Community Forums:

I have a 1996 850 in which I broke and replaced the small vacuum hose 3 months ago. The small hose leads to a "vacuum tree" next to the throttle connection under a plastic cover which must be removed to access the vacuum tree. The plastic line from the flame trap holder to the vacuum tree measures about 5 inches. The small hose and two rubber connectors can be purchased from a dealer for a few dollars. This hose became clogged with baked oil and brittle due to engine heat. Its purpose is to apply vacuum the to the flame trap crankase ventilation system which prevents excessive crankase pressure from building up in your engine and eventually blowing your rear main engine seal. I learned this too late and had to replace my rear main seal due to a blocked vacuum line which caused excessive crankase pressure. Always check this vacuum line for blockage when you clean or change your flame trap.

The large line (from the flame trap holder) goes to the oil trap or oil vapor separator which is located under the manifold and connects to the front part of the engine to return oil and vapors to the crankase. This large line and oil trap rarely plugs unless you fail to change you oil filter regularly.

I was told by my parts guy that the small plastic hose and connectors were a special order item due to low demand for this item. However, rear main engine seals were a high demand item. It occured to me that many a rear main seal could be saved if more owners and technicians inspected and changed this small vacuum line on a regular basis.