Does anyone recognize this symptom - Engine running poorly

[Jetter]

Guys I am posting this because through my own mechanical issues with my Tige, I got some good advice and people here seem to have a great wealth of knowledge so I am putting this out there. My buddy has an 08 Sanger with the Mercruiser black max 350 motor. The boat has under 40 hours on it. That's correct - not a typo! The boat starts and idles fine. Once warm, when he goes to accelerate the boat makes whomp whomp sort of sound and has no accelerating power. It sounds like helicopter blades cutting through the air. If he leaves the throttle on it will eventually "catch" and accelerate and be fine. The boat does not do it every time. It is an intermittent problem.

Just wondering if that sounds familiar to anyone? The local mechanic charged him $600 to "tune up" the motor by changing the plugs and distributor. But he tested in the tank and apparently the problem is still there.

Any ideas or suggestions would be appreciated!!!

Jetter

[JCP]

Sounds like secondary ignition. Verify that the cap, rotor, plugs and plug wires were changed. (you didn't mention plug wires and by distributor I assume that means cap and rotor?)

[Jetter]

The plugs, distributor, and rotor changed but not the wires.

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[JCP]

Plug wires would be next and when you say distributor are you referring to the distributor cap?

[Jetter]

Yes that is correct. Distributer cap.

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[boardman74]

I had a boat do that and it was due to intermittent voltage to the ECM. Caused by a loose wire on the ignition. The dealer had a heck of a time finding it. Mine only did it when you engaged the cruise, but sounds similar.

First thing I would do is remove the flame arrested and look down the throttle body as the problem is happening. If the butterfly is opening and closing rapidly causing the problem then its likely a computer issue like I had. That rapid opening and closing is what causes that whoop whoop whoop sound.

[Jetter]

Thanks boardman74. I'm assuming this intermittent voltage could also be caused by a fault TPS sensor in the throttle assembly. Would that be a reasonable assumption?

[JCP]

Does it have a throttle cable or is it electronic throttle (drive by wire)?

[Jetter]

Does it have a throttle cable or is it electronic throttle (drive by wire)?

I'm pretty sure it is electronic with a throttle position sensor so drive by wire. Looking at the butterfly flap is a good idea. The mechanic should also be able to see smooth increase in voltage if he has it hooked up to computer too. If the voltage is jumpy that should be his problem. I think...

[JCP]

If it is drive by wire, the TPS is most likely part of the electronic throttle body and not individually serviceable, at least that is how it is with automotive applications.

[boardman74]

I can tell you with mine(I worked for the dealership at the time and did the driving for the water testing) running it hooked up to diacom did not show the problem.

I think a faulty TPS could cause the problem.

[TigeFamily]

Just to throw out one other suggestion...if that is carburated...and such low hours on it. Possible that ethenol has crapped out the carb from sitting so much and it needs to be re-built. Years ago, I picked up a 2006 Sea Ray with TPS that had sat for last 3 years and had very low hours and when the carb was pulled apart, it looked like there was sand in it from the ethenol and previous owner swore he only ran ethenol free (yea right). Lot of hesitation when I got it....re-built the whole carb...AND adjusted the timing (which could be another issue) and it ran perfect after that for years.
May be off base here since many know much more than I do when it comes to mechanical issues, but its something to consider if carburated. Dont know much about those boats.

[lancehon]

I would say it's vapor locking. I had the same problem last year. I had winter fuel and on a hot day when the boat sat for a little bit after the motor was hot it had the same symptoms. Winter grade fuel vaporizes at a lower temperature. I'd suggest draining the tank and refilling.

[Jetdriver]

I'd try to eliminate all the cheaper, smaller things first. Replacing the fuel, cleaning up the throttle body and checking all over for loose connections is a solid place to start.

[Jetter]

Throttle position sensor was changed and we think problem has been fixed. Sounds like intermittent voltage to the throttle body was indeed the issue. Thanks to boardman74 and others who chipped in. Hope this can help someone else down the road.