Any merc dealer should know what it is and would be able to order it or just call Bam Marine or Doller Marine. I think they are about $350 for the kit.

I looked through all the records I got when I bought the boat and it appears it already has this kit installed....

I started having the same problems with my 502's just a few weeks back. I got pissed off and yanked everything out, including the check valves on top of the take where the gas is pulled thru. I installed new 90* hose barb fittings, new hose, new electric fuel pumps (Holley RED), removed the fuel filters from the motor and hung them low on the bulkhead, mounted the fuel pumps on the stringers and ran all new gas line. Problem was solved, but I also have carbed 502's

I also had vapor lock issues after converting from carbs to Merc EFI. First I eliminated the VST and ran the return back to the suction side of the lift pump. Still had minor issues after ideling for long periods of time. Then I installed a small oil cooler on the raw water intake line and plumbed the fuel return line thru that. Both engines have ran perfect since. Fuel lines are cold to the touch now!

Just fixed this issue myself without all the pumps and crap. twin 454mag mpi's in a 312 Fastech Formuls. Dumb ass design by Merc... I moved the fuel filter off the motor to the bulkhead of the back seat. then jerked off that fuel line that merc runs right on the motor..(ahh motor runs at 160-170 but they want the gas to be no more than 106 hitting the VST.. So then I ran new S/S braided fuel lines and fitting from the filter to the mech pump then from the mech pump in front of the motor along the fresh cool water line up and over the top of the plenum, actually top-side to the VST.
Ran the **** out of it this weekend. Shut off to let the thing heat absorb and she fired right up and never bogged when I hit the fuel.. Cost me a $129.00 in fuel lines and fittings and about 3 hrs labor each.. and thats me making the lines..

Took the boat to my mechanic today to have a few other things done and he suggested to go from the factory 3/8 fuel line to 1/2. Ran the boat and no vapor lock today..

Problem's back, ran the boat in a poker run last week and it did the same thing 5 times in one day. You talk about one pissed of boat owner, never could run with the pack it would always shut down after about a 1/2 mile of restarting.

Run the fuel rail return to the main fuel tank instead of the vst tank. Plug the fitting on the vst tank. Problem solved.

I don't have a VST tank, I have the cool fuel.

Here is the part number for the Merc booster pump 862264A2....this will fix your problem. I live in the mid west and we have E-90 gas around here and every body has had vapor lock issues. I have installed this set up on many 502s and they never vapor lock again no matter the temp or what they have for fuel