2004 Merc350 mag MPI RMP/ stall

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Over1227

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Aug 3, 2018
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I have a 2004 MerCruiser 350 mag mpi..Engine starts and runs great for a while. Last week ran the boat on and off for almost 5 hrs..pulling tubes, knee board, etc..ran great..anchored in a cove for about 35 minutes. Boat started no problem ran good on plane for about 100 yards. Running around 3500 RPMs then boat stops RMPs drop to idle. Pull back in to neutral then forward backfires.back to neutral boat recovers idles great never dies..back on plane same thing all the way back to ramp. Replaced pickup tube, fuel lines to fuel filter, fuel lines to separater and separater filter. Today boat starts great better. Ran boat WOT for a while then just crusied for a while total time about 2 hrs..stop in channel for about 1 hr..boat started back up and idled no problems..got on plane then issue returns..RPMs drop engine never dies..all gauges read normal. Opened engine hatch manifolds cool to the touch, engine sounds great.. I even opened fuel fill just in case it was vapor locked..only thing that was a bit weird was the oil filter was so hot you could even touch it..ANY IDEAS WHY IT RUNS SO GOOD FOR SO LONG THEN STALLS AND DROPS RPMS AFTER SITTING THEN HEADING IN?
 
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alldodge

alldodge

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Sounds to me like vapor lock and opening the fuel filter doesn't make it clear up. There is a Shrader valve on the fuel rail and when it does this, you need a rag and something to push in on the valve. Turn key ON, pump runs, push in on the valve and there should be fuel and air escape. Do this a couple time till the air is gone, you should be good

The issue, is heat soak. Boat runs for a while and everything gets good and warm. Shut the motor off and heat soaks into the fuel sysem and boil the gas. The gas fumes replace the liquid in the rails, so the boat stops. The fume pressure is greater then the 7 psi liquid pressure trying to come in or sucked in.

Partial fix, run the blower to cool the motor down before shutting down. Open hatch and let air cool motor while floating

Real fix, remove return line from regulator going back to fuel filter and run it back to the fuel tank
 
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Over1227

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Aug 3, 2018
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I am guessing that would involve drilling a hole and inserting another tank inlet??
 
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