Crankcase blow-by
Crankcase blow-by
I was searching topics on Crankcase blow-by...the descriptions were similar to symptoms of my KA (124k), but when I unscrewed the oil cap, the damn thing stalled. I DID feel some air moving tho. Suggestions, or is it time for a rebuild or SR?
Let me clarify: I have a 1992 240sx, KA motor, 125K on the clock. Engine has been idling smooth, but seems to lack power. I saw a post on diagnosing blow by in the crankcase causing loss of compression. I took the oil fill cap off the motor while it was running, and as soon as it was off, it sputtered and died. I restarted it with the cap on, and it started right up, removed the cap again, and quickly put my hand over the fill hole to seal it. I did feel air moving, but I didn't have to push too hard at all to seal the hole and keep the engine running.
Do you think my compression is SO low across all 4 cyl. that taking the oil cap off is enough to stall it, or is there a better explanation? BTW, I hope to compression test it sometime this week. Any thoughts would be helpful.
Thanks
Do you think my compression is SO low across all 4 cyl. that taking the oil cap off is enough to stall it, or is there a better explanation? BTW, I hope to compression test it sometime this week. Any thoughts would be helpful.
Thanks
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Touchdown038
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Someone correct me if I'm wrong, but I don't think taking off the oil cap has anything to do with blow-by gas. The blow-by gases that get below the rings is pushed out of the PCV valve and sucked into the intake manifold to be burned. Saying the blow-by gases are above the cylinder makes no sense to me, of course I could be way wrong...
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Touchdown038
- Posts: 1928
- Joined: Tue Dec 31, 2002 1:55 pm
- Car: Computers, cars, sports
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The only reason I can think of is when you take off the oil cap, you are creating a leak in the system and air is coming out the hole instead of going into the cylinders through the intake valves. But this is just a far-fetched guess, I really have no idea why that would kill it. Maybe some kind of vacuum leak. But you're not alone, when I take off my oil cap it makes my car run really poorly, at least until I put it back on.
Touchdown038 wrote:Someone correct me if I'm wrong, but I don't think taking off the oil cap has anything to do with blow-by gas. The blow-by gases that get below the rings is pushed out of the PCV valve and sucked into the intake manifold to be burned. Saying the blow-by gases are above the cylinder makes no sense to me, of course I could be way wrong...
Well...blow by gases can easily reach above the cylinders, and it absolutely does, this is why you also have a tube from the valve cover to the intake. So the engine can get rid of any gases around the head area also.
Blow by gases isn't supposed to be alot at all. This is why the PCV valve is rather small, because it only expects that it will have to recycle a small amount of blow by in the life of the engine. Now when you have worn piston rings, that's a different story. The blow by can be so excessive that the PCV sytem is unable to suck out all/most of the crankcase gases. On a car with excessive blow by, if you open the oil cap you will actually feel the air blowing through the oil cap itself.
An engine is never supposed to shut off with the oil cap remove, not that you should remove the oil cap on a running engine often. If this happens on your car, then I would suspect that you have a blockage in the PCV system. The PCV is used for idling of the vehicle. The gases are always sucked through the intake even on closed throttle, and this makes the PCV system part of the idling system also. The gases that are sucked through the PCV is metered, and is combusted along with air from the idling gear. Removing the oil cap could release any pressure that was forcing gases through the pcv, and with the bad mixture the car shuts off.
Excessive blow by could be the result of a gas washed cylinder(s). Meaning if your car uses excessive fuel, the extra fuel could be deterioriating the oil on the cylinder walls. The thin layer of oil is needed by the rings to form a proper seal against the cylinder, without the oil there then alow of exhaust can be forced past the rings.