As seen in Speedboat Magazine’s
March 2024 Issue.
OIL PRESSURE ISSUES
Dear Bob:
I have a 39 Cigarette Top Gun with Mercury Racing 700 SCIs. I recently had the engines completely refreshed. At idle on initial startup, both engines maintain 60 PSI on the oil pressure gauge with no fluctuation in pressure. One engine has an issue with oil pressure when it goes above 3,000 rpm. Above 3,000 rpm, the oil pressure drops down to around 15 psi. It takes about 1 minute for it to drop from 60 psi to 15 psi, and the drop is linear. It will stay at 15 psi until the engine goes under 3,000 rpm, and then returns almost instantly to 60 PSI. The water temp on this engine does not get over 180 degrees. In order to validate the electric oil pressure gauge, I placed a mechanical gauge on the engine and was able to replicate the problem. The engine does not use or leak oil. The engine runs smoothly and does not make any unusual noises or knocks. I have concerns about oil pressure being low on high rpms.
Any help would be greatly appreciated.
Bob Campbell
Houston, Texas
First of all, I would not continue to run your engine above 3000 RPM with only 15 PSI of oil pressure. For that matter, I would be nervous about running much above idle speed with oil pressure that low. A rule of thumb for a big block Chevrolet in marine use is 10 PSI of oil pressure for every 1000 RPM.
It is possible that the oil pump pickup on that engine has fallen off or broken off the oil pump. When the crankcase is full, the body of the oil pump can be partially submerged in the oil that is in the pan’s sump. So, if the pickup is missing, the inlet in the pump where the pick-up is pressed into (and usually tack welded) can pick up oil normally when the sump is full. As the RPM goes up, the oil level in the pan drops because of the amount of time it takes for oil to return from the upper end (cylinder heads and lifter valley) of the engine.
One way to test if this is the problem is to overfill the engine a couple of quarts and run a test to see if the drop in pressure is delayed or occurs at a higher RPM, and then recovers sooner. If the test results indicate the above change, it is very likely that the oil pump pick up is laying in the oil pan.
Another possibility is that your problem is being caused by uncontrolled crankcase windage. Your crankcase is equipped with a windage tray to minimize the impact of the windstorm created by the crankshaft rotating assembly. Depending on your engine, the windage tray could be spot welded to the inside of the pan and/or attached to studs that are extended from the main cap bolts. If the windage tray is absent, oil in the pan’s sump is sucked up by the windstorm created by the rotation of the crankshaft. Generally, the oil clings in a tornado-like manner around the crankshaft. As the RPM increases, more oil is pulled away from the sump, which then lowers it below the pickup level resulting in starvation and low pressure. The same effect can result from the oil being aerated by the crankshaft rotation. Usually, if your problem is being caused by windage, the oil pressure gauge will also be fluctuating at the higher RPM levels. It is possible that your engine rebuilder could have inadvertently left the bolt in windage tray out. In a couple of cases, I have seen a situation where a piece of the windage tray that is spot welded to the inside of the pan has cracked off and gets sucked up to the underside of the oil pickup.
When you perform your test by overfilling the crankcase a couple of quarts, if the problem is associated with crankcase windage, it will likely worsen with the higher oil level. In either case, I would advise that you pull the engine and remove the oil pan to resolve the problem. If it were me, I think I would just go ahead and pull the motor to be safe. You are going to find something obvious.
The oil pump pickup on your engine is pressed into the inlet of the pump and then needs to be welded or brazed into place. If this was not done properly, the pickup could be too close to the bottom of the pan causing a restriction in flow at higher RPM.
Your engine has several internal oil galley plugs in various locations. If the engine rebuilder accidentally omitted one or more of these plugs, the result would be an excessive internal engine oil leak which would cause the oil in the sump to be used up more rap-idly. If the missing plugs were up in the lifter valley area, that would cause more oil flowing back over the rotating crankshaft which could also exaggerate the windage problem.
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