502 MPI engine stalling
 

issue with one of the engines,
no issues when engines have been not run for a period of time,but after running boat,then sitting for a few hours,go to restart engine,will run for few minutes then stall,and at the same time the tach will act erratically (jumps up 800 rpm).replaced Map Sensor,but did not fix the issue.
any help on what to look for would be appreciated.

Sounds like Vapor Lock, if you search here you will see many threads with the same issue. If you have Cool Fuel, you can add an electric fuel pump. If you have VST, you will need to return your fuel to the main fuel tank.

^^^ I agree. What you are describing sounds like vapor lock that is VERY COMMON on 454 & 502 mag EFI's. With out redoing your fuel system, a couple things you can try. Leave blower running to help cool the engine. After running at speed/power let the engine idle 3-5 mins allowing it to cool using sea/raw water. Wile idling open the engine hatch to allow heat to escape.

Ultimate fix is you want to route the return line form the fuel rail back to the tank rather the VST.

VST= Vapor separator tank.

To explain whats happening its something like this. The gas in the VST starts to boil and make "vapor" when the engine is HOT, not running or cooling. This vapor pressure gets to high and the mechanical fuel pump can not pump gas into the black tank. Then to make matters worse what ever "hot" fuel you had in the black tank never goes back to the main tank for cooling. It just goes round and round form VST to fuel rail. Once its all gone its gone untill the pressure is low enought in the VST for the mechanical fuel pump to pump gas into the VST.

http://i212.photobucket.com/albums/c...X/IMAG0263.jpg


Google search "vapor lock VST".

thank you n2nsx and gmc271

I have been researching this issue a lot. I have a 95 502, my vst does not look like the one in the picture posted hrre. Can someone talk me through this reroute. I have an unused bung on my fuel tank so I have a place to reroute back into my tank. Now my question is do I just plug the return port on the VST and run rubber fuel line from the fuel rail to the tank. I take it I need to take the plenum off to do this, do I need to change gaskets also.

You shouldn't have to remove the plenum to route the fuel line to the tank. Just plug the return on the tank and you can ger an adapter from Summit or Jegs to adapt the return line (inverted flare) fitting from the rail to AN fitting. Then just run a line from it to the fuel tank. you can use the Aeroquip AQP Socketless Hose that pushes on the AN fitting.
Hope this helps!

Great Thanks!!

my VST doesnt look like the one in the picture above. I have an extra vent line on the top of mine that runs to the fuel pump and to the plenum, that vent line was plugged solid. Is that vent line really needed or is it just a coast guard requirement.

That is the vent line that vents out the pressure from the vst. The other vacuum line opens up the diaphram once the motor starts. With out that line, the fuel vapors in the vst would just vent into the motor compartment, so yes it is a very good safety hose. Also when the fuel pump goes bad it would just start leaking fuel into the bilge, so that hose can keep your boat from blowing up because of the fuel vapors.

Thank you... this is all new to me. I Have always owned cruisers and they were all carb motors. So I apologize for all the questions I try to run a search first before I ask.

No problem, the 2 types of VST are kind of a beast that was only used for a few years because of the problems they had. I have a couple of them sitting around that aren't used anymore, because they were eliminated, and the vapor lock problems were eliminated. 92nsx's description above is exactly the problem the VST has!