Ive been looking at some used Cat machines descriptions. and in their condition notes they mention the 'blow by', some of the descriptions will say Blow By - Yes - Normal or Blow By - No If a machine has any kind of 'blow by' is it asking for trouble? If some have NO 'blow by', which im assuming is normal... than how can you have 'blow by' and have it normal? im lost....
Blowby is caused by interanl wear in the engine and compression getting into the crankcase. All engines will have it to a greater or lesser extent, even brand-new ones. I am surmising here but maybe no blowby means exactly what it says, none at all, and "normal" blowby means whatever would be considered normal bearing in mind the hours on the engine. Still a bit of a weird way of describing it though.
They're all the salesman/ inspector's interpretation on the condition. Unless you take the velocity of the crankcase gas it's an educated guess
A few examples 695 hrs - Blow By - No 170 hrs - Blow By - Yes - Normal 189 hrs - Blow By - No 7425 hrs - Blow By - Yes - Considering 7000+ hrs 501 hrs - Blow By - No 2792 hrs - Blow By - Yes -Normal 1382 hrs - Blow By - No 236 hrs - Blow By - No
I have found relative humidity seems to affect perception of blow by. In some cases I believe it may be steam exiting as opposed to actual combustion gases entering the crankcase. This seems to depend greatly on engine operating temperature but I have been concerned more than once to find a freshly washed engine huffing more than normal
I've seen that report term in a couple of marketing web sites and in that context it means nothing. Generally a perfectly fine running engine will have some vapor coming from the breather tube. A statement saying blow by pops the dipstick out the tube would most likely mean something is very wrong with the engine. Many engines today have positive crankcase ventilation and there is no way to see any blow by. High blow by pressure can be surmised when you see many leaking flange connections, sealed by gaskets, seeping oil. Front and rear covers, oil pans, cam pocket covers and valve covers with leaks that wash the dirt down are specific symptoms of a worn engine. Crankcase pressure on older engines that are vented to the atmosphere can be measured with a water tube manometer or a gauge calibrated for inches of water. Blow by pressure readings can be an objective measurement if the manufacturer provides a specification for that particular engine. An example might be early Isuzu engines had it that a reading with the manometer connected to the dipstick tube of over one inch of water at full load meant the engine was bypassing enough compression gas that a tear down, inspection and possible replacement of pistons, rings and liners or repair of same was warranted. Cummins used to use a calibrated orifice put in the crankcase breather tube with a connection teed above the orifice and readings of up to 18 inches were permissible. Other manufactures use an air flow gauge that is similar to a wind gauge. In that case velocity is the key metric and there are published standards denoting good and bad. Newer engines now have engineered in positive crankcase ventilation systems where the gases from the crankcase are sucked into and through the turbo charger. The system is about the same as the PCV system on a gas engine in a car. I haven't seen enough of them yet that are old enough to draw a conclusion on how they will leak when they get old.
Means nothing unless you hook up a blow by kit and measure it. I have done a few blow-by tests engines and was very surprised and how much blow-by was considered normal. You can get sheets on every cat engine giving acceptable blow-by for that hp ratings. Or you can find formulas as a general rule based on hp rating, hrs on the engine and number of cylinders. I once tested a 385C with a C-18 (8000hrs) that was blowing out dipsticks intermittently and it looked like excessive blow-by coming out of the machine watching it run. I hooked up the meter and according to specs it was mid range or slightly less. I was amazed at how much is allowed before its called excessive. Just for fun I then tested a brand new 385C with 20 hrs and it had more so it goes to show engines do still have to break in. I guess my advice is if your looking at a machine and not sure bring a mechanic along and if he has seen enough engines run he will know if its excessive. All engines have blow by and like I said you would be surprised what is actually excessive. I don't like when the oil is dripping out the breather or the guard below the breather tube is covered in oil thats what sets off my alarm bells to look closer.