1991 w250 90'000 blowing raw fuel ( 1/4 tank in 15 minutes)from the crankcase vent tube. 3/4 inch clear plastic tube. not the water separator. truck started and ran normal. have not found any fuel on the dip stick. how would i go about diagnosing whats failed??

 

Then it's probably dripping off the vent tube, not coming out of it. There's no way to get fuel out of there and not have the oil level rise rapidly and smell strongly of fuel. My guess is the fuel heater gasket or one of the banjo bolt washers.

 

How high was the oil on the dipstick? If fuel got in there, it will still be black, and may or may not smell like raw fuel. It could well have been the ULSD taking out your old seals (front pump seal will let fuel into the crankcase when it goes), esp with your ultra low mileage.

AS for diagnosis, clean it all off real well (simple green and a garden hose with a nozzle works), and watch it for leaks. If it is still blowing out the breather tube, see above.

Daniel