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PCV Valve

28K views 45 replies 17 participants last post by  Intense RT 
#1 ·
Is it possible to eliminate or bypass the PCV valve?
 
#5 ·
that brings me to ask the same thing... why???? the whole point of the pvc valve is to relieve crank case pressure... witch is caused by a small amount of blow by from the rings... so all you really half to do is make sure its not sealed. because if the motor is sealed it will push the oil out all around the oil pan to release the crank case pressure... had it happen, thats how i learned... lol i was young... any way i have just used the push in breathers before and not had a problem, the only down fall is once the breathers get dirty they get your valve covers dirty... just wipe them off and go buy some new breathers... no big deal... the pvc valve sucks the crank case pressure out... but if you just use the breathers the pressure just vents through the breathers... so all i really half to say is just make sure you dont just seal the motor up, make sure it can relieve that crank case pressure somehow...
 
#6 ·
pcv

The reason I was interested in eliminating it is because my balancer shaft oil seal popped out of it's recess. Possibly because of excess pressure in the crankcase. However like you said it should be in working order to effectively do the job.
 
#8 ·
not only that.. but w/o PCV you will have a lot of sludge and other build-up problems inside the crank case.. including having to more frequently change your oil.

if you are blowing seals out.. you might start with a leak down test and make sure you haven't got a cylinder with serious blow-by issues...

i had a worn out chevy 350 that would blow the oil dipstick out of the tube everytime I got on it cuz of the blow by :)
 
#10 ·
I checked the operation of mine because I've developed a rear main leak. From what I could find on the net, it's supposed to pull 1-3 inches of guage vacuum. It had zero and I'm not sure if the spring is too weak (it's a couple years old). I have 18" manifold vacuum and very very little was getting past the valve...at least not enough to pull vacuum and help prevent oil leaks at idle. I'd like it to pull a little vacuum at idle but I'm not aware of "custom" pcv to allow this.


As far as the leak, also, I changed oil brand from Mobil 1 5w-30 to Motorcraft 5w-30 and also went from a 160* thermostat to a stock 192* to have some heat here in NC after moving here from Houston. I'm thinking switch everything back. Maybe the rear main cap will not expand as much and keep the seal tighter against the crank.
 
#11 ·
the PCV valve isn't there to relieve pressure from the crankcase.
the hose to the crankcase filter does that job.
the PCV valves' job is to remove the harmful vapors from the crankcase and only does that at idle and light throttle.
when your on the gas more than about 1/4 throttle, there little to no intake vacuum, but there is more blowby at that time.

also, don't run 2 PCV valves, it does no good to do so and it will cause oil leaks if you don't have a crankcase breather on the motor.
 
#13 ·
Soul_Hunter said:
the PCV valve isn't there to relieve pressure from the crankcase.
the hose to the crankcase filter does that job.
the PCV valves' job is to remove the harmful vapors from the crankcase and only does that at idle and light throttle.
when your on the gas more than about 1/4 throttle, there little to no intake vacuum, but there is more blowby at that time.

also, don't run 2 PCV valves, it does no good to do so and it will cause oil leaks if you don't have a crankcase breather on the motor.
/The pcv valve IS there to Ventilate POSITIVE CRANKCASE PRESSURE. The opposite valve cover provides fresh filtered air to the crankcase. Yes, it does remove aerated oil and unburned fuel/ combustion by products that get past the rings. At idle the pcv is practically closed. Only enough air gets by to make a hissing sound and this is correct operation. When there is little to no vacuum the spring pushes the valve down letting more blow by pressure to be released from the crankcase.

This is how internet misinformation comes about. If you disagree go read some fsm's or aa1car.com. They have a lot of information. That or just understand pressure differential.
 
#15 ·
The PCV system does a lot of good. It sucks unburnt gas vapors out of the engine, and helps keep water condensation out of your oil. The gas vapors returned into the intake manifold make it possible to run a slightly leaner mixture, or if you have a car with O2 sensors on it, the cars computer does it automatically. In most cases, the crankcase on a PCV system operates at a slight vacuum, and this really helps to reduce any oil leaks in the engine.

If you just dump the blowby gasses into the air, through a breather system, you are wasting the gasses, and they really do add a lot to smog, far much more than if the blowby gasses were routed back into the intake, and burned.

For what it is worth, I am working on a 1947 Willys jeep, and it has a PCV system on it.

As far as different emissions systems go, the PVC system is good. It does not rob power. It helps your engine last longer, and stay cleaner. It reduces your cost to run the engine, and helps you get better mileage.

You will be better off leaving the PCV system intact on your engine.
 
#16 ·
DanielC said:
The PCV system does a lot of good. It sucks unburnt gas vapors out of the engine, and helps keep water condensation out of your oil. The gas vapors returned into the intake manifold make it possible to run a slightly leaner mixture, or if you have a car with O2 sensors on it, the cars computer does it automatically. In most cases, the crankcase on a PCV system operates at a slight vacuum, and this really helps to reduce any oil leaks in the engine.

If you just dump the blowby gasses into the air, through a breather system, you are wasting the gasses, and they really do add a lot to smog, far much more than if the blowby gasses were routed back into the intake, and burned.

For what it is worth, I am working on a 1947 Willys jeep, and it has a PCV system on it.

As far as different emissions systems go, the PVC system is good. It does not rob power. It helps your engine last longer, and stay cleaner. It reduces your cost to run the engine, and helps you get better mileage.

You will be better off leaving the PCV system intact on your engine.
x2 ...great advice!
 
#17 ·
Currently I'm trying to find out if it's typical for my vehicle to not pull any vacuum on the crankcase at idle. I'll ask others on another forum with the same engines to find out. May just be an oil pan gasket, etc. leaking air into the crankcase. I would like to see 1-3" of guage vacuum when I connect to my dipstick tube. I'm currently fighting a small rear main leak but I also need to put the cooler thermostat back in and hopefully the rear cap's bore will not expand as much thus less compression of the rear seal against crank.
 
#18 ·
The high vacuum at idle almost closes the PCV valve. Less vacuum at mid throttle, and the PCV valve opens up more, and if the manifold pressure goes positive (during a backfire) the PCV valve closes completely.

I would obviously check the valve first. Does it rattle when you shake it? Can you disconnect the crankcase side of it and feel a vacuum there? If yes, do a leakdown test on the engine, and see if there is a lot of air blowing past the rings.
 
#19 ·
Intense RT said:
/The pcv valve IS there to Ventilate POSITIVE CRANKCASE PRESSURE. The opposite valve cover provides fresh filtered air to the crankcase. Yes, it does remove aerated oil and unburned fuel/ combustion by products that get past the rings. At idle the pcv is practically closed. Only enough air gets by to make a hissing sound and this is correct operation. When there is little to no vacuum the spring pushes the valve down letting more blow by pressure to be released from the crankcase.

This is how internet misinformation comes about. If you disagree go read some fsm's or aa1car.com. They have a lot of information. That or just understand pressure differential.

Intense RT, i know how pcv systems work and what they do.
i gave no misinformation.
but excuse me for not explaining exactly how and why a pcv system works.
the pcv valve is there to vent blowby gases into the intake instead of the atmosphere.
a road draft tube will do the very same thing, only not as efficiently and it vents to the atmosphere.
not all car have a pcv valve, they use an orifice but they do have a crankcase vent for fresh air and excessive blowby.
the pcv system is an emissions device.
a by product of the pcv system is the oil stays clean longer which leads to longer lasting motors.

FSM 1982 chevy pickup pcv system description and operation,
"in 1961, the earliest form of automotive emission control, the PCV system was standard equipment on all GM vehicles sold in california. 2 years later in 1963, all GM vehicles sold in the USA were equipped with this emission control system.
the PCV system prevents blowby gases from entering the atmosphere by routing the blowby into the intake manifold through a vacuum controlled ventilating valve and a hose while allowing proper crankcase ventilation."

FSM 1990 chevy pick TBI pcv system description and operation,
"during certain engine conditions, more blowby gases are created than the ventilator valve can handle. the excess is returned through the air inlet tube to the air cleaner and TBI unit where it is burned in the engine."

FSM 2003 chevy pickup 5.3 pcv system description and operation,
"if abnormal operating conditions arise, the system is designed to allow excessive amounts of blowby gases to back flow through the crankcase vent tube into the engine air inlet to be consumed by normal combustion"
 
#20 ·
Soul_Hunter said:
Intense RT, i know how pcv systems work and what they do.
i gave no misinformation.
but excuse me for not explaining exactly how and why a pcv system works.
the pcv valve is there to vent blowby gases into the intake instead of the atmosphere.
a road draft tube will do the very same thing, only not as efficiently and it vents to the atmosphere.
not all car have a pcv valve, they use an orifice but they do have a crankcase vent for fresh air and excessive blowby.
the pcv system is an emissions device.
a by product of the pcv system is the oil stays clean longer which leads to longer lasting motors.

FSM 1982 chevy pickup pcv system description and operation,
"in 1961, the earliest form of automotive emission control, the PCV system was standard equipment on all GM vehicles sold in california. 2 years later in 1963, all GM vehicles sold in the USA were equipped with this emission control system.
the PCV system prevents blowby gases from entering the atmosphere by routing the blowby into the intake manifold through a vacuum controlled ventilating valve and a hose while allowing proper crankcase ventilation."

FSM 1990 chevy pick TBI pcv system description and operation,
"during certain engine conditions, more blowby gases are created than the ventilator valve can handle. the excess is returned through the air inlet tube to the air cleaner and TBI unit where it is burned in the engine."

FSM 2003 chevy pickup 5.3 pcv system description and operation,
"if abnormal operating conditions arise, the system is designed to allow excessive amounts of blowby gases to back flow through the crankcase vent tube into the engine air inlet to be consumed by normal combustion"
You gave no information there in regards to a "pcv valve and how it operates under varying pressures" and you previously described the operation incorrectly, fact.
 
#21 ·
DanielC said:
The high vacuum at idle almost closes the PCV valve. Less vacuum at mid throttle, and the PCV valve opens up more, and if the manifold pressure goes positive (during a backfire) the PCV valve closes completely.

I would obviously check the valve first. Does it rattle when you shake it? Can you disconnect the crankcase side of it and feel a vacuum there? If yes, do a leakdown test on the engine, and see if there is a lot of air blowing past the rings.
Yes the valve moves and I checked the operation of the spring. Operatively, it functions. At idle it is effectively closed, minus the designed in leakage (a slight hissing). Bump the throttle and it opens, then goes back to closed. On the vacuum guage if you bump the throttle and hold for not quite a second, you can see the the guage go from vacuum to a very slight positive pressure while connected snugly at the dipstick tube.

The engine has 65K miles and runs great, especially when compared to others ET's with comparable mods. I would like to see some vacuum in the crankcase though. :thumbup:
 
#24 ·
You guys are really confusing me. PCV valve on my SBC is connected to a line to the base of the carb. With this line hooked up, I get vacuum @ idle, at the valve cover end of the valve.

I thought vacuum opened the valve, not closed it as someone stated.

QUOTE:
Originally Posted by DanielC
The high vacuum at idle almost closes the PCV valve. Less vacuum at mid throttle, and the PCV valve opens up more, and if the manifold pressure goes positive (during a backfire) the PCV valve closes completely.QUOTE

PCV valves, for the same engine type, can have different vacuum requirements. A pcv valve for a SBC is different for a L-48 motor than for a L-82 motor, mainly due to the camshafts.

Please correct me if I am wrong.
 
#26 ·
PCV valves are calibrated for a certain amount of flow according to the engine combo from the factory. Many aftermarket PCV valves don't really concern themselves too much with this calibration, therefore one part number can cover a much wider range of applications with the same physical characteristics. Generally it doesn't matter that much, but if you replaced a brand new, proper, GM PCV valve with a generic aftermarket one, you'd probably notice a difference. ;) :cool:
 
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