Possible fuel problem?

[gkbid]

Hi, I'm new to the forum and own a 1990 Sunbird Corsca with the 4.3 OMC Cobra I/O. I took the family and boat to Lake of the Ozarks for vacation last week (its first time in the water this year). The boat has been well maintained and was in good running order before taking the trip. The boat ran fine the first day. The next day it ran fine and that evening we piled in the boat to go have dinner at a restaurant on the other side of the channel. When we left the restaurant after dark we idled out with no problem. Once I passed the No Wake and gave it the gas but it stumbled badly and died. It restarted and idled but died again when I tried to accelerate. It would not restart. We were fortunate to get a tow back to the restaurant and tied up. The next day I drove to that location, loaded the boat.

Since returning home I've replaced the fuel/water filter, checked the anti-siphon valve at the gas tank, check the vent line, and checked the small screen filter at the carb. Also, the gas doesn't smell bad. I'm suspecting the fuel pump due to erratic test results. When I first put the fuel pressure gauge on the line it only read 3 lbs per sq. inc., I then tried to observe the volume by directing gas into a cup... it barely moved any fuel. Approx 15 minutes later I repeated the test, this time it read 7 lbs per sq inch pressure and the volume was very good - it filled the solo cup in about 20 seconds. Tonight I tried the test again, it read 4 lbs per sq inch and the volume was reasonable but clearly less than before. I also checked to see how much vacuum the fuel pump was pulling at the gas tank outlet... it was a consistent 6 to 7 inch pounds. I know that a car's mechanical fuel pump to run around 7 lbs per square inch but don't know if a marine pump is any different. Is it common for a fuel pump to have this range of readings?

Also, I'm really surprised at how much more expensive a marine 4.3 fuel pump is compared to an automotive 4.3 fuel pump. Can an automotive pump be used instead? All things being equal I'd prefer to save the $100 difference.

Thanks in advance for any advice...

​Gary

 

[Howard Sterndrive]

Did you consider the anti siphon valve at the tank pickup? or some debris in the tank floating around - covering the pickup intermittently.
Any gas in the clear yellowish tube between fuel pump and carb?
the spec for fuel pressure is 4-7 psi, but not on the trailer. You have to be planing or trying to plane to get any meaningful numbers there.

I'd check for spark before doing anything about a fuel pump.

 

[Howard Sterndrive]

and just a reminder to run the blower while doing any of those fuel system tests.

 

[gkbid]

Thanks for that response Mr. Sterndrive... I did check the anti-siphon valve at the gas tank pick-up - at least I think it's OK. I tried to blow into the line and it completely restricted blowing any air into the gas tank. Conversely it easily allowed me to pull suction/gas from the tank. I've not checked the pickup line... I guess I'm not exactly sure how to do that. Do I simply unscrew the anti-siphon valve to remove the pickup tube, or would I need to access the end of the pickup tube from the fuel gauge sender opening since it's considerably larger? Also, I just got a tune up kit and will be replacing the distributor cap, rotor, points and condenser. I also plan to test the coil.

I'm surprised to read that you can't get accurate fuel pressure readings "on the trailer". I was considering installing a small fuel pressure gauge on the fuel line adjacent to the carburetor - sounds like that would be a smart move.

 

[Howard Sterndrive]

I didn't say you can't get accurate pressure readings. It's just not a valid test of anything. The worst fuel pump in the world can supply a teaspoon per minute. That's what it takes to keep an engine idling. A good fuel pump is one that continually keeps 4-7 psi at the carb when the engine is drinking a solid jet of gasoline like a garden hose on half. (Look down the carb of your engine some day at WOT - it's like the bathroom faucet is on). It takes massive quantities of gas to keep ahead of that demand. At idle, gravity and siphon might even do it- some boats with a bow tank might even idle without a fuel pump.

 

[gkbid]

I'm pleased to report I replaced the anti-siphon valve and the problem is resolved. Also replaced points, condenser, rotor and cap and it's running great.
Thanks again