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High oil temperatures continue to be one of the most frequently reported problems from builders/flyers of Long-EZs. As reported in CP 66, page 4, a rather detailed engine and oil cooling test and analysis was conducted by Bill Freeman. He found that it literally took doubling the size of the oil cooler to keep the temperature of the oil at the desired level. There are three Long-EZs based at Mojave that are owned and flown by employees at Scaled. All three have varying degrees of high operating oil temperatures. These three have all made numerous changes to try to lower operating oil temperatures. Among these changes were: new Vernitherms (thermostat - VERY EXPENSIVE!), different positions of oil cooler in the cowling, various configuration of inlet and outlet ducting to and from the oil cooler, the use of Mobil 1 (an exotic, high temperature synthetic oil), larger flexible hoses between the engine and the oil cooler, etc. None of these changes reduced the operating oil temperature consistently to the 180-190 degrees F that is desirable.
On Mike and Sally's Long-EZ, N26MS, with the oil cooler mounted on the firewall above the centersection spar, the oil temperature would vary from 190F to 230 degrees F depending on the outside air temperature (at similar power settings). Mike put up with this situation for several years because these temperature ranges were within those specified by the engine manufacturer. Recently, a top overhaul was conducted on his engine and after this overhaul, oil temperatures ran at or above 245OF red line.
Power had to be reduced soon after every take-off due to exceeding the oil temperature red line limit. A new Vernitherm made no perceptible change. Having Bill Freeman's test in mind, Mike installed a new oil cooler essentially twice the size of the stock cooler. This was installed on the lower cowl, left side, similar to the plans callout. Oil temperature now remains between 1800 and 195OF even in a maximum power climb to 17,500 feet on a hot summer day. So far, the oil temperature has never gone below 180'F even at low cruise power at high altitude during the winter due, presumably, to a correctly operating Vernitherm. Nor has the temperature gone higher than 195OF and this only occurred in a full gross weight, maximum power climb in the middle of summer in the Mojave desert.
Dan Kreigh owns an 0-235-L2C powered Long-EZ here at Mojave and until he doubled the size of his oil cooler (by simply installing a second cooler in series) he had tried every one of the options in this article with little or no success. His oil temperature consistently ran close to or at the red line. The second cooler has eliminated the problem.
This article is aimed at those builders/flyers who may have high oil temperature problems. If your oil temperatures are normal, please disregard this recommendation.