I don't know anything about this particular engine. In general, some blow by and "smoke" will always be in the crank case. I would check the breather system to make sure it is not clogged or restricted, which would force all blowby (even normal blowby) out the filler cap. I would also take off the breather hose where it enters the intake system and see if it is also blowing out there at the same time. If so, that would indicate to me the breather system is working and there is a lot of blowby. Low or no flow compared to the oil cap would indicate a clogged breather system. Ofcourse, a compression check would be the ultimate tool.

Just some thoughts, may be some one else can provide hard facts.

Best of luck.

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Deezel
87 300TDT
150,000 miles