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Mercury 115 4 stroke fuel delivery issue

M

Mac300

Member
Bought a “new to me” pontoon boat 2 weeks ago. Motor is a 2011 Mercury 115 EFI 4 stroke. Ran great all day first day. Boat has 2 separate fuel tanks. Ran almost all of the 24 gallons out of tank 1 on first day. Second day I launch boat and go half mile and swap to tank 2…or so I thought. Boat started sputtering and cutting out. Swapped to tank 1 and ran great. Did this a few times back and forth and same result. Rode a short distance on tank 1 and went back home. Assumed tank 2 had bad gas…called seller and he confirmed they always used the other tank as it was on same side as his truck gas tank and they seldom used the boat. Drained gas, dried out tank, thoroughly cleaned it, and put in fresh fuel in both tanks. Launch boat, tank 1 runs great , tank 2 cuts out and sputters, go back to tank 1 and it runs rough and sputters. Ends up that it wont run on EITHER tank…had to be towed in…I’m hating my boat! Bring it home and replace the low pressure and high pressure filters under the cowling and put a new water separating fuel filter on the external bracket. Also took apart the brass fuel tank petcock and realized the internal piece was backwards to what the seller told me so when I went to tank 2 I was actually in a “neutral” position and not pulling from the tank. This explains the ORIGINAL issue. I cranked the boat and it ran great on both tanks ( on muffs). I disconnected all 4 injectors one at time and had noticeable drop off at idle so I feel like they are ok. Put it in the water and cranked right up, idled great, went a few hundred yards and opened it up. Within a minute it was sputtering and cutting off…on both tanks. Getting towed back again! Tomorrow I’m going to pull all the fuel lines which are only 2 years old, blow them out, and check for blockages. I’m also wondering if the fuel select petcock could be letting air in the fuel line…is this as detrimental on a 4 stroke as it is a 2 stroke? Maybe when I initially switched to tank 2 and it was not pulling from the tank it sucked some trash from line? I’m at a loss…any advice greatly appreciated.
 
I would first test run with a 5 gallon portable tank ( borrow one ) if you can.------Then you will know if it is a issue with motor or with boat fuel system.
 
Good idea… thanks. I just cranked the boat on the trailer using muffs….ran great. I guess the problem starts when under a load. Don’t know why after it sat overnight it runs good.
 
Racerone…1) I had a similar problem years ago with a 2 stroke that had a tiny leak on a fuel line…only evident when I pumped bulb a lot…are 4 strokes that temperamental to air leaks? I have a tank on another boat…I’ll swap them and go try it out. If that works I’ll bypass my tank selector petcock and go straight from tank to motor. Thanks!
 
If there is an breach of the fuel line ( air leak ) then the pump will pull air.----Makes no difference for a 2 stroke or 4 stroke motor !!
 
Good to know. I just went through everything again. Completely disassembled the brass fuel tank selector petcock. The small o-ring around the stem is badly deteriorated. Hopefully it’s as simple as that. I bypassed that petcock and ran fuel line directly from engine to tank. Will also take another tank as a backup. Thanks!
 
Update-just put boat in the water after bypassing the tank and going on a separate tank. Idled great, had to go a few hundred yards to clear the no wake zone, opened it up and ran great for 30 seconds and started to sputter and quit. Obviously not the fuel select petcock like I was hoping. I remember now why I sold my previous boats.
 
At this point I think I’ll pull the VST and bench test both fuel pumps. They both apparently work with no load but when under load the fuel supply appears to be interrupted. Does anyone know the Acceptable resistance range for these pumps? If I can’t determine one of the pumps are bad I’ll accept defeat and take it to a shop. It’s really weird how it will run well at low speed after sitting overnight but shuts down as soon as I throttle up. And won’t run again until it sits for a long time.
 
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If you can get to the outlet side of the pumps try putting a fuel hose on it and run it into a can to see if both of the pumps are the same and have a decent quanty of flow and don't shut off after a few seconds. There is likely a spec on how much they should flow in a minute but I don't know what it is and just an eyeball measurement should tell something.
 
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