Be careful to check your oil level frequently!!
I have an 84 F150 carbed 4.9. When i did my valves over i decided to change the PCV valve 2 bucks and it has been in since84 so why not.
I put it in and about a week later took the truck on about a 250 mile trip. I stopped for fuel when i reached my destination and noticed an oil film in the back window of the bed cap.
I looked under the truck and the fuel tank was just loaded with oil. I figured The oil filter was leaking since i had just changed the oil the night before.
Got under the truck and noticed oil dripping from everything including the front of the oil pan. Checked the oil and could not find any on the dipstick. I added a quart of oil and checked again could just see a thumnails worth on the bottom of the dipstick.
Added two more quarts of oil to bring it up the full.
Checked the oil filter looked ok, oil seemed to be comming from the front seal.
Bought a couple of extra quarts and kept going.
Later that night i remembered i changed the PVC valve.
Next morning i pulled the valve out before driving the truck. No oil leak.
Put the valve back in drove an hour further checked again down a quart and oil everywhere. Pulled the pcv valve out disconnected the hoses put some duct tape around the hose and plugged it into the valve cover. Ran a little rough at idle but no more oil leaks.
Seems like with my old PCV valve was leaking a little applying a negative pressure to the crankcase all of the time. When i put the new valve in it applied a negative pressure just some of the time allowing a positive presssure to build up and making the front seal leak like crazy.
Since my engine uses almost no oil between oil changes I could have run my engine out of oil because of a $2 part.
Got home found my old valve in the trash and reinstalled it. No problems since.

 

I'll have to watch that. Reason why changing variables is sometimes not a good thing. Changing to Synthetic..replacing fluids that havne't been changed in ages...they can cause problems when all the sudden things change. *shrug*

Still..why is it that PCV Valves can cause you to lose mileage? I still don't understand.

 

I want to say that the engine is calibrated to expect so much air flow for any throttle body position and flows fuel accordingly. For example, if the throttle body is open at say, 30*, the TPS would relay that info to the computer, and it would spray some amount of fuel into the intake runners to match the expected air flow into the engine. The PVC is part of this expected air flow since it is letting air drawn through the engine crankcase to be fed into the intake and burn off any hydrocarbons. If this air flow is cut off, then the computer will still flow the same amount of fuel in relation to the TPS position even though there is less air going into the engine, hence it runs rich. This is for speed density engines(pre 96'), MASS air wouldn't be effected I dont think

 

Logical effect-cause relationship there...thanks man! =)

I just couldn't think of any reason why, but that makes good sense.

I don't really feel like swapping in a new computer...MAF...etc just for one more mile to the gallon and a bit more smoothness. Too expensive to deal with ther...