View Full Version : Diesel Engine BlowBy

02-08-2006, 09:37 PM

I've been told that a reasonable check of engine condition in lieu of compression testing is to measure its "blow-by" with an instrument. What is engine "blow-by", and what type of instrument is used?

Thank you,


02-09-2006, 07:29 AM
blow by is refering to the compression gases being forced into the crankcase on compression / detionation stroke due to worn rings or scored cylinder bores.
Have never heard of any test to analize a engines condition by measuring blow by though.
I suppose if a engine is in really bad shape a visual would be a dead give away. But no substitute for a compression test!

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05-03-2006, 08:48 AM
"Theese kind of test instruments were available some years ago, from amongst BOSCH. It worked the way that the injector was replaced with an air hoose connected to a small compressor and a flow meeter. In the instructions following the instruments there were given roughly how much flow was acceptable. By blowing soap water in the innlet/exhaust ports, valve shafts etc leakage could be determined. Have not seen any one in operation in the last decade and never at a marine shop! Quite expencive and not very accurate."

scott s.
05-03-2006, 09:55 AM
"cylinder leak down tester does not replace compression test. rather, it helps locate source of compression loss by pressurizing cyl. @ T.D.C. of compression stroke, and measuring flow from intake, exhaust, & crankcase. (valves, & rings) available from Snap-on. expensive, & rarely needed"