Blowby Question - PeachParts Mercedes-Benz Forum

rrgrassi · #1 08-19-2007, 01:27 AM

I know my car has some blowby. The cap dances, but is in no danger of falling off or bouncing off.

Here is the question...

Why does the cap lay flat and not bounce when I raise the idle up by hand to say 1500 rpm?

#2 08-19-2007, 01:34 AM

My guess is that you have created a slight negative pressure downstream of the air filter and it's pulling the blow-by through the valve cover vent hose and the oil separator.

tangofox007 · #3 08-19-2007, 09:38 AM

Quote:

Originally Posted by rrgrassi

I know my car has some blowby.

Any car with a reciprocating engine has blow by.

kerry · #4 08-19-2007, 10:26 AM

We had an experiment going a few months ago measuring blowby into a 30 gallon trash bag and comparing the amount of time it took to fill it up. Thread must still be around.
There is a device for measurement of blowby but I've never actually seen one.

tangofox007 · #5 08-19-2007, 10:41 AM

Quote:

Originally Posted by kerry edwards

.
There is a device for measurement of blowby but I've never actually seen one.

It's called a compression tester.

aklim · #6 08-19-2007, 11:22 AM

So far, I have removed the hose from the crankcase to the turbo on my 606. Reason is that it puts an oily mist into the pipes. Well, when it meets the EGR smoke, it becomes a sludgy mess in the intakes. Now it has a copper pipe to lead it to the ground. Learnt that trick from International.

I'd say your problem is not the smoke so much as the vibration causing the cap to dance. At higher RPM, it will smoothen out the engine vibes so the cap is fine to sit.

kerry · #7 08-19-2007, 11:57 AM

Quote:

Originally Posted by tangofox007

It's called a compression tester.

I was referring to a separate device referred to in the FSM.(I think) It only measures crankcase pressure or blowby.

sailor15015 · #8 08-19-2007, 12:47 PM

Quote:

Originally Posted by rrgrassi

I know my car has some blowby. The cap dances, but is in no danger of falling off or bouncing off.

Here is the question...

Why does the cap lay flat and not bounce when I raise the idle up by hand to say 1500 rpm?

Make sure you plug the crank case breather when you're watching the cap dance or it doesn't mean anything because the air can still escape out that way.

rrgrassi · #9 08-19-2007, 09:44 PM

Quote:

Originally Posted by sailor15015

Make sure you plug the crank case breather when you're watching the cap dance or it doesn't mean anything because the air can still escape out that way.

That I did not know to do. I will try that.

The slightly negative pressure sounds right as well.

I would have answered sooner, but I found a nice rust out under the battery. I have it taken care of now, thanks to older posts.

Since I do not have a welder, after preddingthe surfaces, I made a sandwich out of fiberglass mat, pounded out tin can and more fiberglass mat, then topped with jb weld stick and covered with rubber undercoating, until I can do something more permanent. I am also going to go with either an Optima or Odyssey battery. I also replaced the battery tray.

sailor15015 · #10 08-19-2007, 10:48 PM

I didn't know that either when I did my first blow by test. I thought my engine was doing great since the cap didn't move at all. After plugging the breather it still didn't move that much, even when revved so I'm still satisfied.

hofler · #11 08-19-2007, 11:36 PM

I have taken possesion of an 82 300D turbo Diesel and they guy that gave it to me (yes he just signed over the title and gave it to me) claimed that it suffered from bowby. As he described it he claimed that once it got up to highway speed and had been there for an hour or so then it would begijn to smoke and loose power. This is what he called blowby. Is this a manifestation of blowby?

I have not had it out on the highway as I am trying to iron out tranny issues.

aklim · #12 08-19-2007, 11:43 PM

I would think if it had that much blowby, I would wonder how it could get up to speed and more importantly, stay at speed for an hour before the rings gave out. I think your problem lies elsewhere. Best way to check is to run a ompression test and that will tell us if you are having excessive blowby

hofler · #13 08-20-2007, 01:41 PM

Aklim,
Thants pretty much what I was thinking, why would it wait till it got hot to start that? Also I figured that things would expand as they got warmer and that would reduce blowby. Of course I could be completely wrong as this whole diesel thing is new to me.

Anyhow, for the moment she is off to the shop for a tranny evaluation. I monkeyed with as many different things as I could think of and she refused to shift past second gear. So I have to defer to a professional. Lets hope for the best.
I'll hopefully be able to take her out on a good long run when I get her back. Then we shall see first what happens after an hour on the freeway.
Cheers,
R

rrgrassi · #14 08-20-2007, 01:49 PM

Even with the blowby, The car is easy to start, hot or cold.

Hofler...do not admit to the shop you did anything. Remember the labor rate guide...

$100 per hour if we do the work.
$150 per hour if you help.
$200 if you tried yourself and then brought it in.

DieselAddict · #15 08-20-2007, 02:30 PM

Quote:

Originally Posted by aklim

I'd say your problem is not the smoke so much as the vibration causing the cap to dance. At higher RPM, it will smoothen out the engine vibes so the cap is fine to sit.

I'd agree with that description and I have mentioned something along those lines several times on this forum. I would also say the best indicator of blowby is cold-starting ability or a compression test. If it runs good and doesn't consume much oil, why worry about blowby?