Since the crank case vent is connected to the manifold through a the P.C.V. valve, at idle the manifold is going to have more vacuum and so there will be less pressure in the engine, or so it would seem. At full throttle, there would be less vacuum to draw gasses out of the engine so there would be more pressure. So oil leaks would be more likely to occur at full throttle.
I actually know very little about how the engine crank case vent systems work on cars.
So, if I am trying to reproduce an oil leak with the engine at idle, would I be better off disconnecting the hose to the PCV and pinching it off? Or is the pressure in the engine about the same all the time and it doesn’t really matter? Does how it works vary depending on the vehicle?
The crankcase “vent” line is the larger hose without the PCV valve. Shown in the diagram below, the crankcase vent line on the left shows a blue arrow indicating fresh air entering the engine, this line flows in both directions. When the throttle is open more than halfway, there is insufficient vacuum to open the PCV valve, crankcase gases flow through the vent line (shown on the left) to the air cleaner/intake system.
Your post seems to indicate that engine vacuum opens the PCV valve. Engine vacuum closes (reduces) the PCV valve flow.
Is the PCV valve just to limit the flow of air? Basically the same thing as a small hole but resistant to becomming clogged.
If the diagram is uses a carburetor and throttle combination, then some fresh air is going through the engine and past the carburetor. This would make it run a bit lean at idle, but that must be accounted for in the adjustments. If the engine is worn out, then combusted air is going to go through the valve, but since it is on the output side of the carburetor it won’t change the mixture.
If it is fuel injection with mass air flow metering, then It looks like the filtered air is really metered air. It lets some of the metered air go around the throttle body and through the engine. So that increases the idle speed. As blow by increases it won’t be metered so it won’t harm anything.
On many lawn mowers they put the crank case vent on the input side of the carburetor and when it wears out and gasoline containing blow by increases it starts running too rich. Can this happen on worn out vehicles too?
Quick Google and found this:…
The PCV valve regulates how much vacuum can pull through the crankcase. It’s hooked into the intake - when the vacuum is high at idle, the PCV valve stops most of that vacuum. As vacuum drops with an increase in RPM, The PCV valve opens, to allow a higher volume of gasses and pressure to be pulled into the intake. As a result, those gasses are recirculated back into the combustion cycle, and burnt off with the fresh fuel and air, preventing those gasses from condensing in the oil, preventing pressure from causing the seals to leak, and preventing pollution from escaping the crankcase.
What does the PCV System do & why is it important to you? - Dennis Road Automotive | Dallas, TX.
The PCV valve does control the flow of gasses to the intake system, but it also closes to prevent a flame from reaching the crankcase during a backfire. The old advice to shake the valve to see if it rattles; there is a light spring inside holding the valve closed.
What about a 1992 to 2001 Toyota Camry 2.2L? There does not appear to be any filtered air inlet to the engine anywhere. There is just one PCV valve on the valve cover and a short hose going to the manifold. Lacking ventilation, it would appear that the engine could be at intake manifold pressure.
Red circle around the vent hose, blue circle around the vacuum hose.
Thanks! I didn’t see the hose on the car’s left side with the distributer there.
The pcv valve is designed to severely limit how much air goes through that path at idle. Otherwise it would create the equivalent of a big vacuum leak, and the engine would idle poorly or not at all. I didn’t realize that there was reverse air flow through the vent path off-idle, something that seems worth knowing .
Some engines have a simple orifice in the PCV line with no backfire valve, flow is dependent on the level of vacuum.
Where did you think the crankcase gases went during acceleration? Blow-by increases under load.
I’m aware of course that crankcase-vent flow must increase during accel, but I thought the path was still via the pcv valve.
The four-cylinder engine below has 3 crankcase vent lines.
Numbers 1 and 2 are before the turbocharger. Number 3 is after the turbocharger on the “boost” side. Number 4 is the PCV valve. Keeping the crankcase free of pressure on a turbocharged engine is more complicated than the classic car engines.
BTW, when the intake manifold in under pressure, don’t you want the PCV valve to be closed?
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