my mercruiser 502 starboard engine has a problem which would appear to be getting gradually worse after a normal run swithch of got to restart after say about i hour it at first starts then dies of and stalls ,it then takes about 10mins plus of restarting and trying to kind of catch it out on the throttle to keep it running,it a year 2000 mag mpi,i have heard of vapour locks etc but please help where do i start???????????????the lenght of time to get it restarted is now taking longer every time .

Don't run ethonal gas, run the blower at all times. And if you can, for the hour that your shut off, crack the hatch or run the blower, to help get the temps get cooled off. The problem is the engine actually get's hotter for a while, since the waterflow has stopped. That's when you get the fuel vapor in the hardline routed in front of the block.

I haven't worked on the 502 mag, but on the 350 mag there is a schrader valve on the fuel rail. If you're vapor locked, when you depress this valve you'll get air.. (warning, may get fuel also - be careful)

confirm it's vapor lock, then go from there.

hi cloudmaster, i run standard type gas , tried the blower and tried lifting the hatch it only seems to go away when the engine is stone cold but saying that ive noticed it can happen even then so not sure if it is vapour lock or fuel pump?

hi dcq, not 100 perc sure its vapour lock ,where abouts is this valve and what does it look like , sounds dangerous are w e talking boooom here if i get it wrong cheers!

I had a similar problem with 1997 502 MPIs earlier this year. One response to the thread I posted suggested using Marvel Mystery oil in the fuel (it raises the fuel vaporization temperature). I have been adding it since and have not had a stumble/stall/hard restart since then. My engines have VST (Vapor Separator Tanks) which I disassembled and cleaned but did not find anthing unusual. The VST system does not have a schrader valve but there is a plug on the top of the VST which can be removed to release pressure in the tank itself. I was going to try this but the problem has not repeated. Good Luck.

I also have the VST tank, and since using the blower and cracking hatch on a regular basis when shut down, i rarely have a problem.

Do you have the mechanical pump mounted to the sea pump?

There is a bulliten out for those motors with that problem.

I have done several kits at the shop, and it corrected the problem in all cases.

It adds an electric booster pump in leu of the mechanical that Merc eliminated for a small group of serial numbers.

inspector , thanks for that anything is worth a go !cloudmaster cheers ill try the hatch n blower but ive done that before and it didnt help at all if any thing the problem is getting worse.
obnoxus , hi cheers for that where could i read about the bulletin ,ill have to check the sea pump im not sure , the engines are year 2000 mer mag mpi 415 hp straight through exhaust in a baja 342.your solution may be it! lets hope so

I would bet being a 2000 model that you don't have the mechanical boost pump on the sea pump. I have 2000 454 MAG MPI, and mine does not have it.