Positive Crankcase Ventilation

[TandW]

Can anyone explain the internal self contained system the 8.1 has. I sure would like to know if there's an orifice or sumpin' that might sludge up over time. And what should the crankcase pressure be during WOT? Just curious!!

[sknight]

The 8.1 uses a fixed orifice style PCV system. Basically instead of a moving valve it just has a small hole in the nipple of the valve cover to meter the air drawn into the intake.

It "Shouldn't" sludge up over time however if you get behind on your oil changes it could happen. Since it's a short passage with engine vacuum on it it shouldn't be able to get blocked. At least in my time with GM I didn't see any issues.

As for the pressure in the crankcase it will rise considerably during WOT because of increased blowby during high combustion pressure periods and the zero vacuum that will be drawn on the system while the throttle is wide open. It's not an issue though, the system flow reverses and vacuum is drawn on the vent hose that's in front of the throttle body. Whatever pressure builds will just get vented there, it shouldn't build up to a positive pressure.

[TandW]

Don't know what is meant by a vacuum hose on TBI nor nipple hole in valve cover. The 8.1 is listed as having an "INTERNAL" pcv system. Help!!

[sknight]

It's internal as in it's in the valve cover and doesn't use an external mechanical valve.

[TandW]

Can you elaborate further? What was the business about a hose?

[sknight]

A PCV (Positive crankcase ventilation system) uses engine vacuum created in the intake manifold to evacuate excess pressure in the crankcase. This pressure mainly stems from gasses leaking past the sealing rings on the pistons. It also comes from engine heat raising the pressure inside the crankcase. This pressure has to be relieved or the pressure will build up and cause oil leaks and horsepower loss. The system is vented so that excess vacuum won't develop in the crankcase and it also allows combustible fumes to be drawn out.

The system is very simple, there is a hose that attaches to the air filtration system at some point before the throttle body and draws in the fresh air that is positive atmospheric pressure. This air enters the engine, in the case of the 8.1 through the right hand valve cover. The fresh air goes through the engine and draws the fumes out.

These fumes are mainly gasoline vapor. When the piston goes up compressing the mixture in the cylinder the pressure reaches 200+ PSI. A little of that pressure leaks past the sealing rings, about 3% is normal, 5% is the practical limit, 10% will start causing running problems. The rest of the fumes come from heating oil to 250* degrees, hydrocarbons that escape as oil gets heated.

The fresh air and fumes are drawn out of the engine by engine vacuum. The left hand valve cover has a nipple on it to attach a hose. This nipple has a small orifice in it, around .150 of an inch according to my sometimes calibrated eye which reduces the amount or air that can flow through it. If it were subjected to full engine vacuum the engine oil would get sucked out.

This flows through a rubber hose that is attached to the intake manifold after the throttle body which is an environment that lives at 20ish" of vacuum. The fumes are drawn into the intake manifold, it mixes with the regular air that is drawn through the air filter, and enters the cylinder to be burned. This way any gasoline that gets by the sealing rings goes back in and is burned plus the hydrocarbons from the oil get burned lowering emissions.

That's an internal system, a metering devise within the valve cover allows gasses to be burned inside the engine.

On an external system basically there's a filter exposed to the air and what's called a road tube that vents the pressure to the atmosphere. It was the system used on all engines up to the early 60's. While inefficient it's still the system used on diesel engines because of their lack of manifold vacuum to draw fumes through, plus since the fuel in a diesel isn't introduced until combustion is set to occur very little in the way of fumes get through.

Hope this helps, feel free to ask more if necessary.

[TandW]

Still need a description of the GM 8.1 litre engine INTERNAL POSITIVE CRANKCASE VENTILATION SYSTEM, please.

[sknight]

I just laid it out, part, by part, by part, exactly how the 8.1 system works and what comprises it. I worked for Chevrolet for 10 years. I've been to numerous classes, repaired these systems hands on. Repaired driveability problems caused by these that others couldn't. This basic system has been on all GM manufactured engines (Which the 8.1 was until the Tonawanda plant stopped making them.) since around 2004.

If you want a description of how your 8.1 liter internal positive crankcase ventilation system system works I suggest you read post #6 or actually go to a dealership. This is my last post, good luck finding the answers you want to hear.

[TandW]

I was hoping you could describe the flow path as there are no external fittings. There is no fitting between the throttle body and the manifold. There are no external vacuum lines on the engine. I was figuring you might say there were ports for this function between the intake manifold and the cylinder heads and where we would find them during a tear-down. I thought someone might be able to explain the system. Guess I'm wrong. Oh well!

[TandW]

Come on guys! Sooner than later one of us will have a plugged up crankcase ventilation system. How can we fix it without a teardown? All I can find is on the marine engine its internal but having to do with the intake manifold. Driver, you need to know cause you like WOT!! How about it guys?

[mfire1339]

TandW

Don't quote me but I believe it is "built in to the underside" of the intake manifold. Not having much of a memory I believe I saw it when my engine was torn down looking for the missing piston in #7 hole.

Mike

[DriVer]

Quote:

Originally Posted by TandW Driver, you need to know cause you like WOT!!?

I don't think that I could have written a better description other than what sknight offered. I have to commend sknight for putting a lot of thought into his reply.

I'm just going through the service manual and the PCV is not shown as being a maintenance replaceable part. The Type valve is shown as N/A. On the 454 there is an AC part Number.

Inspect the following:
The PCV port (8) for debris or varnish buildup.

Intake Manifold
The intake manifold is a one-piece design. The intake
manifold is made of cast aluminum. The throttle body
is attached to the front of the intake manifold. A linear
exhaust gas recirculation (EGR) port is cast into the
manifold for exhaust gas recirculation mixture. The
EGR valve bolts onto the rear of the intake manifold.
The fuel rail assembly with eight separate fuel
injectors is retained to the intake manifold by four
studs. The fuel injectors are seated in their individual
manifold bores with O-ring seals to provide sealing. A
Manifold Absolute Pressure (MAP) sensor is mounted
on the top of the intake manifold and sealed by an

O-ring seal. The MAP sensor is held in place with a

retainer bolt. The evaporative emission canister
solenoid is located in the front of the intake manifold.
The positive crankcase ventilation (PCV) system is
internally cast into the intake manifold. There is not a
PCV valve. A splash shield is installed under the
intake manifold. The shield prevents hot oil from
contacting the bottom of the intake manifold, maintaining
air inlet charge density.

The L18 has an internal positive crankcase
ventilation (PCV) system, and does not

utilize a PCV valve.

Results of Incorrect Operation

A plugged PCV pipe/passageway may cause:

• Rough idle
• Stalling or slow idle speed
• Oil leaks
• Sludge in engine

There you go ... that's just about all I have on it.

[sknight]

Did a little research and did come up with a design change that shows the fixed orifice is part of the lower intake instead of the left valve cover. No I don't know what years, all it did was move the point of entry and eliminate a hose, the system is still the exact same as I described. Since I left GM in 2008 the newest I would have paid attention to would have been a 2007 model. Anyhow sorry for any confusion that caused, last one I worked on and noticed was conventional in it's setup.

Either way if it gets plugged up, which I've seen well abused trucks with hundreds of thousands of miles still sucking away, you're going to your mechanic or getting your tools out for a long weekend.

The reason they dropped mechanical PCV valves in favor of fixed orifice is because of issues like clogging and general failure. Basically it lowered maintenance a little.

If you want to see part of the system, the silver nipple on the top right of this pic is the vent I described. A hose runs from that to the fresh air inlet.

[gg]

sknight

Excellent post,

In God we Trust - everyone else bring Data!

IF memory serves my correct the PVC replace the down draft tube in the 1963 GM model year. Thus is was the first emisson control device.

gg