Hello, I drive a 1981 Ford f100 with the 4.9 inline six.

About a week ago, I was driving and noticed that my lifter tick had disappeared and my oil pressure had fallen down from a constant 25-ish to a 10 at idle gaining about 10 PSI per 1000 RPMs. I changed the oil using 10-40 about two days later and now the oil pressure reads 20 PSI at idle and about 38 at 2000 RPMs. The tick is almost entirely gone, except upon a cold start and until it warms up.

However, now the truck seems to smell like oil. I'm going to toss this one out as oil I split or didn't clean up when I changed it for now, since I was very rushed when I did so. It also seems to release pressure through the breather tube (the piece of tube that goes from the oil cap to the air filter, right?). It makes a pressure release noise sometimes when I shut down the truck and it seems to be strong enough to blow the breather off the air filter housing.

Today I noticed that I appear to be missing a vacuum line from my PCV valve. That seems like it could be causing something along the lines of increased pressure like I described?

 

There should be a vacuum line from the PCV valve which attaches at the intake manifold vacuum "tree", which is a fitting with several [5?] ports. The PCV vacuum line is one of the larger ones and will be on the larger port. Sometimes, the power brake booster connects in the same area, or it might have a separate fitting at the base of the carburetor. Since one line is apparantly missing, check all other lines for missing, being loose or being cracked due to heat and age. There is a vacuum diagram on the radiator support.

These engines are sensitive to any vacuum leak and will run badly as a result.