correct me if I'm wrong here but I thought the whole PCV idea was to take the oil fumes and add them to the combustion chamber as part of emissions since it is positive crankcase ventilation I never had pcv's on my older cars and only added one to increase vacuum on one side of my head to get better ring seal and the passenger side was the vent? I even noticed when I blocked one side and left the other open I had serious blowby but when I ran the pcv system I reduced it by a large degree and the motor consumed less oil...

 

Yes... a "closed" PCV system pulls the vapors out of the crankcase and burns them in the combustion chambers. The vapors can include combustibles, like fuel that bypasses the rings and oil vapors, and "inerts" like water vapor of CO2. The've been using these systems for at least 25 years, that I am familiar with. It was one of the first "emissions" systems.

The only thing unique about the LT1 system is the fact that in the 1994-1997 models, the air that is pulled through the crankcase to "sweep" that vapor out has been filtered, and then metered by the MAF sensor. And the PCM includes this air in the combustion calculations.

 

here comes the million dollar question------How does the MAF meter the air?

 

How does the MAF meter it when the air enters the engine AFTER the MAF?

 

All the air goes through the MAF and gets measured. Most of it goes through the throttle body ports, into the intake manifold. A small amount enters a port in the TB, in front of the blades, and flows through the line to the valve covers. All of the air ends up in the combustion chambers.

 

How does the MAF know how much air is going thru the TB when it only measures air coming thru the air filter? In other words there is air from the I.A.C, PCV on the left, E.G.R on the left, and throttle blades which is seperate from the MAF??

 

Exactly. It has no way of metering it since the breather tube is at the TB. It is a small amount of air, therefore it does not have much effect on the measurements.

 

Ah, but it is metered. Read Injuneer's reply again as he's got it right. The air that reaches the breather tube in the TB (and runs to the passenger VC) had to pass through the air filter and MAF (where it's metered) before it got to the TB.

Rob
95Z28

 

Exactly... the air that enters the PCV system has been measured by the MAF sensor (when it was part of the entire mass of air entering the intake system), and the PCM uses that measurement to calculate how much fuel it adds to the system.

If you eliminate the tube from the TB, the PCV valve pulls in air that has not been included in the MAF calculation, and hence the PCM does not add enough fuel, and has to use the long term fuel corrections to add the required fuel.

I said... "Nothing that the PCM can't correct for"....

Just trying to help people understand how the system works.

 

thats not how mine is set up the only thing ahead of my MAF is the air filter and the A.I.R pump line. According to the manual removal of the pcv system only creates more blowby and increased emissions due to more fuel being added by the pcm due to less air entering the TB and the MAF readings stay the same. Now if the pcv passed first before the MAF I could understand the relationship between the two.

 

OK.... I'll try this one more time.

The MAF sensor measures all the air that passes through it.

Some of this air goes thru the TB blades.

Some of this air goes through the IAC passage

Some of this air enters the port before the TB blades and enters the crankcase, where the PVC valve dumps it back into the intake manifold.

The PCM can not know how the air was split between the blades, the IAC and the PCV... and it doesn't care. It only knows that ALL the air that went through the MAF sensor ends up in the combustion chamber. It adds the required mass of fuel to match the MAF measured amount of air.

So.... with a closed PCV system, total air in thru the MAF = total air burned.

But, if you REMOVE the vent hose to the valve cover, and replace it with a breather, there is extra air entering the combustion chambers, via the PCV system, that has NOT been metered by the MAF. Hence, you have created the equivalent of a small, controlled vacuum leak. There is no fuel provided by the PCM to go along with the air being pulled into the intake manifold through the breather. The O2 sensors see a slight lean condition, and add the extra fuel using the LTFT's.

What part of this are you having trouble with ?

 

Alright the MAF ONLY measures the amount of air entering the engine and sends a signal to the pcm telling it either WOT exists or part throttle or vice versa and tells the pcm to either go lean or rich or e.t.c which is directly connected to the I.A.C-I.A.T, and T.P.S. When you take the pcv and all related components OUT of the mix it tells the pcm NOTHING!! Because the LT1 is running 10-1 CR it relies on the pcv for proper ring seal. I bet if you hooked up a crankcase evac to your headers on your drag car you will see what I mean!! I'm not trying to doubt you believe me I just read a lot of info on the pcv system and from being in a garage way too much without working there That's also why most draggers pushing 13-15.1 CR they put in a vacuum assisted pcv system and that is basically the same scenario with our LT1's except at higher rpm's or WOT it creates a "vacuum" which sucks the blowby thus creating a better seal for the rings thus also stopping the pressure from blowing the valve cover gasket off the seal!! Although I haven's seen that yet I have watched my friends LT1 start to leak oil through the gaskets because he totally did away with his pcv system and plugged the holes in the cover on the passenger side including the oil fill hole.

 

True.

False. The MAF measures the air that passes through it. That's it. The TPS tells the PCM whether you're WOT or part throttle.

False. The PCM does not dictate rich or lean, it responds to the exhaust gasses as reported by the O2 sensors (while in closed loop and not WOT). In fact, the PCM tries its best to maintain 14.7:1 AFR while in closed loop and non-WOT.

False. It depends on how you accomplish removing those items. If you remove the TB to passenger side VC hose, plug the TB port, and install a filter in the VC.....the result is a vacuum leak in the sense that air is entering the combustion chamber without having passed through the MAF (i.e. the PCM doesn't know about it).

.....and that has nothing to do with the MAF-metered vs. non-MAF metered air.

Yep, that's a likely scenario when you pressurize the crankcase. Let's reference your earlier post about removing the PCV system, after the reading you've done.....I missed the part about oil leaks:

 

grygst76: i don't think you get it... These guys are talking about leaving the PCV system in place, but replacing the crank case ventilation hose with a breather.

Replacing this hose with a breather lets in unmetered air. This air passes through the block, and is then vacuumed into the INTAKE through the PCV valve. From there, the air enters a cylinder and exits through the exhaust.

Next question... Why "unmetered"? Because the air enters through the breather. With the hose in place, the air enters through the filter, MAF, and exits the throttle body just before the butterflies, running through that hose, and into the crankcase (where it exits through the PCV system into the intake, and through the heads, cyliders, and and finally exhaust).

Now, as you were stating, removing the PCV system ENTIRELY, there will be no "unmetered air" flowing through the combustion chamber, but that's not the original intent of this thread.

I'm guilty of throwing a breather in the valvecover too, but that's only because I had been getting oil in the line and didn't know a breather filter like the aforementioned one existed.

 

ok fred has convinced me to put my hose back on....
thanks
Jesse