Pcv valve/ vacuum leak
oilspot
Wagonist
I'm working on trying to track down a vacuum leak. I'm having high idle problems and have already checked cleaned the iavc and fitv and both are working perfectly.
So I start looking for vacuum leaks elswhere.
Thats when i find the the hose on the bottom side of the pcv valve is cracked. Now heres where my questions really come in...
when i put my finger on the bottom of the pcv valve at idle it's pulling a lot of air through the pcv. So I start looking for info to figure out if the pcv valve is bad (it's brand new by the way).
I'm getting mixed info. Some people say that at high vacuum (idle) that the pcv valve should be shut and should only open when the throttle is open and the manifold isn't at high vacuum. That makes sense to me because the crankcase shouldn't be producing much gasses at idle that would need to be burnt off.
If that's true that would mean that I got a faulty pcv valve.
Now other info I'm getting says that a pcv valve should flow freely at idle. Now if thats true that is a shit load of air being allowed to enter the manifold, like one mega intake leak :? .
Now if that's how it should flow is the air flow in the engine case limited. I know that the air flows in from the head via the filtered intake air. Mabey it just can't flow that quick through the engine? Which would keep the air from the pcv from being a "vacuum leak"
So I start looking for vacuum leaks elswhere.
Thats when i find the the hose on the bottom side of the pcv valve is cracked. Now heres where my questions really come in...
when i put my finger on the bottom of the pcv valve at idle it's pulling a lot of air through the pcv. So I start looking for info to figure out if the pcv valve is bad (it's brand new by the way).
I'm getting mixed info. Some people say that at high vacuum (idle) that the pcv valve should be shut and should only open when the throttle is open and the manifold isn't at high vacuum. That makes sense to me because the crankcase shouldn't be producing much gasses at idle that would need to be burnt off.
If that's true that would mean that I got a faulty pcv valve.
Now other info I'm getting says that a pcv valve should flow freely at idle. Now if thats true that is a shit load of air being allowed to enter the manifold, like one mega intake leak :? .
Now if that's how it should flow is the air flow in the engine case limited. I know that the air flows in from the head via the filtered intake air. Mabey it just can't flow that quick through the engine? Which would keep the air from the pcv from being a "vacuum leak"
Comments
That should help a little.
At idle you should feel a vacuum on the bottom of the PCV valve if you unplug the hose, that lets you know that the valve is working properly.
And yes, it does seem like a large vacuum leak, but it's not as big as you think, and it's really kind of a "factory controlled vacuum leak," and it is constant. There shouldn't be that much air moving through the crankcase at all unless you have serious piston ring problems. That air moving through the crankcase is called "blow-by" and it is caused by the fact that piston rings cannot seal the combustion chamber 100%. Also keep in mind that crankcase vapors contain some vaporized fuel and oil, which help aid combustion a little bit which makes this little "vacuum leak" a lot more bearable because it doesn't cause a noticeable change in AFR.
In reality the only time the PCV valve should close is if the pressure in the manifold increases A LOT. This will happen on turbocharged engines, and it will also happen if your engine backfires into the intake manifold. If you have a backfire in the intake manifold, the PCV valve MUST keep that combustion from entering the crankcase. If it doesn't then your crankcase could explode (remember blow-by is air mixed with vaporized oil and fuel). On a turbocharged engine the PCV valve must keep the crazy amount of manifold pressure from getting into the crankcase because it will cause the seals in the engine to hemorrhage oil (the crankcase was designed to work under vacuum not pressure, and pressure will push oil right past the seals).
So long story short, it sounds like your PCV valve is functioning properly, just make sure you replace all the hoses!
Cool. This has been driving me kinda nuts. I haven't had any time to mess with the car. I have a feeling that once I replace the hose that the idle will settle down, and I can start driving the car. i know the rings are good so It's just running lean because the "controlled vacuum leak"..... isnt controlled at this point.
I know it may be a little strange, but I really like knowing and understanding how every component on an engine works.
"the PCV valve itself is designed to allow the most amount of gasses through it, when the least amount of vacuum is present.
at idle, the PCV valve should block about 98% of the air flowing through it. at WOT or atmospheric pressure, the PCV valve plunger will be floating and allow crank case pressure to bypass it and be consumed by the engine.
it sounds like your PCV valve has seen better days."
blowby isn't that much air. Everything else has to come through the head via the smaller passages. I'm sitting inside away from my shop, but if i remember correctly there are only some small passageways that let oil drain from the head back to the block. Other than that.... there's no other inlets for the block to suck in air.
thanks for the help on this one!
Glad to help!