Ive got a 79 F150 with a 351M in it. My oil pressure all of the sudden the other day dissapeared. Now, the pressure has always been low, like on the lowest part of the normal bracket, but now it has gone way down. The needle barely goes up now when i start the truck, its below the L. I believe that the guage is accurate because I reved the engine up and held it at high rpm, and the pressure went up, then dropped again after I let it go back to idle. Im running 20W-50 oil and the oil level is fine, and this is the first time Ive had a problem. What can it be? Do i have a clog somewhere, or do I finally have an excuse to rebuild the engine? Any help is greatly appreciated!!!

 

The cam bearings are what is the problem. The 20-50 didn't help ether. So yes you have a real good reason to rebuld it, hopfully before you hurt the crank, or it strands you.

 

Say what you will but I've gotten two years of daily driving out of the same problem before just by replacing the oil pump with a new one (not rebuilt). For some reason, I've found the stock oil pumps in the 'M' series always give out before the bearings do. The only bad news is that, if it's a 4x4, you're going to have to jack the engine to remove the oil pan (I've done it before with the oil pan hanging but it's a pain). Good Luck.

 

It is probably the cam bearings. You may be able to raise the pressure by using a High Volume oil pump. This will supply more oil, so what leaks out of the cam bearings is replace by the increased flow from the oil pump.

 

Personally, I've found that unless you increase the size of your oil pan, the high volume pump can actually make the problem worse because it tends to use up the stock oil capacity too quickly...In my opinion, stock oil pan =stock oil pump....high volume oil pump=extra capacity oil pan.Just my opinion based on 20 years experience with these creatures.