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Charles, Marine Mechanic
Category: Boat
Satisfied Customers: 3934
Experience:  US Navy (Ret) Certified Mercury Marine Tech with over a decade of repair experience.
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Customer Question

Hi. How does this work?
Submitted: 1 year ago.
Category: Boat
Customer: replied 1 year ago.
I have a 2000 340 Sundancer with twin 7.4 Horizon Mercruisers V-drives. On the port engine, problem will only start to occur after 30 minutes or so of cruising power, will abruptly lose RPMS and will then not exceed 2800 rpms. If I push the throttle, RPMS dip. If I back all the way down and resume, will rev to normal RPM's but quickly occur again. If I back all the way down and give it a minute, it will rev to normal RPM's and last about 6-8 min before problem occurs again. I have opened the gas cap to check for venting issues and I have opened the injector solenoids. Neither fixed the issue.
Expert:  Charles replied 1 year ago.

Hello, my name is***** will assist you.

This sounds like it could be one of several items. You will need to do some troubleshooting to locate the cause. 1. Use a multimeter to test the voltage that is coming from the ALT. Make sure that it is not above 14.5V when the engine is on plane.2. Use an IR thermometer and test the temp of the engine after it has began to slow down. Make sure that it is below 180*F. If it is higher, there may be a blockage in the cooling system, or the raw water pump would need serviced. 3. Remove the oil pressure sender and install a local gauge there. Watch the pressure of the oil as you run the engine. Make sure that it does not drop when the engine slows.