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Tan, MerCruiser Certified Technician
Category: Boat
Satisfied Customers: 10897
Experience:  40 yrs. experience with complete engine repairs both inboard/outboard and MerCruiser Certified Technician.
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We recently purchased a 2006 Searay 340 with Mercruiser 496

Customer Question

we recently purchased a 2006 Searay 340 with Mercruiser 496 magnums 375 hp. Standard cable throttle linkage and around 200hrs. After operating the boat for 20 hours I started noticing the port engine burning more fuel at equal rpms. Starboard FF at 3300rpm=14 to 15 GPH, right on spec. Port FF at 3300 rpm=18 to 19 GPH. FF verified through port fuel tank decreasing faster. FF starts to differ at about 1800 to 2000 rpm and differs at an increasing rate as rpms increase. Port engine runs smoothly across all ranges and all other engine parameters are all the same and in spec. Any ideas? is this difference within spec or abnormal as I feel it is?
Submitted: 1 year ago.
Category: Boat
Expert:  Tan replied 1 year ago.
Hello. My name is ***** ***** I will be assisting you today.
I would start out simple change fuel filters and spark plugs.
Many times a failing spark plug will cause an engine performance issue. The engine being computerized it will change the timing and injector pulse to make up for a failing cylinder hence the extra fuel consumption.
The rpm difference tells me it is just one cylinder. Now the hours you have on the engine it is over due for spark plugs.
Personally if I had just bought the boat I would do a complete service of both engines. This way you are starting out fresh and can make a note of your service intervals.
Does this make sense?
Customer: replied 1 year ago.
Ok, a good place to start. Just to verify, at equal rpm the fuel flows differ, correspondingly at equal fuel flow there would be an rpm difference. The engine does run smoothly but I have no idea if the plugs have ever been changed but filters and lube had been serviced regularly. Thanks for the info and I will report back if this helps.
Expert:  Tan replied 1 year ago.

The engine will run smooth as the computer tries to make up for the loss of power. It will change the timing and the injector pulse. Both of these changes will make the engine use more fuel. Many times you will have to advance the throttle a little more on one engine to keep up with the other.

Post back with findings.

Thank you...Tan

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