9.4 compression ratio and old gas

[The motor guy]

So I looked at the specs for my 350 chevy boat engine. MPI, 325hp The boat holds like 120 gallons of gas and I usually keep it mostly full, but i'd reckon half of that gas has been in there a year, maybe more. The boat gets started up on the trailer atleast monthly if I can't make it to the lake, so it's not like it just sits in the garage neglected all the time. Looks like they say 87 octane is fine, but 89 is actually the recommended gas. I've always just used 87 octane. The engine does have vortec heads btw. I really can't find anything wrong that would cause my hesitation, misfiring, stumbling issue. The spark plugs look fine, the cap and rotor are okay, the fuel vent is clear, etc. I'm not sure what they run for ignition timing since it says computer controlled. Someone did mention the ignition module could cause problems under the distributor cap. To me everything seems to lead back to my old gas. Honestly, I have all kinds of engines that run 1 year old fuel regularly, never had an issue, but maybe the boat motor is under enough load for it to show up. Any ideas?

 

[Wrenchturner44]

I would also consider the possibility of how short the shelf life is of todays fuels before phase seperation starts to occurr. In high humidity climates phase seperation happens pretty quickly. You may also want to test the gas in your tanks to see it contains ethanol and what the percentage might be.

 

[AdmiralYoda]

With the cost of boat maintenance and the cost of marine engine repairs...it might be best to put the best gas you can in there. Not only to prevent the gas from degrading, but also phase separation like @Wrenchturner44 said. Boats seem to be more susceptible to this for some reason.

And on boats you aren't cruising at 2k RPM at a light load on the highway, those engines work hard.

Going forward I'd go with 93 or 91 Octane, whatever is highest in your area. And a double dose of Stabil AND Star Tron Enzyme based treatments. They both stabilize fuel in a different way, so might as well help yourself out for a couple extra bucks. You won't use too much of that stuff, and the benefits of preventing the gas going bad far outweighs the cost.

Fuel, air and spark. Do you have a water separator on your fuel line? If not, you should. Pretty common for water to get in the gas after it sits for a while and it will make it run like crap.

Any chance you can cap off your on-boat fuel tank and connect a separate 5-gallon marine fuel can to the fuel pump temporarily? With good gas you'll know pretty quick if it is a fuel issue.

 

[The motor guy]

With the cost of boat maintenance and the cost of marine engine repairs...it might be best to put the best gas you can in there. Not only to prevent the gas from degrading, but also phase separation like @Wrenchturner44 said. Boats seem to be more susceptible to this for some reason.

And on boats you aren't cruising at 2k RPM at a light load on the highway, those engines work hard.

Going forward I'd go with 93 or 91 Octane, whatever is highest in your area. And a double dose of Stabil AND Star Tron Enzyme based treatments. They both stabilize fuel in a different way, so might as well help yourself out for a couple extra bucks. You won't use too much of that stuff, and the benefits of preventing the gas going bad far outweighs the cost.

Fuel, air and spark. Do you have a water separator on your fuel line? If not, you should. Pretty common for water to get in the gas after it sits for a while and it will make it run like crap.

Any chance you can cap off your on-boat fuel tank and connect a separate 5-gallon marine fuel can to the fuel pump temporarily? With good gas you'll know pretty quick if it is a fuel issue.

I've thought about it, but those barbed fuel fittings are like fish hooks. That hose isnt coming back off once it goes on unless you cut it off

 

[The motor guy]

Is a 9.4 ratio fairly high for 87 octane or not really? Maybe in the case of a heavily loaded boat engine perhaps?