I had 502's and in Florida it was happening all the time. I met one of the crew when Nigel was running a Scarab in Sarasota at the race. I discussed the problem I was having and he told me to run Marvel Mystery oil in the fuel. We were in the boat down for the races so I ran to the auto parts store and purchased some. By the time I got the boat back home the problem went away. I continued to run marvel and never had a problem with the 502's again. In my new boat, I have 496's and continue to run marvel and have not had any fuel related problems. I am a believer and it is cheap. Sam's sometimes has in the gallon container. I usually stock up on a case or two.

Pismo10, If it has the spring on the wrong side of the diaphram, it will mess up everytime on both motors after it sits long enough to heat up the fuel and build up pressure in the VST. It may not be this, but I know they had these assembled wrong during that year. A couple of quick test, should direct him to the problem.
kr1276, that is an interesting fix for vapor lock, that I have never heard of before. note to self!

I want to throw my two cents in. I have 1995 454 mag mpi's w/VST and fuel pump/sea water combo. It's suspicious that both your engines stalled same time...but I do want all to know for future reference what caused mine at separate times to stall. Went thru all the vapor concerns, considered messing with VST and high pressure pump but these parts are pricey. On a whim, I measured the fuel pressure exiting the mechanical pump, it was only 2.5 to 3 psi! I removed fuel pump and found the actuator arm very worn and found a mix of water and oil(that's why it's worn)in the oil resevore area. The seals between sea water pump and fuel pump had failed. Replaced complete system and new fuel pump, now 5 to 6psi and engine runs great again. Checked other engine, same thing was happenning, just not as far along.

Illusive, good point. A lot of people do not change the oil in the mech fuel pump reservoir. I change mine when I change the water pump impeller.

I am coming back to the fact that both engines died. I would check the water separating fuel filters for water.

Can someone give me the maintenance procedure for changing this oil out, i bought my boat in august last year, it has not been done since i've had it. Thanks.

Just to clarify, the early VST's did not have the spring installed on the "wrong" side. They did change the vapor pressure regulator design from a low pressure pump referenced design to an atmosphere referenced design. The design change put the spring on the other side of the diaphragm, but the early ones were not assembled incorrectly.

Dave

What can be done to fix the vapor lock problem on the 96 model vst then? Other than trying to stay away from ethanol gasoline......

Efi/mpi Vst
 

Just a note. There have been many threads concerning VST tank problems. One very common problem has been the VST screen inside the VST tank. This screen is extremely fine and will clog and restrict engine running very easily. A solution I have posted here before still works great for me. I installed an additional Racor fuel filter "after" the standard filter for each engine. The Racor is much finer and catches the finest of contaminants. Placing it "after" the Merc filter keeps me from having to change it so often as the standard filter catches the bigger debris. This doesn't address all the VST problems but certainly addresses a very large one. I also keep a spare VST screen and O-ring set onboard at all times. By the way, after installing the Racors I'm trouble free for over 300 hrs.

You are coorect Dave N, the early VST tanks were not assembled wrong, but when they changed to the new top, some of them were still assembled with the spring on the same side as the old style, or at least mine was. If you look at the overhaul section of the vst tank in the manual, there is a square box off to the side noting that the spring must be installed on the opposite side of the diaphram for a particular style and it describes what style needs the spring moved.