McRoth

Some facts and fallacies…

Blow-by – What is it?
As others have already stated here: Gasses leaking past the piston compression rings during the compression and power strokes into the crankcase. The suction provided by the manifold via the breather hose to the valve cover is sufficient to scavenge the crankcase to below atmospheric press. during the life of a well maintained engine. That is the whole story, just as simple as that! Blow-by is normal and significant on a new engine and would typically stabilize to a min. around 50,000 miles.

How much is too much?
In a nutshell the “break point” is when the breather can no longer cope with the volume of gas and a positive pressure builds up in the crankcase. Excessive blow-by goes hand-in-hand with poor compression. The first clues are usually oil seeping from the filler cap, leaking front or rear oil seals or in severe cases oil blowing out of the dip-stick tube. More subtle symptoms via pressure to the fuel pump stop diaphragm can cause rough idle or loss of power. A healthy engine has a negative crankcase pressure at all times.

So how to check it?
First a couple of fallacies: Remove the oil filler cap while idling and you will see the hot smoky gasses rising out and some oil spray from the valve gear. What you don’t see is the cool clean air sneaking back in simultaneously to replace it. If you have a sooty exhaust you will have sooty blow-by and this is no indication of the blow-by volume. Of course if your filler cap blows off when undone you have a shot engine – so read no further!

There has been some discussion here on how many seconds it should take to cut the fuel when blocking the breather pipe 5sec 10sec…whatever. Before you try this please consider the following. A crankcase pressure of 10psi will exert a load of 2.5lbs on your thumb. 10psi will exert a vertical load of 1200lbs on your valve cover and considerably more for the horizontal component. The story is worse down south. The time taken to stop depends on a number of variables of which blow-by vol. is only one – the strength of the rack return spring, the pump diaphragm / linkage friction, idle RPM etc. If there is excessive resistance you are just seconds away from blasting off the valve cover and at best you will distort the cover.

A number of years ago I had an Apprentice assigned to me in the R & D test shop at Perkins Engines whom I gave the task of recording data on a prototype engine we had on a water brake rig. Perhaps boredom led him to investigate what would happen if he blocked the breather. It blew off the timing case and the engine destroyed itself along with the Apprentices prospects at the company.

A simple, accurate and non-destructive method
With engine hot and off remove oil filler cap. Wipe cover clean and smear grease around the filler hole. Take a piece of thin polythene sheet ( an old bag is fine) and lay over the filler hole sealing with the grease. Start engine and let idle. The sheet should not have blown off. Now increase revs slowly to about 2000 and the sheet should now progressively be sucking into the engine indicating a negative crankcase pressure. In other words the manifold is sucking off the blow-by faster than gas is leaking past the pistons.

Once you have established that you have a neg. crankcase pressure as above you can take it a stage further if you wish and test under load (i.e. at max. torque) using the same method. This time you will need to tape the polythene in front and behind so that it sits snugly over the filler – since you will be driving the car. ( masking tape works ok). A short hill climb will reveal whether pos. or neg.
My own 20 year old 300D with 780,000km on the original engine still passes this test.

My humble apologies to Jim for the length of this post

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Beagle