As seen in Speedboat Magazine’s
Sept/Oct 2024 Issue.
FUEL Starvation
Dear Bob:
I have an older Fountain with MerCruiser 502 Magnum engines. Last year, I noticed that it felt like I was down on power and could not achieve full RPM compared to the year before. I think the engines have been getting progressively worse over the years, so I felt that it was just a result of them getting tired.
This year, I took the boat out and the engines seem to be run-ning out of fuel at about 3,000 rpm. They idle OK and I can rev them but when under load, the engines feel like they are running out of fuel.
There is a mechanical fuel pump mounted on the raw water pump. I checked with a local auto parts store and they have a similar pump, but the lever appears to be different. Could this pump be causing the fuel starvation problem?
Randy Sherman
Dallas, TX
It is possible that the mechanical pump mounted on the raw water pump could be a problem. Being that both the motors are doing the same thing, it seems unlikely that it is the mechanical fuel pump. But to be sure, you can change them. The correct pump is a Mercury part number 861677T. It comes with a gasket but not the fittings that are on your existing pump. You will need to remove the fittings and install them in the new pump copying their orientation. The threads are 1/4 NPT and proper sealant such as Loctite 567 PST should be used on the treads. Do not use thread tape because it can dissolve in fuel and pieces can get into the fuel system.
The mechanical fuel pump mounts on the raw water pump body. The lever is actuated by a cam eccentric on the pump shaft. The res-ervoir in the body is filled to the sight plug with stern drive lube for lubrication. If the gear oil is diluted with fuel, it must be drained and new Quicksilver 85W90 oil added to the level of the sight plug in the housing. This is also an indication that the fuel pump diaphragm was leaking fuel.
There are basically two generations of the 502 Magnum MerCruiser engines. If yours is the early version with a Vapor Separation Tank (VST), it is more likely that the problem is related to this system. The VST is located above the valve cover at the port rear of the engine. Fuel is supplied to it by the mechanical fuel pump on the raw water pump. The fuel supply line goes to a needle and seat assembly which fills a reservoir. A float similar to that in carburetor bowl regulates the fuel level in the VST. Inside the VST is a high pressure pump that provides fuel to the fuel injector rails. On the bottom of that pump is a filter. The inside of the VST has a coating that we have found is dissolving due to the Ethanol in currently available gasolines. This debris can plug up the filter on the pump and cause a flow restriction. One version of the VST also has an inline filter between the VST pump and the injector rail which can also get plugged up and restrict fuel flow. I feel the fuel restriction problem is related to the VST. It is either a restriction in the float system, a problem with the high pressure fuel pump, a clogged pump filter, or if it exists: a clogged inline filter. If the coating on the inside of the VST is coming off, it can cause all the problems identified above. You can carefully remove the top of the VST and inspect it and possibly clean the inside and the filter.
The bad news is that the VST system and its parts are no longer available or serviceable. If you cannot find a good one, the only choice is to convert the engine with a “cool fuel” system similar to what is used on the Generation VI version of the 502 Magnum and the 500EFI and above Mercury Racing engines. You should be able to find a system that has been taken off. It will require a modification in fuel plumbing and routing the engine cooling water through the cool fuel unit that usually mounts under the oil pan in the front of the engine.
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