Intermittent Blue Cloud
 

I've been chasing down an oil leak for a bit on this truck ('98). Maybe someone has had a similar issue.

How it presents itself... About every two-five miles a cloud of blue smoke will come out from the rear (not tail pipe) as I'm driving down the road and last about 5-10 seconds then go away. Constant speed between 50-65. Can't exactly tell where it's hitting the exhaust. No leaks when is is sitting idling.

Have replaced oil cooler lines (verified only one o-ring per fitting), front crank seal, and cam position sensor o-ring (it had turned into Bakelite). Really thought I had it with the cam position sensor o-ring as it took longer to start fogging for mosquitos, 25 miles, but once it started it was back to 2-5 miles. Going to check the oil pressure valve o-ring and it that not it, yanking the front cover and dropping sump. Front / bottom is always wet with oil. The cooler lines, front seal, and cam sensor were all leaking. Don't believe it's a rear main. Had the bell housing access cover off for the front seal replacement and everything in there was dry as a bone, no slung oil or anything.

I don't have a lot of history with this particular engine, yet. Just a quick h.g. job. Clock is showing just shy of 340k put it pulls to strong for it to be original and not opened up and gone through at some point.
Hoping to keep this one going for a year or so. Want to yank the one out of my '97 and preform the 4.6 conversion ceremony one foggy moonlit night. Have to wait a bit on that project, don't have the necessary sacrificial goats and/or chickens.

The front crank seal is a common leak point and not difficult to replace.

Maybe ensure the PCV path is not plugged. If plugged, pressure can build up in the crankcase and push oil out.

In addition to the PCV, is it at all possible it is a valve cover gasket leak allowing oil to hit the exhaust manifold?

Johns idea above sounds like a really great place to start looking.