Supercharger first start vacuum in crankcase

[Travis017]

Finally got my transmission back and installed with punisher valve body and circle D torque converter. Fired the truck up for the first time with the supercharger finally installed. Everything seems good and is working like it should on the lift. Went to check the oil after a short idle run and noticed I had a suction at the dipstick that released when I pulled it out. Checked while running and it’s pulling vacuum. Not sure if I forgot to do something. Installed a breather oil cap, removed all the emissions and capped the lines, installed a new pcv valve and screen under it. Could my pcv valve not be working right?

 

[FMOS Racing]

The PCV valve has a flow orifice in it that is supposed to limit the amount of "draw" it has on the crankcase. High vacuum isn't really an issue, though, except for drawing oil into the intake manifold. The real intention of the PCV system is to clear positive crankcase pressure caused by warm up and what gets past the rings.

However, if you have a breather, there shouldn't be any way for there to be any appreciable vacuum at all. That's pretty perplexing.

 

[Travis017]

I’m wondering if the vented oil cap isn’t actually letting air through. When I remove it while the trucks running, the vacuum at the dipstick goes away. The weird thing is I don’t believe I had the issue before the charger was mounted. I had already installed the vented oil cap and removed the line from the throttle body to valve cover and had driven it with no issues without the charger. When the trans was pulled I finished up the rest of the charger install which included removing the smog tubes off the back of the engine, removing egr, and capping any unused vacuum. Installed a new pcv valve and the screen below it as mine was missing as well. Installed the charger and oil feed and return lines at that time. Would the charger at idle create my vacuum issue and that’s why I’m just seeing it?

 

[FMOS Racing]

Nothing should create a vacuum with that vented oil cap. If you remove the cap and the vacuum goes away, it's the cap.

Unplug the nipple on the oil fill neck for giggles and see what happens?

 

[Travis017]

So the vented cap has a check valve in it and won’t let it pull air in but will let it breath out. I think I have the wrong cap. The place I bought it from only has vented caps with a check ball in to prevent it from sucking air in. I guess I need to find a different cap. Any suggestions?

 

[oilwell1415]

There's supposed to be vacuum there. If you feel like it's too much, check the PCV valve to make sure it's actually liming like it should. There are some PCV "valves" out there that aren't valves at all, they are just fittings to connect the crankcase to vacuum. They don't limit anything.

 

[Travis017]

There's supposed to be vacuum there. If you feel like it's too much, check the PCV valve to make sure it's actually liming like it should. There are some PCV "valves" out there that aren't valves at all, they are just fittings to connect the crankcase to vacuum. They don't limit anything.

You can hear it release suction after shutting off the truck and then pulling the dipstick. I have a brand new oem pcv valve and a new one from Carquest. Both do the same thing. I didn’t notice it before I bolted on the supercharger but I also installed the new pcv at the same time. I put a new screen filter in below the pcv also as mine was missing. I’m wondering if that has something to do with it.

 

[FMOS Racing]

On a stock truck, there is no restriction on the airflow at the filler neck. The tube across the motor attaches upstream of the throttle blade, so at that point the air in the intake stream is really close to atmospheric. There's no vacuum, and no restriction.

The reason this doesn't work with a supercharger is that connection gets positive pressure on it, and that then puts positive pressure in the crankcase, and then oil starts coming out of places where you don't want it to come out.

I always had issues pushing oil up the breather neck with an open element filter on it, which then dripped down the side of the valve cover and onto the header. Now I have a hose that connects to the stock nipple, runs to an oil separator, and then to a fitting between the air filter and the turbo inlet. The whole PCV system works as intended.

 

[Travis017]

On a stock truck, there is no restriction on the airflow at the filler neck. The tube across the motor attaches upstream of the throttle blade, so at that point the air in the intake stream is really close to atmospheric. There's no vacuum, and no restriction.

The reason this doesn't work with a supercharger is that connection gets positive pressure on it, and that then puts positive pressure in the crankcase, and then oil starts coming out of places where you don't want it to come out.

I always had issues pushing oil up the breather neck with an open element filter on it, which then dripped down the side of the valve cover and onto the header. Now I have a hose that connects to the stock nipple, runs to an oil separator, and then to a fitting between the air filter and the turbo inlet. The whole PCV system works as intended.

Good info. I guess I’ll run it some and see what happens. If I start drooling oil from the filler cap I’ll look into a catch can

 

[oilwell1415]

You can hear it release suction after shutting off the truck and then pulling the dipstick. I have a brand new oem pcv valve and a new one from Carquest. Both do the same thing. I didn’t notice it before I bolted on the supercharger but I also installed the new pcv at the same time. I put a new screen filter in below the pcv also as mine was missing. I’m wondering if that has something to do with it.

If you are hearing it release after the truck is turned off there is definitely something wrong. The crankcase should be vented and should not hold vacuum after the truck is off.

 

[rusty_408]

Are you mass air? If so, I think it's best that the crankcase is at vacuum when idling. The PCV design needs special consideration for mass air and a SC. The design ultimately needs to (1) avoid boost pressure in crankcase, (2) avoid intake of unmetered air, and (3) vent fumes flow blowby. This is my design for my novi.

 

[Travis017]

Are you mass air? If so, I think it's best that the crankcase is at vacuum when idling. The PCV design needs special consideration for mass air and a SC. The design ultimately needs to (1) avoid boost pressure in crankcase, (2) avoid intake of unmetered air, and (3) vent fumes flow blowby. This is my design for my novi.

No still on speed density.

 

[rusty_408]

oh. that's easier... if i sketched it like the one if posted, you would just need an open breather on the driver's valve cover, or an open line to the suction side of the SC. no check valve. option of a catch can if you start getting oil puke

 

[Travis017]

oh. that's easier... if i sketched it like the one if posted, you would just need an open breather on the driver's valve cover, or an open line to the suction side of the SC. no check valve. option of a catch can if you start getting oil puke

I have a breather on the passenger valve cover at the oil cap I just need to see if I can remove the check ball from it. Not sure I understand where the open line on the suction side of the sc would go?

 

[FMOS Racing]

I have a breather on the passenger valve cover at the oil cap I just need to see if I can remove the check ball from it. Not sure I understand where the open line on the suction side of the sc would go?

Same layout I described on my turbo truck. I ran a check in mine for a while, but then realized I didn't need it.

 

[Required]

I believe open line to suction side of the SC would be your air filter more or less. Or your intake tube. I run one into my airbox from the drivers side.

 

[Travis017]

I believe open line to suction side of the SC would be your air filter more or less. Or your intake tube. I run one into my airbox from the drivers side.

So could I run a hose from the passenger valve cover at the existing port on the filler neck to the air filter?

 

[Travis017]

Ok I was able to take the vented oil cap apart and remove the check valve. This solved the vacuum issue in the crankcase. Time will tell if I get any oil drooling from it.