Mercruiser Mag 350 temporary no start

KD4UPL

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Feb 13, 2010
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I've got a 2004 Rinker bowrider with a Mercruiser Mag 350 and a B3. I've had it 3 seasons now and it's run flawlessly. Today we launched and ran a ways out in the lake then shut the engine down to swim a while. When I tried to restart the boat it would crank but not fire. After 3 or 4 attempts I thought maybe the fuel tank vent was clogged and it was pulling a vacuum so I loosened the fuel filler cap. 2 more cranking attempts and still no start. Looking the engine over again I noticed that the drive lube reservoir was low; it had maybe an inch of oil in it. Thinking I'll try anything I topped up the reservoir. 2 more cranking attempts and the boat started. It ran fine back to the dock. Tied to the dock I started it 5 times in a row with no issues so we went back out on the lake. I started it probably 3 more times with no problem.
I'm stumped as to why it wouldn't start. I really don't think it was the drive lube; I think that sensor just gives a warning buzzer. I did have the gas cap a little loose for the rest of the day.
Any ideas? I'm a little afraid this problem will return.
 

alldodge

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Bad connection
The oil lube and gas cap had nothing to do with it

What is the motor serial number
 

KD4UPL

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S/N is 0W005823
After doing a lot of reading I think I'm leaning toward vapor lock. It was in the low 90's and I had just run out into the middle of the lake with 7 people on board. I shut it down so we could float and swim a while, maybe 30 minutes. After that is when it wouldn't start.
The thing is I didn't do anything differently then I have the last 2 years or the first part of this year and I've never had the problem before.
 

alldodge

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Maybe, but normally takes longer then 30

If that is what is happening, take a rag, safety glasses and small screw driver. Take cap off schrader valve and push in on it when key is turned ON. If there is air come out then probably is. Do this a couple times and it will then fire back up
 

KD4UPL

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An auto mechanic friend of mine mentioned it might be the crank position sensor starting to malfunction. Any input on that?
 

alldodge

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Doubt it, but if it was the CPS there would be a code thrown
 
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