What is blow-by?

I have a general idea what it might be, but I'd like to know what it really is. Ok, today, after driving, I opened the oil cap while the engine was still hot. I saw light wisps of white smoke coming out of the oil hole. It wasn't a thick cloud, but the smoke was noticeable. Is this blow-by, and if so, is this normal? What causes it? Thanks.

Cossie,

If you remove the radiator cap immediately after shutting down a hot engine, it is normal to see smoke. It is basically oil vapor and not necessarily a negative indication.

Blow by is basically a condition where an excessive amount of cylinder pressure which is produced from combustion "blows by" the rings into the crankcase. Whenever the blow by becomes really excessive, it will be blowing oil out the valve cover vent into the intake, drawn into the cylinders and burned. It also can cause problems with the crankcase ventilation system causing leaks and other problems. This is typically associated with a high mileage or improperly maintained engine.

Hope this helps,

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Larry Bible
'84 Euro 240D, 516K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in
Mercedes automobiles

Cossie,

Larry is 99% correct. He made a typo and said "Radiator Cap". I only mention this in case you did not realize it was a typo and if you open the radiator cap while the engine is hot, you could get severe burns from the hot radiator fluid and steam. Removing the oil cap while the engine is hot is not a problem.

Regards,

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Deezel
87 300TDT
150,000 miles

Deezel,

Thanks, I had my head up my .... when I answered the question. That mistake could definitely cause a safety problem.

Many times I'm answering something like this when I'm waiting on a compile or rebooting another machine or something. It's kind of sobering to think what could possibly happen as a result.

Thanks,

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Larry Bible
'84 Euro 240D, 516K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in
Mercedes automobiles

[ncarter]

So, is there an easy way to tell if I've got too much blow-by? By easy, I mean requiring no $1000-dollar tools, and requiring no more than a couple hours of work; or possibly by evaluating the car's performance, oil consumption, etc.

I've got the same symptom as described above - the wisps of smoke upon removing the oil filler cap from a hot engine - but the crankcase pressure is also causing some minor oil leaks.

I think I'm going to plan on rebuilding this engine next summer.

Thanks.

- Nathan
'83 240D, 250k miles

ncarter,

Blow by is not really a measurable thing. The best way to determine engine condition is a compression check and oil usage.

Good luck,

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Larry Bible
'84 Euro 240D, 516K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in
Mercedes automobiles

If you take off the cap on your valve cover while the car is running, and some serious smoke comes out - you might have a problem.

Harvey Sutlive

Cossie,
I don't know if you are experiencing what I have on a previous car but if you are then you may want to consider the following. I had previously owned a 81 300TDT that was blowing oil through the crank case vent tube into my air breather. It was so bad that I was having to change the air filter with each oil change. In an effort to remedy this problem, I thought long and hard as well as looking for some help. I concluded that the only reason for this was that the oil was not flowing in the passages that is was intended to and as a result found the path with LEAST RESISTANCE. With that in mind, I came across a product that seemd appropriate...STP Engine Flush. I began using this product on a car that had 250,000+ mi. with some reservation...as most product of this nature actually recoment not using on cars over (?),000 miles. After the first use, I noticed immediate change. By the time I had used the product five times consecutively (which is not necesarilly recomended on the prod.) all of the "blow by" as I refer to it had stopped and no additional oil leaks were incurred. I discontinued the use of the product and the car continued to do very well. I later sold that car in order to purchase a 300SD and am treating it the same way with excellent success! I hope that you can resolve your problem as I have.