mercruiser 3.0 crankcase pressureized

jkl1q · Nov 24, 2012

i went top up engine oil after doing an oil change with motor runnning, and once i removed the filler cap, air was coming out. there was enough pressure that it was blowing the oil i was trying to pour in all over the place. i assumed i had bad piston rings so i ran a compression test and came up with 152,148,150,150 respectfully. what could be causing this and what is my next course of action

thumpar · Nov 24, 2012

Re: mercruiser 3.0 crankcase pressureized

There are always blow by gasses. That is why there is a vent in the rocker cover.

jkl1q · Nov 24, 2012

Re: mercruiser 3.0 crankcase pressureized

i understand there is always some blow by, but enough to blow a stream of engine oil seems excessive to me

Another retired guy · Nov 24, 2012

Re: mercruiser 3.0 crankcase pressureized

Hi. It sounds fairly normal for an older engine. To avoid overfilling with oil it shouldn't be topped up while running. Stop the engine and let it sit for a minute or two to allow the oil to flow back into the pan, and then check the level.

thumpar · Nov 24, 2012

Re: mercruiser 3.0 crankcase pressureized

I have never tried to add oil to a running engine. Why are you trying to add oil while the engine is running? How would you know how much to add since you can't measure it on the dip stick while running?

Bt Doctur · Nov 24, 2012

Re: mercruiser 3.0 crankcase pressureized

mercruiser 3.0 crankcase pressureized
i went top up engine oil after doing an oil change with motor runnning

Makes no sense, oil level is checked with the motor off.Sure hope the manual dosent say check it that way.

MarkSee · Nov 24, 2012

Re: mercruiser 3.0 crankcase pressureized

jkl1q said:
compression test and came up with 152,148,150,150 respectfully.

Welcome to IBOATS

Very nice compressions numbers.....keep taking care of that baby.........

Mark

tinkerguy70 · Nov 24, 2012

Re: mercruiser 3.0 crankcase pressureized

Yes they are! I wish mine had numbers like that!
FWIW, mine has pressure like you described while running with the oil cap off. I wouldn't worry about it.

I'm curious though, why would you want to add oil to the engine while it is running?

Another retired guy · Nov 24, 2012

Re: mercruiser 3.0 crankcase pressureized

Considering the OP's original comment re oil blowing out of the filler, and his practice of adding oil to a running engine, it is possible that those nice compression #'s are due to an overfull crankcase, and an abundance of oil splashing around the cylinder walls!?

jkl1q · Nov 24, 2012

Re: mercruiser 3.0 crankcase pressureized

so ill be honest im not really sure why i was adding oil with it running. it was a few weeks ago that i did it. i had probably started it up to verify i had no leaks and rechecked oil level via dipstick and due to it being running, was a little low. obv a mistake on my part. the pressure issue was something that was bothering me the more i thought about it, so it wasnt until today that i did compression check. i will reverify that im not over filled, but i doubt that even if it is im over by that much to make a difference. remember i couldnt add oil with it running becasue of the pressure. the question still remains why its pressurized and is that normal. ive worked on a few car engines in my life and never seen anything like that

Another retired guy · Nov 24, 2012

Re: mercruiser 3.0 crankcase pressureized

The two most likely causes are worn compression rings or a blown head gasket - but your compression readings indicate that isn't the reason. The one thing that can "hide" worn rings is oil in the cylinders.

thumpar · Nov 24, 2012

Re: mercruiser 3.0 crankcase pressureized

But if the oil was "hiding" the ring issue he would also have less blowby coming out the oil filler.

I don't think the amount is anything to worry about. Back in the day when my friends and I could only afford cheap cars the first thing we did was take the oil cap off to see how much and what color the blowby was. Even today I have never (including the moderately new vehicles we have) seen a car with no blowby. You have to think about it, you have a maybe 2" hole that has air coming out of it and you try to poor a stream of oil down. It doesn't take much air to throw that oil around. Go out on a windy day and poor oil into a pan and see how much it moves in the wind.

mercruiser 3.0 crankcase pressureized

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