As seen in Speedboat Magazine’s May/June 2020 Issue.
525SC Overheating at Idle
I have a ’97 Fountain 42 with three 525SCs. It has been parked and not run for two years. When I started one of the engines on a hose, after about ten minutes the temp gauge went up to 140 then dropped back down to below 100 and the overheat alarm started going off. I have replaced both the thermostat and sea water pump. Water pressure is normal (compared to the other two engines) while running. I also installed a gauge where the alarm sender is located in the intake manifold and verified the temp is around 190 there and the water is in fact cold around the thermostat. I am not sure where to look now.
Santa Fe, TX
Most of the 525SC engines were equipped with the Mercury version of the GIL exhaust manifolds and risers. The carbureted HP500 also shared the same system that has hoses going to the distribution fittings at the bottom of the manifolds, and also has hoses going to the risers. The thermostat housing is unique in that it has a pair of spring-loaded plastic balls installed on a rod in the upper tee. The purpose of the balls is to allow water to bypass to cool the exhaust when the thermostat is closed. Once the thermostat opens, the spring tension causes the plastic balls to close off the upper hose flow and directs raw water to be directed to the circulation pump mounted on front of the engine block. Then the water flows through the engine and out of the other hoses, cooling the exhaust and exiting out the exhaust stream at the end of the risers.
I believe that your plastic balls are worn out or stuck open for some reason. Remove the two upper hoses from the tee on the thermostat housing and inspect the condition of the balls and springs.
As a side note, it is crucial that the thermostat, sleeve, and gaskets are installed in the correct sequence. Make sure you have done this. The fact that your thermostat housing feels cool when the motor is overheating indicates that the raw water is not getting to the inlet of the circulation pump.
One of the least effective windage control designs, especially for stroker crankshafts. The windage tray is attached to studs on the main cap bolts.
I have a 540-cubic-inch big-block Chevy engine. What weight oil do you recommend? I have installed a 10-quart oil pan. When the RPM goes from 4,000 to 5,000, the oil pressure fluctuates from 60 to 40 psi. What do you believe is the cause of this?
Colorado Springs, CO
Most high-performance marine engines require a higher viscosity motor oil than traditional automotive applications. Some factors that dictate the proper oil include the bearing clearances that were used when the engine was built, the efficiency of the oil cooler being used, and the climate and water temperature that the boat is normally used in. Supercharged engines usually require oils with higher viscosity because the oil tends to get diluted a little by fuel. For many years, high performance engine builders preferred straight weight oils, but the trend now is using multi-viscosity synthetic or synthetic blend oils. We have had good success with full synthetic oils. Currently, we feel the best all around
oil for high performance marine engines is the Mercury Racing 25W50 synthetic
blend. In tests, this oil seems to cling to parts in extreme conditions and excels in environments where there is increased fuel dilution. This is the preferred and recommended oil for all Mercury Racing engines and we have adopted it for all high performance engines we build whether they are naturally aspirated, turbocharged, or supercharged.
Fluctuation of oil pressure at a sustained higher rpm is an indication of poor windage control in your oil pan. This is a dangerous situation because the fluctuation of pressure is an indication of your oil being aerated. This means that even though your gauge indicated that you have 40 psi oil pressure, it may also be partially air which results in less lubrication protection.
The fact that your engine has a stroker crank makes the situation worse because of the increased radius of the crankshaft. In the crankcase of the engine, a windstorm is created by the rotating mass. I can only describe this phenomenon as a tornado of oil circulating with the crankshaft that does not return to the sump of the oil pan. So basically, your oil pick up is picking up what is left from a nearly empty sump. This problem can be worse if you have excessive oil going to the valve train above, or for some reason the oil is unable to flow back to the oil pan sump quickly. Remember that all oil that runs over the camshaft then falls onto the rotating crankshaft.
Years ago, it was popular to have crankshafts with “cross drilled” mains to increase the oil supply to the main and rod bearings. We haven’t used cross drilled crankshafts for a few decades now because the amount of oil flow that is required can consume the capacity of the wet sump oil pan.
It is likely that your problem will be resolved by installing an oil pan that incorporates a good windage system that helps to return the oil to the sump. We have done tests on various oil pan windage control combinations to learn what works best for wet-sump, big-block Chevy applications. In the process, we also learned that on a 600-horsepower big-block Chevy engine there was a 50-horsepower gain by going to the best oil pan and windage control system compared to the worst. Oil temperature was also lower when the aeration is controlled. Most importantly, stable oil pressure and lubrication was achieved.
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