Blow by
 

Does anyone know how much blow by an m44 motor should have?

Yes new motor fully broken in 2%-3%. Good motor but has mileage 5%. good core high mileage 8%-10%.

This is done with a leak down tester. I set my blow in pressure gauge at 100psi and and look for 97psi to 98psi on the leak gauge.


If you have a cylinder that has high blow-by you can check it further by squirting some oil in the spark plug hole and re-testing that cylinder. If the leak-down improves it is most likely cylinder, piston ring issues. If it does not improve it is most likely valve issues.

There is no acceptable amount of blow-by to measure with the car running but a leak-down is a great way to teat blow-by

Correct me if I am wrong, While the car is running if you pull the oil cap off there shouldn't be ANY air blowing out from it?

All crankcases have some pressure.

If you pull the oil cap off while the engine is running on these motors, they usually won't idle.

From what I have been told you want very little pressure, if there is a good flow the valves are not seating right or are worn. I was more or less just curious. I have had three of these motors and all had a surprising blow-by.

That is normal. You don't want a smoke-stack out of that oil cap hole but a fair amount comes out of there. As the pistons move up and down in their bores they create positive and negative crankcase pressures which will blow-through the oil cap if it is off. You will see the crankcase gasses escape from the oil caps' hole but won't see the quick periods of lack of pressure or almost negative pressure.. Remember all piston rings in perfect condition have a gap that allows 2%-3% on the compression stroke blow-by, imagine the percent of blow-by when that pistons compressed charge is ignited and those rings take thousands of more psi then a simple 100psi from the leak-down tester...

John Smith