Puzzled.....

[Beau]

Is it normal, that when the engine is running (IDLING) , and you take off the oil cap for there to be pressure coming out from there...When revving, there is only a small bit of pressure.

From what I know of there should not be, unless I have very worn valve seals.

Help....

 

[BobMurphy]

I would say its normal . .

The pressure isn't coming from the combustion side, via the oil seals, its the pumping action of the underside of the pistons down in the crankcase. That area (around the crank & above the oil in the sump) is connected to the cam box via the large oil drain holes on the offside.

At tick-over there is a lot of air movement, as the revs increase this becomes confined to the crankcase as there isn't time for the pressure waves to make it to the top of the engine before the opposite pressure wave is encountered. As a piston descends it creates positive pressure in the crankcase, as it rises it creates negative pressure.

You would think that with four pistons moving in pairs that it would all cancel out, but it doesn't seem to at low revs.

I've encountered it on other engines in the past.

My twopence-worth

Bob.

 

[Beau]

Thanks for the info. Just wanted to know if it was all normal...