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Discussion Starter · #1 ·
Blow By Test


A better way to check engine condition is by a blowby test. This is what Cummins uses and is very simple.

You need a blowby tool. a blowby orifice tool 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. A simple manometer can be made by looping into a U 6' 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

New engines numbers are;
63 liters per minute(2.5" water rise = 5" of water) @ 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

The valves could also be out of adjustment.

Another way, 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. Use sharp tipped felt marker to mark the water level with the engine off, have someone 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 then multiply each by 2.

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

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Good info, Thanks!
 

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Also what if you have good compression from a regular-old compression tester? This is probably a good indicator that hte engine is healthy correct?
 

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Discussion Starter · #6 · (Edited)
Also what if you have good compression from a regular-old compression tester? This is probably a good indicator that hte engine is healthy correct?

dont confuse compression with blowby.

compression can easily be checked by using a blank in place of an injector.........with a psi gage screwed into it ofcourse.....turn the motor over and ya should show about 350psi..........I have read peeps getting 450-475 though but my manual says 350

I think the important thing would be to have all cylenders roughly the same psi
 

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Compression and blowby, both, to me seem to be a reasonable indictor of engine strength. If an engine has good compression then it should not have unusually high blowby, correct? Or is it possible to have good compression and still have poor blowby?

Basically in a nut shell, what is the advantage of a blowby test versus a simple compression test? I did a compression test and it showed about 350-360 on all cyls, which for me made me very happy at 186K miles from a used engine that I had no known history from. But I am wondering if there are other indicators that I may be be missing...
 

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Discussion Starter · #8 ·
Compression and blowby, both, to me seem to be a reasonable indictor of engine strength. If an engine has good compression then it should not have unusually high blowby, correct?

correct...they work hand in hand


If you are doing a leakdown test, then you want to see no more than 10-15 percent leakage. If you are reading cranking compression, then you look for no more than a 7-10 percent variance between cylinders......per the manual
 
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