Extreme Crankcase Vacuum: Yes or No?

[savatreatabvr]

I took off the oil cap on my mid 70's 454 while it was running yesterday and there was a huge amount of vacuum. When it's not running the cap comes off easily off but when running there is so much vacuum the cap or PCV is hard to get off, is this normal? I'm leaning towards no, what causes this?

[mud.man.rj]

The pcv will have whatever vacumn the crankcase has which when pulled running is quite a bit of force same as the open port it comes from will really suck your finger in if blocking the port. So I would say all cool.

[savatreatabvr]

Really? With the oil cap on and the PCV pulled yeah the PCV sucks pretty good but I'm talking about when first pulled the suction is more than expected.

Originally Posted by mud.man.rj

The pcv will have whatever vacumn the crankcase has which when pulled running is quite a bit of force same as the open port it comes from will really suck your finger in if blocking the port. So I would say all cool.

[1LowToy]

Check your crankcase breather tube/filter

[beamn7]

Originally Posted by 1lowtoy

check your crankcase breather tube/filter

x2....

[donut]

Originally Posted by 1LowToy

Check your crankcase breather tube/filter

x3

Your breather should be allowing air into the engine, it shouldn't sound like a Coke can opening. V in PCV is for ventilation, not vacuum.

[savatreatabvr]

I'm lost!

Originally Posted by donut

x3

Your breather should be allowing air into the engine, it shouldn't sound like a Coke can opening. V in PCV is for ventilation, not vacuum.

[mud.man.rj]

Ok pcv is for crankcase ventilation which is why it sucks at the end of valve, it sucks crankcase vapour back into the bottom of carb base which has a vacumn port, you had it backwards. Just like the port for a brake booster.

[mud.man.rj]

Smaller the cam lift the more vacumn at idle especially for brake vacumn, the bigger the cam the less vacumn, so awesome engine, great vacumn.

[raceman6135]

There should be a tube that runs from the passenger's side of the air cleaner housing into the valve cover.

Inside the air cleaner, this tube pulls air through a small filtre. The filtre just sits in the plastic "tray" on the inside of the air cleaner housing.

If this filtre and/or tube are plugged, air cannot get into the crankcase. Your PCV will continue to apply a vacuum, but without air able to replace it through the filtre/tube, you'll experience higher than normal vacuum in the crankcase.

[mud.man.rj]

Originally Posted by raceman6135

There should be a tube that runs from the passenger's side of the air cleaner housing into the valve cover.

Inside the air cleaner, this tube pulls air through a small filtre. The filtre just sits in the plastic "tray" on the inside of the air cleaner housing.

If this filtre and/or tube are plugged, air cannot get into the crankcase. Your PCV will continue to apply a vacuum, but without air able to replace it through the filtre/tube, you'll experience higher than normal vacuum in the crankcase.

The tube pushs air into the aircleaner housing and reburns the crankcase vapour, the old days the tube vented down the side of tranny and was called a blowby tube and when an engine wore out it blew a lot so as an emmision thing they vented it into the carb to be reburned. Most people run an breather cap which vents, not sucks.

[mud.man.rj]

Yep as wilkin250R mentioned, normal.

[savatreatabvr]

Ok makes since but what if I'm using an after market Moroso air cleaner? There is a place under the air cleaner to hook a hose too but nothings there, I only have the PCV hooked up to the Valve cover and base plate of the Quadrajet. A friend talked me into using the breather filter in the driver side valve cover for crankcase ventilation, should I keep the breather filter, switch back to the original oil cap or run a hose from the valve cover to the air cleaner?

Originally Posted by raceman6135

There should be a tube that runs from the passenger's side of the air cleaner housing into the valve cover.

Inside the air cleaner, this tube pulls air through a small filtre. The filtre just sits in the plastic "tray" on the inside of the air cleaner housing.

If this filtre and/or tube are plugged, air cannot get into the crankcase. Your PCV will continue to apply a vacuum, but without air able to replace it through the filtre/tube, you'll experience higher than normal vacuum in the crankcase.

[wilkin250r]

During normal operation, there is a small amount of vapor that gets past the piston rings, it's called blowby. The vast majority of these vapors are unburned hydrocarbons, which will pressurize the crankcase and build up, causing problems.

The PCV valve is the answer to this problem, it stands for Positive Crankcase Ventilation.

The PCV valve isn't just normal ventilation, it doesn't just vent to air. It should be connected to the manifold to allow the manifold vacuum to "suck" these vapors from the crankcase. So yes, you will get vacuum pressure at the PCV.

If normal manifold vacuum at idle is somewhere around 17-20 in-HG, this corresponds to about 9-10psi. If your oil cap is about 1.5 square inches, that's as much as 15 lbs!

I'm leaning in the direction of "this is perfectly normal".

[raceman6135]

In theory, the K&N-style breather you have on the driver's side valve cover should flow more than enough air.

Is that filtre plugged? Is there something inside the grommet that is preventing air to flow into the crankcase? I've seen instances where a closed grommet (one that is supposed to be cut open to the proper size by the end user) was used in error, which prevented airflow.

As for which way is better between a stand-alone breather or one that is plumbed into the bottom of the Moroso filtre housing, I think it's just a matter of preference.

With the tube connected to the inside of the filtre housing, any excessive oil mist (blow-by) that the PCV system cannot handle will be deposited inside the air filtre housing. With the open breather, the oil mist accumulates on the top of the valve cover, making it look messy.

[savatreatabvr]

I love this forum, you guys have given me some excellent advice over the last 5 months and I'm sure you've saved me lots of money so Thank You, I sincerely appreciate it!

Originally Posted by raceman6135

In theory, the K&N-style breather you have on the driver's side valve cover should flow more than enough air.

Is that filtre plugged? Is there something inside the grommet that is preventing air to flow into the crankcase? I've seen instances where a closed grommet (one that is supposed to be cut open to the proper size by the end user) was used in error, which prevented airflow.

As for which way is better between a stand-alone breather or one that is plumbed into the bottom of the Moroso filtre housing, I think it's just a matter of preference.

With the tube connected to the inside of the filtre housing, any excessive oil mist (blow-by) that the PCV system cannot handle will be deposited inside the air filtre housing. With the open breather, the oil mist accumulates on the top of the valve cover, making it look messy.

[mud.man.rj]

I usually run a pvc to valve cover and run as breather cap as you are and if air cleaner is equipted with vent plug or remove it. If engine is to see high rpm all the time like a solid lifter cam 283 or 327 then I run a pcv and valve and a 2 breather caps to make sure I get lots of ventilation, in most cases almost all a pcv and a breather are great.

[gmhardwick66]

I ran the PCV valve on one valve cover to base of carb. For the "breather" on the other valve cover I ran a hose to bottom side of my 14" air breather. I have had no problem with oil seapage coing from the "breather" side valve cover. No oil in air cleaner base either. No matter which route you choose to go ...you must have a PCV on one side of engine and a breather on the other side of engine. This will allow fresh air to be pulled into crankcase while at the same time allowing all crankcase vapors [blowby] to be pulled into the PCV valve and drawn into your intake charge.