Why Is My Diesel Engine Pushing Oil Out the Dipstick?

Oil dipstick tubes come in two types, submerged and open. An open dipstick tube pokes a little way down into your block, but likely not past the bottom of the block casting and certainly not into the oil itself. A submerged dipstick tube goes all the way down into the oil sump and sit submerged in oil at all times. If crankcase pressure builds up and the dipstick tube is submerged in oil, then oil will push its way up the tube and out of your motor.

Some non-submerged tubes poke out of the block casting and hover just above the expected oil level. Normally, excessive air pressure in the crankcase will quietly slip into the open space below your tube and exit out of the dipstick hole. However, overfilling the oil sump can submerge an otherwise non-submerged tube, sealing it and forcing oil to crawl upward in order to relieve pressure.

Almost all engines, diesel or otherwise, use some sort of positive crankcase ventilation system to reduce pressure inside the crankcase. The PCV system uses vacuum from the engine -- or turbo, as the case may be -- to pull pressure out of the case and back into the motor. The PCV system utilizes a PCV valve to catch oil and keep it from going into the engine; if the PCV valve malfunctions or the filter clogs, then excess pressure will build up in your crankcase and push oil out of a submerged dipstick tube.

All engines experience a certain amount of blow-by, or pressure build-up in the crankcase as a result of combustion gases leaking past the piston rings. Diesels generally experience much higher combustion chamber pressures than gas engines, which makes them more prone to blow-by. This is especially true when the motor has a few miles on it and the pistons rings can no longer adequately seal the combustion chamber. The only solution to this problem is a complete rebuild with an over-bore, new pistons and new rings.