few more RPMs and it almost stops dancing...

__________________
'79 300D "Jaeger"
'61 VW Baja "Frosch"
'92 Honda Shadow 600
'80 Honda 185 (cafe project)

All the time actualy. My 603 with 254k on the clock has a slight amount of suction if you stick you hand over it. So does my friends 606 with 160k on it. Only shot or tired (or green engines maybe?) diesel engines have blow by.

As for the original queston as long as it runs good forget about it.

The only fix is either to rebuild the engine or sell the car.

__________________
1999 SL500
1969 280SE
2023 Ram 1500
2007 Tiara 3200

I think whether smoke comes out of the oil filler hole or not is meaningless and depends on the weather condition. I think a lot of it is water vapor and the same stuff that comes out of your mouth when you exhale in cold weather. As I recall, my nearly new Jetta had a bunch of smoke coming out when I did this test while my 240D was smoke-free but that was on different days and they're different engines. The E300 had a little bit coming out. All 3 cars needed varying amounts of engine speed to make the cap stop dancing. The TDI engine doesn't have as good engine mounts and it vibrates a lot so I had to rev it over 2000 RPM to make the cap stop moving. The E300's cap on the other hand was just sitting there even at idle. The 240D needed a little bit of a rev to make the cap stop moving. I'd say if you can't get the cap to stop moving no matter how high you rev it, then you may have a problem.

__________________
2004 VW Jetta TDI (manual)

Past MB's: '96 E300D, '83 240D, '82 300D, '87 300D, '87 420SEL

Almost every old MB diesel I have ever seen has blowby. While they probably did not have much if any when new, these cars are over 20 years old now. As long as the car starts and runs well, then dont give it much thought.

__________________
1981 300D 147k
1998 VW Jetta Tdi 320k
2001 Dodge Ram 2500 141k
1979 300D 234k (sold)
1984 300D "Astor" 262k(sold)
Mercedes How-To and Repair Pictorials
I love the smell of diesel smoke in my hair

This is what I have heard:

They designed these automobiles with a PCV system complete with breather hose, oil condensor, etc. to deal with what the engineers knew was happening FROM DAY ONE.

Blow-by gets worse when the rings wear, but it was always there.

Yeah, actually on the larger engines they measure crank case pressure. They have this pretty cool guage, I have seen it done once. As the crank case pressure goes up the rebuild time grows near.

I don't know if you can do this with MB's, I know they do it with large Detroits, Cats, and Cummins ect.

Thats why the Walker Airseps work so well, the create negative crank case pressure and reduce oil leaks.

__________________
1999 SL500
1969 280SE
2023 Ram 1500
2007 Tiara 3200