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Intermittent Stall at Speed on ‘96 Sunesta 210

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Hey, all. 

TL;DR - Need help diagnosing intermittent stall at speed.

I am at the limit of my knowledge here. My 1996 Sunesta 210 with 4.3L Mercruiser Alpha One randomly dies when I have the boat running. Boat idles fine, shifts gears without issue, and I can’t make the problem re-occur, but it does happen every time I take the boat out. The boat may run for 30 minutes without issue, but then it’ll die. 

Engine starts right back up after dying, but once it stalls, it’ll likely stall again in the next ten to twenty minutes. 

Things I’ve tried or fixed:

- New fuel filter

- New anti-syphon valve

- Flame arrestor is decently clean

- Checked all fluids

Fuel was left in the tank over the winter but with fuel stabilizer. Looked fine when I replaced the fuel filter, so I don’t think that’s it. 

Read elsewhere it could be the distributor cap, but I’m not entirely sure how to check that. 

Any insight would be super appreciated!


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Random loose electrical connection. Should be the key switch. Or if your engine has a large multi pin plastic plug connector on the engine. It has SEVERAL split pins inside of it. The split pins LOOSE tension with age & how well they were originally spread apart. Etc.  It can be the best ……. Chase your butt forever trouble. Been there with my old master mechanic. He knew EXACTLY what was wrong. Took him 1.5 minutes to correctly spread ALL the pins a TINY BIT.  With the small blade of his pocket knife.

To ADJUST the gap of the pins is ……….VERY VERY...……..RISKY !!!!!!!!     BATTERY SWITCH   OFF !!!!!!!!!!! There is very high electric current on some of the pins !!!!!!!!!!  A touch BETWEEN ANY 2 pins can be total disaster !!!!! A welded or melted copper blob is the result !!!!!!!!

I would disconnect 1 battery lead of any battery you have in the boat....You can NOT trust the main battery switch in OLD BOATS. 

A quick test of the ignition switch & wires on it.  Is the boat running at normal speed. BANG the dash and jiggle the key itself to see if that causes a shut down. 

The shut downs will just occur faster & faster as the pins connection become weaker.

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The first step for me would be to eliminate the various interlocks and run the engine direct from it's own "hot wire". Locate the gang plug atop the engine (the one encased in rubber and with a hose clamp holding the half's together.).  Locate pin #5 purple wire, and splice a temporary hot into that wire. Now your ignition is hot. Crank and run the boat and see if the problem is still there. If not, then your issue is with the kill switch, ignition switch, and/or boat wiring harness, or even a shorted gauge. If the problem is still there, then the issue is with the actual engine harness, ignition module, coil, fuel pump, or something else related to the actual engine itself. Next step is fuel pressure gauge.  W

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  • 2 weeks later...

Thanks, guys!

I did bang the dashboard as @cyclops2 suggested. I did not check the hot wire as @Wingnut suggested, as I was a little uncomfortable with splicing in wiring.

That said, I wanted to come back with an update. I ended up taking the boat to the marina and having their mechanic look it over. His response this morning was that he thought it was the fuel pump overheating and causing vapor lock.

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No charges if a new fuel pump does not fix the problem ??  He is supposed to know what is wrong. Not run up a massive parts & labor bill with bad guesses.

 No vapor lock engine I have had Ever restarted easily. It could barely crank over slowly at first. 

 I thought only CARBURATED engines can vapor lock ???

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I have a 2000 sunesta 232 with 5.0 mercruiser. Runs 10 -12 minutes from launch perfectly. Throttle back and then back up and it acts like it's starving for fuel. I have replaced the fuel filter, fuel pump, fuel cooler and regulator. Fresh fuel. Better performance but still doing the same thing. Just replaced both relays. Could this be the ECM?

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