Okay, I've got one for you. My truck has 125K on it. Just in the past couple of days, she's developed a problem....and to be quite honest, I'm stumped! Here ya go; 1990 F-250, 5.0 engine....she is building up engine pressure and blowing oil out the dip stick! Now, you know in order to do that, that has to be a great deal of engine pressure. I've changed the PVC value, oil cap (vent issue?), and given all of my years of experience, have never seen anything like this. Engine runs great, oil pressure good, not running hot....but she builds up pressure. Any thoughts?

 

Might check compression.

 

If the problem occurred slowly I would suspect normal ring wear, and if it was sudden as you say but accompanied by a dropped cylinder, then I would think broken ring, but you say it still runs great. Might be worth it to do a comp. check anyway, just to see if all 8 are within 20% of each other and at a decent level.

If that checks out ok, I'd be interested to know if you have a lot of blowby coming out the oil cap if you run it with the cap off (not to drive it like that, just parked and working on it). How about the hose from the PCV valve to the intake? You say you replaced the valve, but is the hose in good shape, not plugged up or rotten inside? Or underneath the PCV valve, in the engine, is it plugged up with sludge, blocking the passage? -TD