More Oil Pressure Observations

While racing at Road America back in September we blew an engine on Rick Morris’s TR3A. It wasn’t really the result of anything strange, but rather a miscalculated BMEP (Brake Mean Effective Pressure).

However, when the engine was disassembled we found that, while the engine had made a few dozen turns after the blow-up, it was packed with debris, particularly in the oil galleries behind the main bearings. While this stuff made its way into every nook and cranny of the block, the oil filter was almost free of junk.

Of course, the big question was, how did all this debris get past the clean oil filter, clean oil cooler and back into the engine? There had to be some type of interconnection.

On all Purolator filter heads there is a pressure relief valve in the housing, near where the filter canister seats against it. It looks like a small ball bearing. It takes only a few ounces of pressure to open, which allows the unfiltered oil to bypass the engine, go into the main oil gallery and back to the bearings. Further, when the oil is cold and the pressure high, most of the oil bypasses the filter (and the oil cooler if so equipped).

For racing, we usually plug the hole and remove the ball, but even for street use it allows a good portion of the oil to by-pass, and a much stronger spring should be used. This by-passing of the oil filter has been linked to accelerated cam and lifter wear along with cylinder and piston wear. On a street-driven car in winter time, the relief valve stays open until the oil warms up, which could be never in cold climate environments.

If you put in a stronger relief valve spring, you can determine if it’s too strong by watching your basic operating oil pressure on startup. If it’s lower than normal then the spring is too strong. 

If you are concerned that your bypass valve may be malfunctioning, or if you are just plain nosy, take off the brass plug and spring. You can leave the ball in the housing. Drill about a 1/8" hole through the top of the brass plug and push a length of about 18 gauge insulated wire through it, solder the inside end to the spring. then insulate the spring with tape, except where the ball rests. Reassemble the ball, spring and cap. Apply some silicone sealant around the wire where it comes out of the hole in the plug to prevent oil leaks. Connect the wire to a test lamp and then to the battery, and be sure the oil filter is grounded. When the ball comes off the seat, the test light should go out. Just be sure everything is properly grounded.