I've got twin 502's with 400 hours. I've had problems several times with starboard side vst tank. It keeps blowing raw fuel thru exhaust. 1st time i was told injector was stuck. Next time the diaphram was broke. They repaired it and told me they rerouted a hose?? so that if did it again it would spread the fuel evenly to all cylinders??? Third time third mechanic, pair blown head gaskets and plenty of fuel on block. was taking water in #3 cylinder. Port motor no problems at all. Can this tank be replaced with something else or eliminated? I don't really understand it but is it not a secondary fuel pump inside a reservoir ? These are 1994 502 EFI' does mercury still use this system ?

i had a few of those come in with the same problem.check the needle and seat in the vst tank,that has always been the problem on the ones we fixed.

sterndrive is right I bet, that system uses a mechanical fuel pump[on the sea pump], that pulls the fuel from the tank and supplies low pressure fuel[6psi] to the VST tank[high pressure elect. pump] which supplies your injectors with say 38psi of fuel pressure, they have problems with vapor lock, because no fuel return back to the fuel tank, but your problem sounds like a needle and seat , are trash is in their causing a stuck problem

If in fact you are having VST tank trash problems, it usually will manifest its self in a clogged VST screen. That will limit max fuel flow and your engine will run at a lower RPM. I experienced continuous VST screen problems until I installed a RACOR spin-on filter with metal drain bowl after my standard Merc water separator filter. Since I installed the RACOR, I have run over 200 hrs without fuel flow related problems or any debris on the VST screen. The VST screen SHOULD catch anything prior to it getting into anything else. The RACOR will catch many things the MERC will no stop.

My recommendation based on my personal experience !

Anyone with VST tank version of EFI should strongly consider installing a RACOR filter. It will eliminate virtually all debris problems in that system. As you have experienced, there are many problems that can come directly from debris in the VST tank/system. The standard MERC water separator alone simply is not fine enough a filter to really help the VST screen.

Good luck.

thats good to know about the racor, thanks

FYI,

I had the same problems with my VST tank being actually plugged to the point of leaning out the motor...was scary! Anyway, I bought a used VST tank off of ebay and have inspected and it seems to work perfectly if you are interested. There are also small screens at the injectors just like the screen for the high pressure pump inside the VST tank....you may want to pull the injectors and clean them as well. Be careful not to damage the O-rings on the injectors...Merc is pretty proud of them as a kit is $10 per injector (and no other o-ring matches it).

PM or email if you would like more information!

Dave

Thanks for the help. we're waiting on the heads to get back. I don't actually do any of the brain work, just try to get info and do the grunt jobs. I think I'll try the racor filter this is getting to ba an annual event that I don't really enjoy. Thanks again for the help