frantzevan

I purchased a 99 Mack E7/400 about 6 months ago and recently noticed milky sludge on the underside of the oil cap. The dip stick doesn’t show any sludge when it’s pulled. The only other place that I see sludge is on the vent tube after the wire mesh filter. I opened the mesh  filter and it was all but clean no sludge there just in the tube after it.  It doesn’t appear to be losing antifreeze out of the pressurized side of the tank. The reservoir side had a cracked cap and is empty. How Could it be  pushing coolant into the oil  but stay filled to the pressurized cap? It is winter and the temps have been ranging from 20 to 35. The truck is used on short hauls and is shut off for an hour or so during the day before making another run. Could this be condensation from the heat or blow by?