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2005 mercruiser 350 mag stalled on the water, got it started

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2005 mercruiser 350 mag stalled on...
2005 mercruiser 350 mag
stalled on the water, got it started and ran fine till next time out. Ran great all day,then after a 20 minute swim, would not start. Took to shop ,
Replaced Idle Air Control. Ran 4 minutes when I put it back in
Changed fuel pump, Ran 20 minutes, after swim ,hard start ,would not come on plain ,and rough idled back almost to dock before giving it up. mechanics are stumped. they had it another week. I put it in yesterday and ran well between swims for a couple of hours. then hesitated coming on plane, but pulled thru
after next swim ,would not start,
other symptoms
backfire within engine compartment when trying to start
smells almost like ether
have also replaced water separator,
thanks
Submitted: 2 years ago.Category: Boat
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Answered in 17 minutes by:
8/31/2015
Marine Mechanic: Jason, Marine Mechanic replied 2 years ago
Jason
Jason, Marine Mechanic
Category: Boat
Satisfied Customers: 16,291
Experience: Degree in Marine Technology. Gas and diesel marine mechanic.
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Hello my name is ***** ***** am I speaking with?

Are you working on this engine yourself, or just trying to get an idea of what might be going on so that you are better prepared in dealing with your own mechanics?

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Customer reply replied 2 years ago
Marine service has been diagnosing and changing out parts, just want to be informed before i take back. I am capable of doing some wrenching, but very limited on guages and diagnostic equipment
Marine Mechanic: Jason, Marine Mechanic replied 2 years ago

Gotcha, let me ask you 4 more question here.

1. Has this been doing it for a while? Or did it literally just start the other day?

2. How old is the fuel in the tank? When did you last fill up?

3. Has the engine been run on a portable tank yet?

4. When was the last time the engine had a full tune up? That would be replacing the cap, rotor, plugs, plug wires, and most especially, all filters?

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Customer reply replied 2 years ago
2005,320 hours, fuel last fall with stabil, 100% gas,no ethanol, in glass jar fuel looksand smellsas it should, no water or particulets,
boat runs strong at idle ,mid, and full throttle until at some points at startup after swimming for a while it will not, i assume you read what has been changed out to this point, Thanks
Marine Mechanic: Jason, Marine Mechanic replied 2 years ago

If the fuel is from last fall then it's a safe bet the gas is no good. Lots of things happen when gas ages. Not only does it start to break down into it's base chemicals. (that's why the smell is different) But it also looses octane, which is resistance to spark knock and pre-ignition. (the backfiring). And it also becomes less volatile. Volatility is basically how hard or easy it is for the gas to "light" properly inside of the engine. The older gas gets the harder it is for it light properly. That lowers the amount of power you get out of gasoline, and it also makes the engine harder to start. It also vapor locks at a lower temperature than new gas

If your mechanics know what they are doing then running the engine off of a portable tank to see if it runs better should have been one of the very first things they do. I keep a 6 gallon portable outboard style tank in my tool kit for just that reason.

There is no question the fuel is going to be part of the problem. Whether it is just part of the problem or the whole issue will only be known when the engine is run off of another fuel supply. What I would do if you brought this to me is first run the engine off of another tank. If it does not clear up 100%, then I would look into the tune up items. Cap, rotor, plugs, wires ect.

Does that all make sense? If yes, did you have any further questions on the issue?

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Marine Mechanic: Jason, Marine Mechanic replied 2 years ago

I think that should do it. But I work for tips so I do want to make sure you are happy with my service before you go. If you had a further question on the issue by all means feel free to ask. If not, Just in case you do not understand the way the website works (and some folks do not). You do have to put forth a positive rating in order for it to credit me for helping you. When ratings are not done, the website simply keeps your deposit and they will not credit me. The ratings box is located at the top of the screen. To rate, you must select the star you wish and also confirm it. Please let me know if you run into any problems or errors when trying to do it. If you do have a problem, or if you can not see the ratings box which is at the top of the screen. Please reply back "I rate Jason's service _______" and fill in the blank.

Questions do not close out, so if you have to come back later on with follow up questions to the issue you still can even after doing a rating.

Thank you
Jason

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Customer reply replied 2 years ago
Thanks, ***** ***** you read my entire description of the problem. It will start and run as if new,strong,no hesitation,no misses, no rough idles,no backfires, top speed same as when newly broke in. then after several run and cruise cycles,after a swim or whatever it will start and run rough,barely idling, or will backfire or not run at all ,, sorry, but doesn't sound like bad gas
Marine Mechanic: Jason, Marine Mechanic replied 2 years ago

I wrote that I would start with running it off of another fuel tank and then taking it deeper from there only if needed.

You have all of the symptoms of vapor lock. And old gas is going to vapor lock wayyyy quicker than new gas is.

Do you know what vapor lock is?

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Jason
Jason
Jason, Marine Mechanic
Category: Boat
Satisfied Customers: 16,291
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Experience: Degree in Marine Technology. Gas and diesel marine mechanic.

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