Can Pressure in Fuel Tank Cause Starting Problem?

I have a 30 HP Mariner, 1982 vintage. Just getting it back in service (had a mechanic do some work on it - new impeller, rebuilt fuel pump, cleaned carb, etc.). I have a 6 gal. Attwood "ventless" fuel tank I bought earlier this summer at Walmart.

Tuesday hooked up the battery and fuel lines, bought 5 gal. gas and mixed w/oil & Staybil, hooked up the muffs and pumped up the primer bulb. Started right up and seemed to run OK, but, no water from tell-tale. Shut it down with the idea of looking further into the cooling water situation next day. I was able to rest my hand on the water jacket so I'm thinking it didn't overheat.

Wednesday went out to check things out. Thought the cooling water problem might be the thermostat since the mechanic who did the work said the old one was bad and he had put in a used one he had. Pulled the thermostat and fired the engine up. Oops. Not a hint of it turning over. Cranked away like either no fuel or no spark. I did notice some pressure build up in fuel tank. Spent the rest of the day reading the fourms and my manuals.

Thursday came out fully educated and ready to do a careful and insightful diagnosis. Noticed the fuel tank was bulging under pressure, so gently released said pressure. Hooked up trusty Harbor Freight in-line spark detector to #1 cylinder and had a friend crank it over. Good healthy spark - and it started right up (fortunatly I had read on the forum about not turning the motor over "dry-foot" so had the muffs hooked up).

Aside from pulling the thermostat, the only thing I did differently from Wednesday to Thursday was to release the fuel tank pressure on Thursday (when it did start). Is there a reason the fuel tank pressure would cause hard or no start? Or, should I be looking for another problem?

Re: Can Pressure in Fuel Tank Cause Starting Problem?

Maybe the extra pressure in the fuel line was causing it to flood?

Re: Can Pressure in Fuel Tank Cause Starting Problem?

I don't get the "ventless" concept, when the motor draws gas from the tank air must be let in to keep the flow going or else a vacuum will be created that will collapse the tank sides. Maybe the cap has to be turned slightly open.

Re: Can Pressure in Fuel Tank Cause Starting Problem?

Agree with Merc850. Pressure in the tank will overcome the 6psig or whatever the float valve in the carb can control and flood the engine. Only engines that used a pressure tank feed were OMC's back in the '50's give or take 10 years. They had a dual fuel line and the engine had no fuel pump. The philosophy was that you used the pump on top of the tank to pump pressure into the tank and that pressure sent fuel to the engine...course leaking fittings and all played .........with another of OMC's lost ventures. Had one so I know.....I run Mercs now.

Mark

Re: Can Pressure in Fuel Tank Cause Starting Problem?

merc850 said:

I don't get the "ventless" concept, when the motor draws gas from the tank air must be let in to keep the flow going or else a vacuum will be created that will collapse the tank sides. Maybe the cap has to be turned slightly open.

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The lable on the tank says it complies w/USEPA regulations and "... has limited pressure venting capabilities." The only way (that I can see) to release the tank pressure is to slightly loosen the cap. However, it seems to run OK with the cap tightly closed.

Re: Can Pressure in Fuel Tank Cause Starting Problem?

The new regs require that a pressure relief valve hold at least 1 lb of pressure on the tank before venting to atmosphere. That would be enough to force a little fuel past a weak needle and seat and flood the engine prior to starting it. It wouldn't cause any trouble with a normal engine otherwise.

The cap on your can has that valve, and a valve to let air in so a vacuum is not created.

If government bureaucrats were capable of learning, they would have learned from the OMC venture, but that would be a cold day in ****. There'll be much spilled fuel, and also fires and mayhem caused by all this newfangled junk.

Re: Can Pressure in Fuel Tank Cause Starting Problem?

Yep, your right. I took a closer look and there is a little valve in the cap that I assume lets air in but restricts outflow.

Also right on the spillage. When I remove the quick disconnect at the tank when it is under pressure there is a squirt of gas released. Probably already released more than would have been released with a vent.

Re: Can Pressure in Fuel Tank Cause Starting Problem?

And if you don't disconnect it, you'll spill more through the engine and do some damage, mostly to your quality boating and fishing time.