Blow By ??

What is that, inches of water?

 

Basically each engine does have an allowable spec for blow by. The manufacture specs aren't very flattering. A special tube is connected to the blow by or road draft tube that you connect a water manometer to. A manometer is long tube in the shape of the "U" that sits up and down about 3 feet long. The tube is filled about 1/2 way with water. One end of the tube will connect to the special blow by fitting, the other end is just open. With engine up to temp and at load the blow by will push down on the water and you read how much. From where the water sits with the engine off is your starting point. Most Manometers have a ruler or something to measure inches. So with the engine running at full load, if the water is being pushed down 5 inches you have 10 inches of blowby, (you always add each side because one side is pushed down 5 inches and the other gets pushed up 5 inches). There is a bit more to it, but that is the basics. I don't know for Detroit off hand, but for an ISX engine with less than 100k should not have more than 12", engine with more than 100k 18" of water is max allowable. The 880 and 900 numbers you are seeing are probably mm of water. I would say a good engine will have less than about 5 - 6" water, or 150 mm.

 

Haven't heard that before- could be truck sales doubletalk?

Some drivers get carried away with the oil additives. It makes the rings stick in the grooves, causing too much blowby. A gallon of fuel in the crankcase and ten minutes of low idle cleans the engine out good. Sometimes this will stop the blowby, sometimes it won't. If it won't, it's a problem. If it does, the last owner(or driver) didn't change the oil enough or loaded the engine up with oil thickening additives.

I put a gallon of fuel in the crankcase every time the oil needs changing- the techs at Speedco don't mind me waiting 10 minutes outside the door, and the engine stays cleaner.

I just look at the blowby when the engine's at operating temp on a used truck and I have passed on trucks that had a lot of blowby.

 

I read on a different forum that a few quarts of ATF added to the engine for a day or two prior to changing the oil works wonders in cleaning up the internals....Anyone ever heard or tried this?

 

I have heard the ATF thing too, but never tried it. I have changed a few injectors over the years that were leaking fuel into the oil, and boy, those engines were clean inside!

 

I just bought an 07 pete with a C15. Had it dyno'd and blow by checked. It had 823 as blow by mechanic said that was great a new engine is around 800-810 a rebuilt is around 820-825 and over 1100is getting into rebuild territory

 

If that's mm 825 would be 82.5 cm which is about 32 inches.

Must be some apples to oranges here.