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Crankcase pressurizing like a balloon :(

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I think this is probably going to be straight to the toilet for my motor.


I fought a problem with my valve cover vent and such a while back where I had excess blow by from the valve cover vent.


A few months back I ran my valve cover vent to my air cleaner, but the oil spray covering my carb got quite annoying so I decided I'd route it to a TEE on my PCV valve to crankcase vent to draw everything out via vacuum @ the PCV valve..



Only issue is that now, I've noticed more then ever that the crankcase is pressurizing itself. I've got oil leaking from fresh gaskets and when I pulled the oil cap off, it was like letting air out of a balloon. (Engine running of course.) Almost as if the PCV was doing nothing. The truck has NEVER burned oil (or smoked) but as expected, did lightly after running the valve cover to PCV.




Now of course, the PCV valve is only a few months old but that doesn't mean it's any good. And the old "shake it" technique isn't really accurate.


I know I need to run a compression test again, but last time I checked the average compression was around 160psi with no more then a variance of about 6-7 between cylinders.


Where do I start next? Valve adjustment, compression check, etc??

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Could the vent be blocked?

It's 6 from the pick up, held by screws



beat me to it ^^ ... i noticed a couple of my l-series block vent filter mesh were caked up pretty good [engine apart]


I'm not sure ... but stuck rings is only other thing i can think of it *might* be ...


if not rebuild


usually i have only had this on low-compression/abused engines so i no no

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It's supposed to be vented top and bottom.


Top is for fresh ait

Bottom is to draw it out


Do not tee the top end into the bottom end.


Sounds like the engine has too much blow-by. Do both WET and DRY tests, as a regular test won't tell you enough.

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At an idle I have blowby out the valve cover on my fresh LZ23, but when I hit the throddle it goes away.

If it is tired, and it is not vented to the air(airfilter is to the air) like it's supposed to be, you will start blowing out gaskets everywhere, I don't beleave that these engine crankcases were meant to be pressurized, and the PCV valve cannot, nor was it ever meant to suck that much air into the intake, fact is that the crankcase/valve cover air is the same air, so if the PCV was able to suck that much air, it would have done it without putting the tee in there.

If it is running good, and your compression test fails/doesn't fail but you need to keep driving it, make somekind of catch can that the valve cover vent can go to, that catches the oil, but vents to the air or back to the airfilter after the oil is caught.

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Jesus or run a catch can so you dont blow shit all over the road and into the environment lol ya bunch of savages


I remove the screen one very L that I rebuild. They seemed to get caked up way to easy.

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I pulled out of the neighborhood, hit second gear, and blew the crankcase vent off :confused: Just re-routed a few things and it will be okay.




When I changed my oil pan gasket, I inspected it and it looked perfect. Although, it's possible for the oil pickup tube to be clogged from the inside right?And not visible from the outside?


I'm gonna compression test it tomorrow afternoon and see what it reads. If not good, I'll just start looking for an L20B or L18 that I can rebuild on the side. If it reads okay, i'll get another pan gasket and drop the pan on my off day and pull the vent to clean it.




The motor was supposedly rebuilt sometime in the last few years, and everything I've found so far reflects that, so maybe I'll just get lucky and it will be a stuck ring?





Anyways, I bought a catch can so I'll put that on and see how it goes.

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I wouldnt use brake fluid to clean anything. It will permeate and contaminate after the fact. Regular old gasoline will clean anything dirty from oil, grease etc... petroleum distillates break down other petroleum products.

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YOU sure you explained this right?

You will NOT run the valve cover vent to the PCV tube. YOu route it under the carb aircleaner.A weber DGV has ythat white elbow.THis is closer to atmosphere pressure

Routingfrom valve cover to the PCV I seen ecess air cannot escape


YOu have a datsun Book/Manual???????


The crankcase tube goes to the PCV valve.


At idle you will always have air come out. As the motor speeds up it should start sucking in. At least at the crankcase if you had it open vented.

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If the engine has good rings, the PCV valve can actually pull a slight vacuum in the crankcase, at idle. Their is a higher vacuum in the crankcase at part throttle, and it is only at WOT, that the PCV valve not able to keep up with the blowby.

Check the compression, that will give you a clue. You probably have a L-20-B engine in your truck. You will have to look up the specs for your engine.

Another test is a leak down test. You actually pressurize the cylinder with compressed air, and the leak down tester measures the percentage of air leaking past the rings, and the valves.

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Okay, so here's my compression test results.


Cylinder 1


Dry- 130




Cylinder 2


Dry- 110




Cylinder 3


Dry- 130

Wet- 150



Cylinder 4


Dry- 145

Wet- 150



I'm gonna start by adjusting the valves. A friend and master mechanic recommended me something called AutoRX to try freeing any stuck rings. He's an old Datsun nut. Pretty awesome guy to talk to. Cylinder 2 is a pretty big jump from the rest, so I'm gonna start there and assume the issue lies primarily there.

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Stupid Question: What Is Wet And Dry??


Dry = compression test performed dry with no oil added into cylinders. Record readings.


Wet = compreswion test performed with 1-teaspoon of oil added per cylinder. Record readings.


If significant difference between dry and wet ... = tells you level of how worn the piston rungs are.


Use these tests to help determine engine health while diagnosing or buying an engine/car.


You should check yours sometime :)



A leakdown test is pretty good to perform though.

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I do not know what the compression should be on a L-20-B engine, but a L-16, with an open chamber head should be 159 to 171.


More importantly, cylinder two is more than 10% below cylinder 4.


But I suspect all the rings are worn.

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