16. Bypass restriction in a late model cooling system

As engines age and cooling passages in the block and head become less than perfectly open (even after flushing), it becomes just as easy for water to flow up through the bypass loop as to flow through the engine. At this point, temperatures will creep up to approximately 180 degrees. At 180 degrees, the normal 140 degree thermostat is fully open and mechanically restricts the flow of by-pass water entering through the top of the thermostat housing. In cases where cooling systems are reasonably well maintained, temperatures can be brought down closer to the thermostat's range by installing a partial restriction in the bypass loop.

Universal recognized that, except in very new engines, the bypass type of system did not provide very positive control between the time the thermostat began to open (at 140 degrees), and the time that is was fully open (at 180 degrees). An early service bulletin suggested that the hose barb of the brass elbow in the inlet to the thermostat housing be filled with lead and then opened up by drilling an 1/8” hole through the lead. In our judgment, this procedure causes a bit too much restriction. There are other ways to put a restriction in the bypass loop. Some folks install a small ball valve in the loop which gives an element of control to the restriction.

Moyer Marine markets a bypass restriction kit which uses a spring loaded check valve to create a nominal 2 to 3 PSI back pressure. This amount of restriction causes a slight preferential flow through the block as the thermostat begins to open to maintain temperature more toward the lower end of the thermostat's range.

When designing a restriction for the bypass loop, it does not pay to get greedy. Too much restriction will force water past the thermostat and result in “over-cooling” and carbon buildup will be somewhat accelerated. - Updated: November 4, 2003