5.0L carbureted Merc with weird stalling issue

[TheSheesh]

Hi all - Great forums. Been stalking them for awhile, and now I need to ask a question!

I just typed a ridiculously long post, which got eaten when I pressed submit. Must have been a warning from the Forum Gods not to be so verbose. So, here's the short version.

2000 Maxum w/ 5.0L Merc, 2BBL, Thunderbolt V. I've run and maintained it for 2 years. 1st year ran flawlessly, and most of the second.

End of last season, we took the boat out on saturday, and ran all day. While pulling a towable that afternoon, the motor dropped from 3000rpm to near idle, but didn't stall. I throttled back and slowly brought it back up. Shortly after, it stalled completely. Couldn't start it again for an hour. Idled back to the slip.

The next morning, we asked the mechanic for advice, bought a new fuel filter, and anti-siphon valve. He checked the old valve and said it was bad - voila.

After 2 hours on the water, the same stall happened, and this time we needed a tow.

Left the boat with the mechanic. $450 in diagnostics and labor later, mechanic says we had a bad (ignition) sensor. We take the boat out the next weekend, and it sure ain't the sensor, since we stalled again. HE towed us back this time.

So - help?

Fuel - clean, fresh, gas, not old winterized stuff. You can smell it when you crank the ignition long enough when it isn't starting

Air - Throttle is opening when advanced - air should be getting in.

Spark - plug wires are new, sensor is new (obviously), mechanic checked out other components (though I don't trust him). I've ordered a new cap, rotor, coil, and other bits.

Could it be a carburetor issue? Something changing size or sticking when it is very warm? Float valve perhaps?

 

[cooter2506]

Re: 5.0L carbureted Merc with weird stalling issue

If your smelling gas when you crank it then its not your carb. Your losing spark. There is a diagnostic section for your ignition system in the manual. You can downlaod it

 

[cooter2506]

Re: 5.0L carbureted Merc with weird stalling issue

If your smelling gas when you crank it then its not your carb. Your losing spark. There is a diagnostic section for your ignition system in the manual. You can downlaod it in the adult only section at the top of the page. Also I think there is a section in theAdult only sticky on checking your igntion system.

 

[capecodder116]

Re: 5.0L carbureted Merc with weird stalling issue

I had a similar issue which plagued me for a couple of boating seasons and made me loose complete confidence in my boat. It's been two seasons and I am now feeling confident that I found my fuel related problem and corrected it.

First you should go to the first post in this section and read #3 concerning checking your fuel system. You will gain a wealth of knowledge from this post and this site in general.

Second, take the boat out again with a helper, and if your lucky enough to have the problem again let it run until it stalls by itself. Without trying to restart, shut the key off, go immediately to the engine remove the spark arrestor (air cleaner) and then tell your helper to put the shift lever in the fast idle mode not in gear, and pump the lever like your just stepping on the gas in your vehicle. While helper is doing that look down into the carb to see if you are squirting streams of gas into the carb.

From my experience with the similar symptoms you described just going along and all of a sudden you loose rpm's and stall, then you'r able to restart but only idle into port....it could be a fuel issue and it could be the needle and seat inside the carb that is sticking closed not allowing fuel to come into the carb bowl.

We rebuilt my carb and although everything appeared normal when we got it apart the rebuild seems to have solved my problem....

However as I suggested, you should read the post I mention above and go from there...you have come to the right place if your looking for knowledge and help with your boating problems...

Good Luck and welcome to the forum...

 

[J. Mark]

Re: 5.0L carbureted Merc with weird stalling issue

bad ignition coils break down when they get hot . . . they are cheap and easy to replace

You can even test them with an ohms meter- test when cold, and then test again when hot

check the resistance between the side terminals of the coil with all of the wires to the coil disconnected. 0.75 to 0.81 ohm of resistance is about right.

Check the resistance between each side terminal and the center high tension terminal. Readings of 10,000 to 11,000 ohms are what you are looking for.

 

[180shabah]

Re: 5.0L carbureted Merc with weird stalling issue

Several ideas mentioned above, just make sure you do the testing WHILE you are having the stalling/no start problem.

 

[TheSheesh]

Re: 5.0L carbureted Merc with weird stalling issue

Thanks all, for the quick responses. I will try them all! New coil is coming in the mail in a couple of days anyway, so I'll throw that on. My assumption is that I need to rebuild the carb as well, since I've heard a lot about the ethanol in fuels contributing to varnish and just gumming up the works. That could be a fun weeknight project anyway...

 

[MikDee]

Re: 5.0L carbureted Merc with weird stalling issue

One more item to check, it could be a bad ignition (spark) module in the distributor, breaking down when it gets hot. You need to try, & determine if it's fuel, or spark, that's missing when it dies.