Topic: Can someone check something for me?

[Rob the plumber]

Can one of you guys remove your oil fill cap while your engine is running, and tell me if you have any air blowing out? Mine has a pretty decent amount of air coming out of there. I checked my friends 4.0L, and there is zero air blowing out on his. His actually pulls vacuum when you crack the throttle.

My engine has done this since I got the Jeep, but runs fine.

[oldjeep]

Mine does that and so does my kids 4.0L

[flyingbrick]

Same here.

[jcsanders79]

Me too.

[Rob the plumber]

Thanks! Good to know. It passed a compression test, but it still had me wondering. I have a spare 2.5L raedy to go just in case. :)

[jagular7]

The only thing about the compression test is it tells you the condition of the pressure capability of the cylinders. The cylinders should be considered separate from the rest of the engine. If you break that seal between them, then there will be considerable air movement. However, even with the cylinders isolated, the air movement out of the oil fill cap will still be large due to the amount of air within the engine itself. Consider the engine as a container. Inside the container, the cylinders are just considered to be blocks. The other moving parts of the container is what makes the air move around. Since the pistons move a great distance up and down compared to the other moving parts of the engine, they generate the air movement.

[Rob the plumber]

I kinda figured that with such a small crankcase, there would some air movement. I imagine on a V8, the opposing bank of cylinders probably kinda displace each other.

[aw12345]

Not really since 2 pistons go up when 2 pistons come down rendering that statement mute. The air that comes out of the oil cap is blowby. Is it to much? The easy check is simple if the engine coats it self in oil and you really cannot find the source of the leak after repeatedly replacing gaskets and so forth, you can pretty well say that it has to much blowby. A compression test isn't really the defenitive answer for checking blowby, performing a leak down test with a leakdown tester is a much better test takes more work, but if all cylinders have less than 10% leak you can pretty well asume that it does not have to much blow by and the piston rings/ cylinder walls and valves are all still sealing properly

[Bounty Hunter]

Air coming out could also be normal crankcase pressure/fumes that aren't properly being scavenged by the CCV system.  With the cap on, pull the airbox end of the hose between the valve cover and airbox.  It should have vacuum if working properly.

[aw12345]

Very true. However on hardworking engines like our beloved 4 bangers there isn't much vacuum to go around when going down the high way since the throttle is pretty well open all the way. A lot of engines that have to work hard there like to oil up the air filters really not all that much you can do but add a breather filter / can to the equation I used to like to use the ones of a chrysler v8 those things really helped chevy engines from oiling up and plugging the air filters

[Jeffy]

Very true. However on hardworking engines like our beloved 4 bangers there isn't much vacuum to go around when going down the high way since the throttle is pretty well open all the way. A lot of engines that have to work hard there like to oil up the air filters really not all that much you can do but add a breather filter / can to the equation I used to like to use the ones of a chrysler v8 those things really helped chevy engines from oiling up and plugging the air filters

Which is why I did this quick thread: http://4bangerjp.com/forums/index.php?topic=3586.0.

Blow-by happens on every engine to some extent.  Adding a catch can is pretty easy to do.  Unlike a filter the catch can doesn't restrict flow airflow.

[Bounty Hunter]

Hardworking they are, but something is wrong if you're puking oil from a CCV tube that should be pulling vacuum.  Catch can is a bandaid fix.

[Jeffy]

Hardworking they are, but something is wrong if you're puking oil from a CCV tube that should be pulling vacuum.  Catch can is a bandaid fix.

Who said it's puking?  Vaporised oil will travel down the tube and condense as it cools leaving an oily mark.  It should not be pooling.  The emissions system is designed to do this.  The 'restrictor' may people pull out of the airbox is actually a venturi which is also where the evap. canister and the CCV vent gets pulled from.

Adding a catch can would catch any oil vapor, allow it to condense and collect in the can rather then get sucked into the intake for burning.

[neale_rs]

Who said it's puking?  Vaporised oil will travel down the tube and condense as it cools leaving an oily mark.  It should not be pooling.  The emissions system is designed to do this.  The 'restrictor' may people pull out of the airbox is actually a venturi which is also where the evap. canister and the CCV vent gets pulled from.

Adding a catch can would catch any oil vapor, allow it to condense and collect in the can rather then get sucked into the intake for burning.

There seems to be some misconception about the CCV system. The vacuum routing can be seen on page 25-3 of the '95 YJ FSM. Here is a quote from the FSM:

On 2.5L 4 cylinder engines, a fitting on drivers side
of cylinder head (valve) cover contains the metered
orifice. It is connected to manifold vacuum.
A fresh air supply hose from the air cleaner is connected
to front of cylinder head cover on 4.0L engines.
It is connected to rear of cover on 2.5L
engines.
When the engine is operating, fresh air enters the
engine and mixes with crankcase vapors. Manifold
vacuum draws the vapor/air mixture through the
fixed orifice and into the intake manifold. The vapors
are then consumed during combustion.


So if the CCV does not provide enough vaccuum, the vapors can go into the tube from the valve cover to the airbox, which sometimes makes a catch can necesary. 

[Jeffy]

Whoops my mistake.  I was thinking the CCV was further back on the airbox.  That Tube is called the Fresh Air Vent, IIRC.  The CCV is the small orifice that on the side of the valve cover that leads to the side of the intake.