Smoke On The Over-run

[Styria]

Hi all, looking for advice and possibly some re-assurance. Most of us will be familiar with this situation. Drive at higher speeds on the Freeway, back-off, or go down a long hill with foot off the accelerator, and you'll see large clouds of blue smoke behind you. Quite embarassing, and also you feel humiliated - you have just passed a few cars at the rate of knots, you slow down and bang - smoke everywhere - well, not quite that bad.

The problem is obviously one of oil leaking past the valve stem seals which, incidentally, I had checked some two years ago and I was told that there was no need to replace them. Now, the car uses very little oil between oil changes, if any at all. The question I have is this : Other than the embarassment factor, am I causing any damage to the engine generally if I contine to use the car without replacing the seals ? I know that the petrol mixture can be influenced by 'oil contaminated' fuel through the system, and I have also used ATF on occasions in the fuel tank to keep the fuel system clean. It could well be that the ATF has done too good a job. Regards, Styria

 

[Mercules]

There is a possibility that I am not well aquainted enough with the Mercedes Benz engine... but in my experience large clouds of blue smoke on over-run is generally a terminal illness for most engines Styria!!!

Please confirm something for me - this engine, does is cough ourt a heap of smoke on hot or cold start-up?

Cheers,
Cam

 

[koan]

I'm inclined to go along with mercukes on this one, smoke on over run is an indication of oil being sucked past the rings into the cylinders due to the high vacuum.

Smoke on startup after standing for some hours is what is usually associcated with valve stem seal problems.

If rings are leaking there would be significant blowby, signs of which are oil around the crankcase ventilation hoses on the cam covers and oil in the air filter housing where the vent hose connects.

Having said that, valve stem seals are $2 or $3 each so there is no great expense in giving then a try.

koan

 

[Andrew280SEL]

Having said that, valve stem seals are $2 or $3 each so there is no great expense in giving then a try.

Well, 'phew' for about the surprising cheapness of valve stem seals and phew about the fact my 350 doesn't seem to have over-run smoke.

My 350 appears to have the classic valve stem seal symptoms - It tends to blow smoke when starting after standing for a little while, and it blows a nasty little cloud when 'booting' it. I think mine are really bad, so I'll have to replace them pronto.

You don't even want to know how fast the thing uses oil unless I drive it sedately!

Surprisingly, my high mileage 280 is quite different. I can boot that thing to the redline and there's no smoke that I can actually see...

 

[koan]

You don't even want to know how fast the thing uses oil unless I drive it sedately!

I'd love to know!

But have a look at the handbook that came with your car if you have it. It might have an oil consumption figure, and you might be surprised by how high it is.

If the oil level wasn't falling in an old engine I'd be thinking oil dilution.

koan

 

[SELfor50]

Been having the same problem Styria, glad you posted this.

Gonna get the VSS's done and hopefully that will fix it.

It usually happens when i'm holding it in first at bout 4k on the tacho then give it a bootfull and it dumps the smoke out the back.
Also happens on engine braking. However! Ol' Smokey the green one - pumped out f*kn TONNES of black smoke on startup and never stopped.


Styria - do you also have a bit of a smell of un-burnt fuel at all?? Or is that maybe oil. I know my mixture is a bit rich atm but in the situations mentioned above i seem to get the smell of un-burnt fuel.

If anyone tell's me it's the rings - don't tell me. I'll cry.

 

[WGB]

I think this sort of problem is endemic with old cars that are seldom driven.

The valve stem seals harden and seal less well.

If you are concerned enough to do anything about it (I probably wouldn't if the car is otherwise consuming little oil) I would do a compression leakdown test to check rings and valves and if OK consider changing the stem seals.

As a counterpoint - one of the "thrills" of my youth was my father telling us kids to look out the back window as he got to the bottom of a hill so that we could gasp in awe as the smoke poured out the back tail-pipe of the family Austin A30. I know he purchased the car new in 1955 and sold it with 30,000 miles on it so it must have been like this since new.

Bill

 

[Bandolero]

Re: smoke on over-run

A compression test will not tell you how good the oil-control rings are.
It will only tell you how good the compression rings are.
Also, if you are not using much oil, the oil rings should be ok.
I wouldn't worry about it.
My 280SE has the same symptoms as your car and I drive it everyday with no problems.
In fact I have done about 40,000 kms with the car doing this.
1, Bottom of hill, plant the foot and lleave a small cloud of blue oil smoke.
2, Start the car (after switching it off for a short while) after it is warmed up and it blows a small cloud as well.

 

[Styria]

Thanks for your response guys - some of it liked, others I didn't ! - for very obvious reasons. As most of you know, I keep a pretty good eye on what happens to my "Gleaming Beauty" to ensure she runs well, but I am also a firm believer in leaving things alone and not to hit the panic button.

Whilst I know there is wear in the engine, compression is reasonable and still comfortably within tolerances and readings as specified in the Technical Data Manual. Judging by the colour of the plugs, the fuel mixture is spot-on, the car is an instantaneous and smoke-free starter in the morning, but lately I have noticed that, in the afternoons, I have to churn the engine for about four to five seconds and it takes another ten or so seconds for the engine to run cleanly.

However, I have worked out a way to overcome this by priming the fuel pump for about five seconds or so, and this seems to eliminate the hesitant starting. I do this by pulling the selector lever into reverse, hold the key in the start position for about five seconds and whilst doing that, move the selector back to 'Park'. It could well be that the accumulator (not replaced in 12 years) might be getting a little tired. Also, it is some time since I have changed the fuel filter, and it could well be that the engine is just not getting a comfortable quantity of fuel, causing a lean mixture from time to time which, in my opinion, may cause the upper cylinder area to run somewhat 'dry'.

One of our new members, Enodia2, has also got a slight hot-start problem with his very nice 450SL, and perhaps he may try my method of priming the system in the manner referred to above. This is just an aside.

I am going to keep an eye on my situation, and will report further in due course. Thanks guys in the meantime. Regards, Styria

 

[WGB]

Styria - Sounds like it might be time for a new Fuel pressure accumulator or new plugs.

My 6.9 starts the same way as yours but my 450 starts the instant the starter bites even if it has been sitting for two weeks.

Bill

 

[Styria]

Hi Bill, I suspect you're pretty right - time for both items. As I said, will keep you abreast of the situation. Regards, Styria

 

[koan]

Styria - Sounds like it might be time for a new Fuel pressure accumulator or new plugs.

I wonder if that will fix the starting difficulty?

I've been doing the Styria priming trick for years to get a good start after standing overnight.

Replaced the fuel accumulator and the primary regulator, the injectors and the cold start injector don't drip.

There is a drip test at the fuel return connection at the K-jet body, I checked that and it's in spec.

With a pressure gauge connected pressure slowly falls but again it's in spec but by the next day pressure is zero.

About the only possible place left for a leak is around the metering piston which would drip into the inlet manifold.

koan