Originally posted by wcbcruzer at Diesel Truck Resource
Blowby tester - Lowes style...
I give full credit to Bill (Infidel) for the homemade u-tube manometer idea (open-ended). It's kinda hard to visualize what it really is through text only, so I thought I'd share how I made mine for anyone else who wants to make one.
Size of clear tubing doesn't matter, amount of water in the tube doesn't matter (as long as enough remains in both sides of the "U" under pressure). This one is about 5 feet tall, but you can make it 3 feet tall or so as well. Before starting you have to plug the crankcase breather hose on the bottom of the truck with a 1/2" nipple and cap with a 15/64" hold drilled in it.
Then fill the manometer with water about halfway and hook it up to your oil dipstick tube and run the engine at 2.2k, 2.5k, and 2.8k rpms. Measure how many inches the water moved up in the left column. You can also buy a manometer gauge but this homemade one is very accurate! With water in there it's accurate to 1/100th of a psi (not that you need it that accurate for testing blowby).
10' - 3/8" I.D. clear PVC hose
10' - 3/4" PVC pipe cut in half w/ bends, etc.
Flexible measuring tape for sewing/crafts
1/2" brass nipple
1/2" brass cap
Cost about $10 for the manometer, and about $7 for the brass nipple/cap. Total under $20 from Lowes.
Crankcase pressure is not very high (less than 1 psi) so water works well for this, but for higher pressure applications you can use a denser fluid like 2-stroke oil, vegetable oil, or mercury (not recommended).
Keep in mind that if you live at higher elevations, your numbers will be skewed, but it shouldn't matter. If you're engine is toast, you'll know... lol.
Physics behind it all...
(P - Po) = pgh
(P - Po) = Pressure difference (crankcase vs. atmosphere)
p = Rho (density) (water = 1 kg/L)
g = Gravity constant (9.81 m/s/s)
h = Height difference between water levels in tube