I am changing from a carb to mpi efi system on a 502 bbc . The efi has a VST fuel system. I have a cool fuel system I may want to use. My sea pump has the belt driven fuel pump from my carb system. Do I need the cool fuel pump system with a VST? Does cool fuel add HP? is it Better? is the VST not used with cool fuel? Any help would be great! Thanks Rick















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Get rid of the VST. Use your mechanical pump to supply the cool fuel pump and return it to the separator. The VST doesn't flow enough fuel for a performance application.

Listen to Bob, he knows what he is talking about. BTW, I wish I would have gone EFI to Carb when I did my build.

i don't believe the mechanical pump is what you want for the mpi system. the cool fuel set up has the hi press pump you want to drive the injection... if anything at all , use the mechanical pump to pump to a 1 qt or so accumulator and let the efi pump draw from that reservoir and run the return to IT and from there back to the tank. that way the hi press pump is always supplied and you have a good lift pump in the mechanical sytem.

Merc got rid of the mechanical pump and vst resevoir for a reason,it was problem prone and had a extra handful of redundant parts to go bad. Design your build after the newer cool fuel system,use a suction line from the tank to a water seperator/filter,pull from the filter with a cool fuel pump or even simpler,a decent electric pump,charge the rail with it going thru a small fuel cooler before the rail and use a regulator to bleed off the extra fuel back to the suction side of the pump,Smitty

A lot of the newer efi marine engines use a low pressure boost pump. Merc even went back to the mechanical boost pump on 500 efi engines. It will do away with the vapor lock problems so often encountered with the cool fuel setups. Since he already has the mechanical setup, I'd use it.

I would lose the VST. I think you could still use the mech boost pump with the coolfuel pump. As others have said, some newer Coolfuel units use an electric boost pump, so I dont see why you couldn't use the mech pump.

As far as return line, instead of running from the fuel rail to the VST, it will run from the Coolfuel unitback to the f/w seperator.

Assuming you are using the MEFI 1 ECM from a VST system, the system is designed to run at 36 PSI fuel pressure. If the Coolfuel system you have is off a MEFI 3 (43 PSI) system, you will need to change the pressure regulator or the engine will run very rich. Also make sure that the injectors are the Kiehin (used on 36 PSI systems) and not the GM (used on 43 PSI systems).

Dave

even tho merc does it from the factory, returning the fuel to the filter is a bad idea.. it just put hot aireated fuel directly in the system... i route mine straight back to the tank... fuel temps went down 10 deg.

The coolfuel system cools the return before it sends it back to the seperator. In my area it's not super hot during the summer, but I pretty much never see any vapor lock issues with this sytem.

Dave

Having no provisions to return mine to the tank I return it to the inlet side of the filter,even running big power its never presented any problem BUT I'm using a fuel cooler the size of a small oil cooler,Smitty