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PLEASE help 502 mag mpi efi dying out after run

Old 08-02-2011, 02:14 PM
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Does sound like vapor lock. I'm having same issue with one of my 502's (port side).

Make sure you do not use ethanol blended fuel. Even with that the pipeline companies are "blending" so much butane and other crap in our fuels that the VL problem is getting worse. that and the VERY hot weather.

I have been circling and opening engine hatch and running blower for about 5 mins before I go into dock area to drop engine temp from 185 down to 165. Keep hatch open while docking. Not ideal but seems to be helping except on the hottest days then nothing really works.
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Old 08-02-2011, 09:51 PM
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Vapor locking. Seems more of a problem when hot summer temps are up there as they have been lately and the boat has sat a short time after ran.

A buddy tried the Merc water check valves to keep the hot engine water from flowing back into the fuel pumps and heating the fuel there. Proved to be a waste of money.

Both my motors did it this past weekend. 90*+ temps and humid. 1 time it was 1 motor wouldn't start, next time the other. I simply had to idle around with hatch fully open for seveal minutes till the dead motor would start. Then they would run like no issue. 1 time both motors wouldn't start. Had to put ice bags on the pleniums to get the fuel to flow and start the motors. My pumps are the newer designs that are supposed to aleviate this issue.
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Old 08-03-2011, 10:17 AM
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Wouldn't a return fuel line solve that?
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Old 08-03-2011, 10:39 AM
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Have been told that it would.
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Old 08-03-2011, 02:04 PM
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great great info thanks all... my buddy just said ever wonder why everyone lately has their hatch open its not to show off its to cool it down cause this issue is growing more and more

just reading that gas regular or eythnol blend can boil as low as 140f this sound true? but the ethynol blend (pump gas) 178f and my boat runs 180.....

hopefully since this being new boat to me and dif motor setup its just the way i am driving it.. the merc bulletin does say idle 3-5 min before shut off and keep hatch open much as possible.

i pulled sender and looked in tank with flash light surprised it actually looks really good in there thank god.

put on new main fuel line from tank to mech pump and dissected that..being an original hose from 96 yr of boat it looked great inside when i sliced it open....

also think the napa water separator is wrong i put what was on it when i got it a napa 3225
putting quicksilver on today and draining old into glass to see if any presence of water.
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Old 08-04-2011, 07:40 AM
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There is no doubt in my mind I would run that return line all the way back to the tank. Maybe a good winter project if you can make it work a little better to get you through the summer. If it is set up properly you will never have to worry about it again. Hatch up, down won't matter it will always start back up.

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Old 08-05-2011, 10:29 AM
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Tried the return line trick , Did not work, the VST pump emptied tank faster than inlet line would fill it. To make it start faster hold an old towel under front plenum and bleed vaporized gas by depressing schrader valve while someone else cycles the key.Once liquid fuel is seen it will start. Works for me but be careful.
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Old 08-05-2011, 02:37 PM
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Where is that Schrader valve. I cannot locate on my engine. Can you snap a pic or give me description of where it is?
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Old 08-05-2011, 11:20 PM
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Its above the thermostat housing dead center front of engine on bottom of intake mIne has brass cap looks like tire valve stem cap same setup as tire valve you really need to look under intake to see it or teach and feel it hope that helps
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Old 08-13-2011, 08:13 AM
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have a quick summer fix for ya.. I have a 312 that used to vapor locked all the time... then I pulled the ****ty fuel lines off that run on the motor..dumb design. Moved my water fuel seperator to the back of the back seat off the motor. ran a new s/s braided line from the mech. pump to seperator, then from there back along water pump hose and then over the top/side of MPI plenum then right to VST.. Has helped allot..

Last edited by silent lucidity; 08-14-2011 at 08:25 AM.
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