Help 4.3 MPI Problem Running - Vapor Lock? or Something Else

[SCJeep]

My motor is 2016 4.3 MPI.
I experienced my first issue with my boat. Had the boat at the lake over the weekend and one time out it started fine, idled away from the dock fine. Stopped the motor to load kids on the tube went and restarted motor, motor restarted fine. Put it in gear and put the throttle down and NOTHING, it just kept idling. Put it in and out of gear a couple of times by this time I could tell the idle was rough. Restarted the motor, no change. At this point I turned us around to try to limp the 100 yds back to the dock. During this time the idle became progressively rougher , sputtered some and even died once, but it would restart. Made it back to the dock and started investigation.

I have VVM so I grabbed my phone and synced it up. Popped the sunpad checked the motor, oil, visual checks, etc. Everything was in order, no codes were thrown and I never got any alarms. Restarted the motor at the dock and put it in neutral and engine revved up fine this time. I proceeded to go for a quick run around and all was normal. We ran the boat the rest of the weekend without issue, pulling loads of kids around on the tube, etc. I probably put ~2-3 hours more on the motor since the initial problem.

So did I experience vapor lock? Or is something else going on?

It was warm 90-95F this weekend. The time that this occurred it probably had been sitting after running hard for maybe 1-2 hours heat soaking. When I started the weekend I filled up with gas at a Sonoco and put ~11 gallons of 93 octane to fill the tank. Unfortunately I couldn't fine Ethanol free which is what I try to run exclusively.

The boat has run fine thus far and I have never experienced this issue. I just changed the fuel filter at the beginning of the season so I don't really suspect it. Boat has been run numerous times after pre-season maintenance.

I lost some confidence in the boat and I am supposed to take it to Florida in less than two weeks. I would love to hear anything that I should check.

 

[BillK2632]

That sounds like vapor lock. If it happens again, a trick is to pour water on the cool fuel unit and fuel lines (since you are in freshwater). This will cool the vaporized fuel in the fuel lines and should resolve the issue. Basically what happens with vapor lock is after shutdown, the fuel sitting in the fuel lines heats up to the point of vaporizing. The fuel pump cannot pump vaporized fuel, can only pump liquid fuel. So anything you can do to cool down the fuel lines will resolve the vapor lock. Also, the high octane ethanol fuel might have contributed to it, high octane and fuel with ethanol in it will vaporize at a lower temperature than mid grade non-ethanol fuel.

 

[aerobat77]

correct me but i do not think a mpi knows something like vapor lock since fuel is delivered at high pressure. at least my 5.0 mpi will run straight away regardless of outside temperature. further it would not start at all with a vapour lock , right ?

is this a dts engine ?

 

[BillK2632]

This is a good read regarding vapor lock. The short answer is any gasoline marine engine that is in an enclosed engine room is subject to vapor lock - carburetor, TBI, MPI it can affect them all. Car engines don't typically vapor lock because 1. The engines are not enclosed and 2. Most modern car engines have a high pressure fuel pump IN the fuel tank that pushes fuel to the engine - with a boat engine, the fuel pump is outside of the tank and draws fuel out.

https://www.perfprotech.com/blog/articles/mercruiser-vapor-lock

 

[SCJeep]

My engine is not DTS, just MPI. It is however a 2016 model. So can the newer models (like mine) experience the vapor lock? I thought that this was mainly an issue in earlier models? I had a friend with an 08' 4.3 MPI that it would vapor lock but he couldn't get it to start for like several hours. Warranty eventually gave him parts to add a booster pump. I'm just trying to build some confidence in my boat or do I need to look deeper into the problem. Its not like this was my first time out in warm weather, but was it just the unicorn of conditions that allowed this to happen?

 

[scoflaw]

That's not VL.. With VL once the vapor clears out and the motor starts it will run normal not rough

 

[BillK2632]

Maybe a call to Mercruiser? They are pretty good about talking directly to owners, not just a dealer. As far as the perfect storm, I think you had it last weekend - 90's and very humid, plus you used 93octane fuel with ethanol which is more likely to cause vapor lock than a lower octane non-ethanol fuel.

Vapor lock is a funny thing, my 290da vapor locked a few times, always during the first few hot days in the spring and when I had winter blend fuel in the tanks. My 185 has vapor locked once, and I have no idea why, it was just an ungodly hot day here in Charlotte - @100deg. An old dock mate had a 340, it vapor locked all the time, he used take his transom shower and hose down the cool fuel to get it running when we were anchored out. Drove him nuts.

 

[SCJeep]

So I did call Mercury customer service line, just for sport. Everything I described to them indicated vapor lock. As noted by members here and other documentation online it was to extend blower running time, open hatch, inspect fuel lines for 90 degree fittings, run ethanol free, etc ...

I am curious if anything changed between older MPI models like a 2008 and the newer catalyzed 2016 like mine. I have a friend who had a 08 4.3 MPI and he had severe vapor lock, he would have to wait hours for it to cool. The dealer/Mercruiser gave him a booster pump under warranty which he ended up installing. After that he hasn't had the issue. So what was different about his MPI fuel pump arrangement and mine that "improved" the situation. As far as I can tell it looks like the more late model MPI engines like mine don't really experience this issue.

I looked at my installation in my boat and the fuel arrangement looks to be optimal. There is one fuel line that comes right from the tank and goes straight into the fuel filter, that might be 18". There are two insulated lines that exit the fuel filter which appear to be OEM and lay on the bilge moving to the Starboard side and the cool fuel arrangement. I imagine that everything from the filter to pump is OEM and the boat builder (SR) just installs the short line from the tank to the filter inlet.