Fuel issue
 

I have a 96 502 mpi fuel injected, had the boat out this weekend ran fine for about 30 min, turned the boat off then when I fired it back up its sounded rough, once I gave it throttle it bogged down and shut off, took about 5 minutes to finally start back up then it ran fine again. 2nd time it's done this. Has new fuel water separator and running non ethanol gas. Any suggestions?

vapor lock?

^^^^^ that's what im thinking...I have a side by side boat and the port engine that the fuel line runs between the motors used to do the same thing

sounds like typical vapor lock, does your boat have cool fuel setup ? not sure if your year did or not...

Fuel pump and raw water pump are same assembly. Outside temps are mid 70s would that still cause vapor lock? Anyone have this same issue I've read it's common for the 502s

Are they 502EFI's with VST's or 502MPI's with cool fuel????

Mpi with vsts

Ambient temp does matter, but its the temp inside the bilge that counts, if you don't have good air flow through the hatch, could be a week fuel pump, next time you stop after running open the hatch and let it breath, see if that helps, if not you need to get a fuel pressure gauge inline on the rail and see what the pump is putting out,

Edit: could be a sensor going out , I would also have it scanned for fault codes

My 2 Cents -- I'm not a mechanic

To me this pure and simple vapor lock

I had this REAL bad the 1st year we had our boat. What I've found best to help - never eliminate without a cool fuel system - which I would never get approved by the CEO is to use a good quality mid grade fuel. I've have put "jip joint" gas knowing I was going to burn thru it in short order. This appeared to be the worst. I try (please don't pound on my) to use Shell or Amaco mid-grade and the vapor issues have been reduced. Also, If it is hotter than "satan's nards in leather pants" out when we come into the dock I open the engine compartment and let it air out/cool down.

I know this is very frustrating- maybe someone can chime in regarding that cool fuel upgrade - but the cost of that in my world is kinda high

All the best and be safe out there

3pointstar

IMO the cool fuel system is more prone to vlock because the pump sets clear down under the port motor mount, the vst sits up on top and may result in a little cooler temps, the cool fuel bypass is easy to do, I ran bypass regulators on my 540's and never had an issue, returned unneeded fuel to a large water separator filter head.

If it's vapor lock is there anyway to avoid it? Run the blower all the time maybe. So annoying that the boat runs great then out of no where it wants to act up, then runs fine again.

do a search on vst problem,s.you will find ton,s of information about it and how to cure the problem.

Also some of the biggest common problems with GEN II cool fuel vapor locking is weak fuel pumps and NO ONE ever completely cleans off and reapplies the heat transfer compound on the heat shield against the fuel pump.

If ALL Merc EFI / MPI engines vapor lock from brand new to many other engines with 2 and 3 year warranties MERC would have been broke from vapor lock claims alone. Certain engines did get the boost pump kit added for GEN II cool fuel apps. This help as well for the engines that vapor locked from the start.

Also poor venting of the engine compartment increases vapor lock issues. Also many people pack their engine compartments with a lot stuff in which makes venting and incoming fresh air worse

The VST did vapor lock especially as it aged and or had its own problems. It was only around for 3.5 / 4 years as Merc had to come out with a better system.

Gen II and GEN III cool fuel systems are still on Merc products currently - and they are install on catalyst system engines, so talk about heat producers and in my area its all E 10 gas and 100 to 110 degree temps and very very warm water temps. These engines run all day, all weekend without vapor lock period..

I worked on 236 boats for 2016 and NOT one boat customer of mine experience any form of vapor lock. Not one. Maintaining systems with the boat and engines is another key. Also all these boaters use land gas in which is all E 10.