Blow-by: What is it, how do I check it, ect?
 

Well, I've been seeing a lot of threads talking about how to fix excessive blow-by and whatnot lately, but I haven't seen anything about what it is, what causes it, what damage it can cause and all that good stuff. Still relatively new to the diesel world, and just lookin to enlighten myself. You know, get my learndedness on.

Thanks!

When one of the engine's cylinders fires and begins the power stroke the cylinder pressure goes way up. Some of that pressure leaks past the rings into the crankcase or "blows by" the rings. The slight pressure rise in the crankcase is relieved through the blowby tube or crankcase ventilation tube. This is just smoke from combustion and all piston engines make it.

A severely worn engine produces a lot more because the bore and the rings are worn and less affective at sealing the upper cylinder pressure.

Much more is produced when the engine is under load.

It's a normal thing and gradually gets worse as the engine wears. Hard to say what is too much. Some engines blow a cloud when the oil cap is removed (the venting happens there with the cap off and through the blowby tube with the cap on) but still run fine.

As the blowby passes through the crankcase it will also pickup vapors from the oil or condensation, and carry that out too.

My old Mercedes used to have the blowby routed through the turbo so I never really had any idea how much it made till the oil cap was off, and then wow!

Gas engines don't show much because they have lower compression and they have a crankcase ventilation system that sucks most of it into the intake and back through the combustion chamber to try and clean it up.


John

Pull the dipstick, place a 3/8" x 6' clear plastic tube over the dipstick tube such that it seals tightly - crank the engine, let it idle, place the other end of the tube in a pail of water - if engine is ok it can bubble a little - if it bubbles and froths like a bunch of piranha on a kitten, yer lookin' at some blowby.........

Awesome, thanks for the description Raspy.

Haha bunch of piranha on a kitten... Nice mental image. Thanks gm, have to try that dipstick test.


Now I thought that that orange tube that hangs down below the engine on the driver side right about even with the oil pan had something to do with blow-by? Haven't figured out what that tube is actually for lol. Imagined it had something to do with oil over flow and whatnot...

That is the crankcase breather tube - also related is that, as the pistons move up and down in the cylinders, they displace, or pump, the volume in the upper cylinder above the piston - they also displace the volume in the cylinder below the piston, so crankcase volume is always changing when the engine is running - that tube allows the crankcase to 'breathe', along with all the vapors created by crank splashing up the oil, hot oil, blowby, etc

That should be your crank case vent tube. It does releive the blow-by.

I will add :
blow-by is normal, and all engines do have it to a certain extent.

When blow-by becomes an issue is when it becomes excessive to the point that you are loosing enough oil to cause damage to the engine. It can also indicate that there is a pressure problem in the crank-case that can effect engine performance and fuel mileage.
Excessive blow-by can be the result of worn pistons/rings, bad valves or very badly set valve lash, or a inoperative or poorly working vacum pump. Poor air-intake/exhaust flow can also cause it.

How much is too much..... well, from what I have learned here, thats a debatable subject.. I have heard anything from folks loosing a quart of oil in less than a month down to a half teaspoon or less a day as being "acceptable"... kinda depends on your engine and conditions....

Making your own blowby measuring tool is cheap and easy.
The blowby orifice tool Cummins uses is simply a tee with one .221" (15/64-in) outlet. Connect one end of the tee to the end of the blowby tube. Put a manometer on last tee outlet. That is your blowby tool. They sell them at the Cummins, but I have made my own plenty of times, less than $10. A simple manometer can be made by looping into a 'U' 6 feet of clear tubing with water in it half way. Measure how high the water level rises with a tape measure, multiply it by 2, convert it to LPMs.

Rough conversion is 1"= 27 lpm, add 3 lpm for each one inch (1/2'' of rise in the tube) of water

The reason for multiplying by 2 is that inches of water equals the water rise in the open end of the tube plus the inches the water is pushed down on the engine side of the water tube. For simplicity my numbers below are the measurement of rise only.


Cummins new 5.9 engine numbers are:
63 liters per minute(2.5" water rise) @ 2200rpm,
76 L/Min (3.5" rise) @ 2500rpm
85 L/Min (4.5" rise) @ 2800rpm.

Worn engine that needs rebuilding are roughly double i.e.
126 L/Min(10.5"rise) @ 2200rpm
152 L/Min(14.5"rise) @ 2500rpm
170 L/Min(17"rise) @ 2800 rpm

Beside indicating a compression problem the valves could also be out of adjustment.

Another way (mine), same idea, is to block the blowby tube with a 1/2'' pipe nipple with a cap that has a 15/64 hole drilled in it. Use 3/8'' id looped clear tubing with water in it slipped over the oil dipstick tube. Other tubing end remains open. Use a sharp tipped felt marker to mark the water level with the engine off, have a helper start an already warmed up engine and run the rpms up to 2.2, 2.5 & 2.8k rpms. Mark each water level with the pen, measure the distance from engine off mark then multiply each by 2.

This is all very simple to do, just hard to explain with words.

Great idea infidel. Think I got the basic idea of how it works. Though I think a nice guide with pictures would definitely be stickey worthy (if you haven't already done one that is)!

What Infidel says is right about the orifice sizes for the ISB, but for less than 250 horsepower use a .221 , and for 250 horsepower and above Cummins wants you using a .302 orifice for Blow-by checking with a water manometer.

Here's my solution for a home-engineered water-column manometer, for those too lazy to just stick a tube in a bucket of water -
Take an 18" tall by 6" wide by 1/2" plywood board, mark a full-length "U" on it, to within 1" of each side and 1" of bottom.

Mark a line clear across both vertical legs of the U at 1" intervals from the bottom of the U, starting at the beginning of the vertical rise of the legs, using a square and scale for accuracy, then label the bottom mark as 0, label next mark as 1, next 2, etc - mark the increments up the center of the U so they are easily visible

Using some of that clear vinyl 3/8" tubing and suitable romex wiring staples form a u-bend on the board, stapling it at the 1" divisions - don't crimp the tubing with the staples, just tight enuff to hold the tubing in place - make the legs about 14" tall, since we're dealing with a Diesel engine.

You can nail the vertical board to a piece of 2x6" so you can sit it on a flat vertical surface - it must be below the level of the dipstick tube opening, to prevent the indicating water from siponing into the crankcase

Now, you have a simple water-tube manometer - color a cup of water with red easter-egg dye, or equiv

Position the manometer exactly vertical, fill the tube to the 7" level with the colored water

Connect the manometer to the open dipstick tube with enuff length to keep the manometer well below the level of the dipstick - don't want any red-colored water siphoning into the engine.

Place the orifice plug into the breather tube, have someone crank the already warm engine, noting the level of the water in the tubes at OFF, then idling.

Holding the manometer exactly vertical to the ground, have the helper rev the engine to 2000rpm, noting the rise of the water level in the open side - the opposite side is connected to the dipstick tube, where crankcase pressure will be venting outward into the manometer tube

Now, keeping the manometer exactly vertical to the ground, add the rise in the open tube to the drop in the other tube to get the actual crankcase pressure level : if the water rises 3", add that to the 3" drop to get 6"H2O

Try it - you'll like it............

Old thread back from the dead [coffee]

Where did you find cummins saying use a .302 orifice ??

On Cummins Quickserve for 6.7 and 5.9 dodge they list Tool 3822476
with .221 orifice.. See pictures:

Attachment 44951

Attachment 44952

Am I the only one not making any sense out of this? :confused:

I am not getting the math to work either in my head or on a calculator. With that first example with 2.5" of rise at 2200 RPM. 27 LPM per inch x 2.5 inches of water rise = 67.5 But the specification shows 63 LPM, which is where I'm getting confused.

Maybe I'm overthinking this?

I am trying to figure this all out as well because I would like to perform the test on my truck. I have all materials and have made the "T", but i still have a few questions. jrs, check out this link Measuring Blow by - The Real Test For Internal Engine Condition it seems to me that the 1in = 27 lpms is a very rough conversion as the chart at that link states that 5in of water would be 64 lpms, which is basically right on for the other numbers in the post. Now what I want to know is how much water do you put in the manometer? wouldn't the amount of water change the inches of water reading that you get because of its own weight? For instance, 8oz of water should rise higher than 16oz at the same pressure from the crank case. How is this test accurate without more details??

At any amount of water in the manometer, the level in both tubes is the same at 0 pressure. Pressure then pushes the water down in one tube and up in the other tube. The measure of pressure is the difference in level between the tubes. The change in one tube is 1/2 of that if the tubes are the same diameter.

inches of water against a fixed orifice wouldn't be a linear function for blow by because increased blow by will have more suspended oil particles in it.

One thing we know. If all the water blows out of the manometer, it's time for a rebuild.[coffee]