Blow By Definition - PeachParts Mercedes-Benz Forum

McRoth · #1 01-07-2003, 09:47 AM

I keep seeing the term blow-by, but I've never seen a definition of what exactly it is. Would someone please explain it to me? I tried searching for it, but couldn't find a definition.

Thanks!!

engatwork · #2 01-07-2003, 09:58 AM

It is compressed fuel/air that is above the piston "blowing by" the rings into the lower part of the block below the pistons.

Nate Stanley · #3 01-07-2003, 09:58 AM

Blowby is caused by the movement of combustion gases in the cylinder getting past the piston rings and into the crankcase area.

A healthy engine's piston rings react to the pressure of the burning fuel and to a great extent are forced outward against the cylinder walls during the power stroke as the piston is pushed down- but as the engine wears , ring grooves, pistons and cylinder walls lose their ability to run within their working tolerances, and blowby increases.

Start your engine, let it warm up and open your oil filler, and you will see it as a vapor coming out of your valve cover.

Some of this is normal, lots of it indicates a worn engine, though in a diesel engine , it's not as critical as in a gasoline engine. Why? Burned blowby leads to carbon deposits that foul the spark plugs, but the diesel just sees it as more fuel and burns it.

Fisherman · #4 01-07-2003, 10:14 AM

So can someone tell me what is considered lots of blowby?
I just picked up a 123 300D and when I pull the oil filler I get a light cloud of vapors rising out of the filler opening. It kind of reminds me of an old steam locomotive but without the heavy chug, chug. More like when it is at rest at a stop.
It definitely is less than my 240 which sounds like a constant brrrrrrrrrrrrrrrrrr sound when I pull the cap.
What is considered normal or reasonable for blowby?

I tried a little test this morning. 16 degrees and I went out, put the key in the new car and did not allow the GPs to cycle but instead just turned the key to start.
Rotated for about 6 seconds and then it caught and fired on all cylinders. 1/4 throttle.
I think this is good, yes? I ask because I have a rough spot just above idle when in gear aned low power but apparently high compression. I know it might be one bad hole but this usually would show up in the blowby test wouldn't it? Also when I crack each injector line they all seem to make teh engine really go rough. I would thin that if it were one bad hole that the engine would not change significantly when that injector line was cracked. At least I guess this means that all cylinders are helping...
This one is going to drive me nuts I'm sure...

McRoth · #5 01-08-2003, 09:11 AM

Well, I checked the blow by last night & it seems to be an awful lot! Could the valves being out of adjustment cause excessive blow-by? I have a good set of wrenches coming in this week so I can re-do the adjustment because it won't start when left outside at night when it's below freezing, but otherwise runs fine. Is there anything besides new rings/pistons & liners that can help?

Thanks!

franklyspeaking · #6 01-08-2003, 04:27 PM

Blow by is increased whenever you allow the compression to escape the combustion chamber during the power stroke. The two most likely locations are past the rings (worn or broken) and past the not fully seated valves (bad seats/out of adjustment). Both of these conditions show up as low compression numbers when doing a compression test.

Triffin · #7 01-08-2003, 05:23 PM

My "Blow By" definition

What most vehicles are doing to me
as I head down the highway in my
non-turbo E300D ..

Jim in Ct ..

P.E.Haiges · #8 01-08-2003, 05:42 PM

McRoth,

Actually valves with not enough clearance betwen the cam and cam follower will reduce the Blowby. This is because the valves leak some of the the combustion gasses which reduces the pressure inside the cylinders and thus less of these gasses will escape past the rings.

Only replacing worn rings and resurfacing the cylinder walls will reduce Blowby.

P E H

rickg · #9 01-08-2003, 06:11 PM

I noticed that my engine has less blowby now then when I bought the car. I'm figuring it's because the rings are wearing in. The engine only had about 7000 miles on it when I bought it. Now has 32,000.

Nate Stanley · #10 01-08-2003, 06:49 PM

That's totally normal.

A couple of stories, please don't try this with your BENZ!

My father was a Cat tractor mechanic most of his life, and when he had to re-ring a Cat diesel in the desert, he would assemble the rings & pistons DRY into the cylinders. Ring seating was nearly instantaneous, and you ran less risk of gouging your cylinder walls due to sand sticking to oil-lubed parts during engine assembly.

A friend of mine in high school was heavy into dirt track race bikes. He would fire up a newly re-rung engine, and blip the throttle while letting the engine suck up a small amount of Comet or Ajax thru the carb throat. He always swore the engine was race-ready at that point, with rings being fully seated.

McRoth · #11 01-10-2003, 08:50 AM

Maybe I have something wrong, but wouldn't gasses escaping past the rings go into the bottom of the engine? Do they come back up from around the timing chain? I haven't seen one apart so I'm not sure if it's open through there or not.

Thanks!

P.E.Haiges · #12 01-10-2003, 03:43 PM

McRoth,

Yes the blowby gasses come up thru the chain cavity. That is why there are breather hoses on top of the valve cover to send the blowby gasses into the intake system to be burned.

P E H

Beagle · #13 01-11-2003, 01:21 PM

McRoth

Some facts and fallacies…

Blow-by – What is it?
As others have already stated here: Gasses leaking past the piston compression rings during the compression and power strokes into the crankcase. The suction provided by the manifold via the breather hose to the valve cover is sufficient to scavenge the crankcase to below atmospheric press. during the life of a well maintained engine. That is the whole story, just as simple as that! Blow-by is normal and significant on a new engine and would typically stabilize to a min. around 50,000 miles.

How much is too much?
In a nutshell the “break point” is when the breather can no longer cope with the volume of gas and a positive pressure builds up in the crankcase. Excessive blow-by goes hand-in-hand with poor compression. The first clues are usually oil seeping from the filler cap, leaking front or rear oil seals or in severe cases oil blowing out of the dip-stick tube. More subtle symptoms via pressure to the fuel pump stop diaphragm can cause rough idle or loss of power. A healthy engine has a negative crankcase pressure at all times.

So how to check it?
First a couple of fallacies: Remove the oil filler cap while idling and you will see the hot smoky gasses rising out and some oil spray from the valve gear. What you don’t see is the cool clean air sneaking back in simultaneously to replace it. If you have a sooty exhaust you will have sooty blow-by and this is no indication of the blow-by volume. Of course if your filler cap blows off when undone you have a shot engine – so read no further!

There has been some discussion here on how many seconds it should take to cut the fuel when blocking the breather pipe 5sec 10sec…whatever. Before you try this please consider the following. A crankcase pressure of 10psi will exert a load of 2.5lbs on your thumb. 10psi will exert a vertical load of 1200lbs on your valve cover and considerably more for the horizontal component. The story is worse down south. The time taken to stop depends on a number of variables of which blow-by vol. is only one – the strength of the rack return spring, the pump diaphragm / linkage friction, idle RPM etc. If there is excessive resistance you are just seconds away from blasting off the valve cover and at best you will distort the cover.

A number of years ago I had an Apprentice assigned to me in the R & D test shop at Perkins Engines whom I gave the task of recording data on a prototype engine we had on a water brake rig. Perhaps boredom led him to investigate what would happen if he blocked the breather. It blew off the timing case and the engine destroyed itself along with the Apprentices prospects at the company.

A simple, accurate and non-destructive method
With engine hot and off remove oil filler cap. Wipe cover clean and smear grease around the filler hole. Take a piece of thin polythene sheet ( an old bag is fine) and lay over the filler hole sealing with the grease. Start engine and let idle. The sheet should not have blown off. Now increase revs slowly to about 2000 and the sheet should now progressively be sucking into the engine indicating a negative crankcase pressure. In other words the manifold is sucking off the blow-by faster than gas is leaking past the pistons.

Once you have established that you have a neg. crankcase pressure as above you can take it a stage further if you wish and test under load (i.e. at max. torque) using the same method. This time you will need to tape the polythene in front and behind so that it sits snugly over the filler – since you will be driving the car. ( masking tape works ok). A short hill climb will reveal whether pos. or neg.
My own 20 year old 300D with 780,000km on the original engine still passes this test.

My humble apologies to Jim for the length of this post

engatwork · #14 01-11-2003, 06:46 PM

No apologies are needed here - that is some good stuff and is exactly the kind of information I enjoy reading

and like to see posted.
thanks Beagle

McRoth · #15 01-12-2003, 06:50 PM

Thanks again for all the explanations!! They really helped alot.

Well, I did the valve adjustment this afternoon & got my brother to help out. That made it MUCH easier!!! 5 or 6 of the valves were out of adjustment, all of them were tight I think. It started up easily after I got everything back together & it seems like it smokes less too. I probably won't be able to leave it outside overnight untli Friday night to see if it will start in the morning. I'll post the results when I do.
I wasn't able to check the timing chain stretch because I don't have a dial indicator yet. I did find one weird thing though, a bolt holding a timing chain guide, the one in the center right above where the valve cover sits, was loose. It had come out as far as the cover would let it! It still seemed to be holding the guide in, but probably not very well! It had a cylinder over it to space it. Anyhow, I tightened it back down, hopefully no damage was done.

Thanks again for everyone's help!