502 MPI Vapor Lock Issues
 

It has been very hot here for the last few weeks. I have had vapor lock issues in my 1997 PowerQuest 380 Avenger. Boat is equipped with 502 Mag MPI's. The last two Sunday's I have had vapor lock issues. Lasy Sunday, both motors, yesterday, just one. Ambient temps over 100degrees.

I have been running with the blower and trying to open the hatch to allow for cooling. Yesterday, boat fired up when we left the sandbar and we were drifting downriver. It was a windy day, so I was only using the starboard engine to move back into the channel as needed. About the 6th time, SB engine would not start. Fortunately, the port engine was cool and we were able to get back to the marina. Was lots of fun backing into my slip with the wind on one engine with no power steering!

These engines are equipped with VST. I hear there is a pump kit to resolve. Anyone with experiences here?Thanks

Some use an electric boost pump by the tank to do it (the kit is no longer available.) Or you could run the return line from the regulator back to the tank instead of the VST tank, this would also stop it.

Had the same issue last year. Bought the merc pump kits, solved all my issues. You can still get the kits through merc call there tech line and they can give you the part number or you can pm me and I can sell you them, I think they run about $400 a price. Thanks Eric Sallinger

Are they easy to install?

Make sure you are not using ethanol fuel. It makes them vapor lock much easier.
To get them to restart, just keep cycling the key and only letting it try to start for 1 second. It should eventually bleed out the vapor locked air after about 10-15 times of doing this. Don't use any throttle when trying to start it.
Another trick is put a chunk of ice on the water sep fuel filter and maybe the VST tank.

I made my own kits with Aeromotive pumps, better pumps than Merc. will sell you, at about 100 bucks cheaper. There easy to install if you have someone else do it. Even tho I have cool fuel, the principle is the same. I have two Holey 12v pumps that are used but intended for this.

I do not use ethanol. Was told to give it 2/3 throttle to restart. I have found there is not enough fuel available at 2/3. I can get it to start for a second or two by cycleing the key as you mentioned, but never tried it quite that many times. I was also told to clean out the VST screens?

Do I just simply install an electric pump before the mechanical pump? Do I wire it up so it always runs, or just a toggle switch to use when needed?

Dont use the sticks on a MPI/EFI let the fuel pump do the work

I installed mine right where the gas line comes through into the engine bay, and push the fuel towards the fuel seperator filter, the wiring can be the tricky part, it needs to be put in line with the key, so it goes on and off with the ignition.