posted 08-24-2015 11:07 AM ET (US)        

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I have a 2002 Ventura 18 with a 135 Optimax.The boat is new to me this season. This weekend I was cruising at 3,000-RPM for about 15 minutes or so when the c.2002 Mercury OptiMax 135-HP outboard engine began to stumble, and then the engine speed suddenly went down to 2000-RPM. I quickly moved the throttle to idle speed, then moved it back to 3000-RPM. I used the boat for several hours with no other [unusual behavior shown by the engine].

Before this happened, I recently topped up the oil tank with oil [recommended for use with the] Optimax, and used Starbrite fuel additive in the fuel tank after the last fuel stop. The fuel tank level gauge reads around 3/4-FULL. I had a similiar occurrence with the engine stumbling about one month ago. [Invites readers to reply with] the possible cause of this condition. I am just curious if I need a engine technican to explore [the unusual loss of engine speed and stumbling by the Mercury OptiMax 135-HP outboard engine]. Thanks for your help--Gerry

posted 08-24-2015 01:56 PM ET (US)            

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There are two general areas that cause an engine to lose engine speed or have difficulty accelerating under load (or what you have called "stumbling"). The cause can typically be either related to an electrical problem or a fuel problem.

Problems caused by electrical malfunctions tend to have very distinct and sharp onset, then stop just as quickly. Problems due to fuel-related causes tend to have a more gradual onset or disappearance.

I think your approach to treating the fuel with an additive is a good move. Whether or not just running fuel treated with additives will provide a remedy is hard to predict.

Problems that only appear at intervals separated by many hours of normal running are going to be difficult to have their cause diagnosed. If you were to take the engine to a technician, the engine may never display the unusual behavior for the technician to observe.

If it were me--and I have been in a similar situation with a very intermittent problem--I would continue to use the boat and keep watch on the behavior of the engine, noting when and under what circumstances the problem reappears--if it indeed does. The cause could be something as simple as some water in the fuel or other fuel contamination.

Other areas to check are:

--condition of the fuel in the tank; age of the fuel; contamination with water

--proper filtering of fuel; use of a fuel-water separating filter; condition of the filters

--fuel deliver problems; poor fuel lines or fittings; degraded fuel lines.

If any of the fuel lines on your boat have a gray outer jacket, I would be very suspicious of their integrity. There are many reports of loss of fuel flow though gray-color fuel hoses, particularly hoses associated with installations of Mercury engines. The inner liner of those gray hoses is often seen separated and clogging the lines. This is often attributed to the effect of using gasoline fuel that contains a blend of ethanol and gasoline.

posted 08-25-2015 10:48 AM ET (US)            

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Might be a fuel injector that is turning faulty. It was common issue of those Optimax motors. A skilled technician with a diagnostic tool should be able to confirm. Mercury had two different fuel injectors kicking around at that time. The originals and then a newer improved version.

At the same time I might ask him to look at the compressor to make sure it's not slightly loose or wobbling around. From what I understand the symptoms you describe are not part of faulty compressor but if you're hiring a technician I would have them look at it just for safety.

posted 08-25-2015 05:10 PM ET (US)            

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The only thing I will add is that on many boats, running at 3000 RPM is in that "in between" range where you're neither just poking along nor planning, but mostly plowing along bow high, throwing a huge wake but not going very fast. This can be very stressful on the motor. If that is the case, you should either speed up or slow down.

Tim