Frustrated with 454 Horizon

[murraybeck]

On my 2000 model 420 Sea ray, the Mercruiser 454 Horizon EPI port engine performs great until 45 minutes to 2 hours of running at 3400-3800 rpm, when it suddenly losing RPMs and begins running very rough. If I pull it back to 2000-2500 rpm, it runs smooth, but misses above that. Fuel pressure gauge shows a constant 45, per spec. New coil, plugs, plug wires. Checked for water in fuel, none. Starboard engine running great during all of this. Mechanic and owner really frustrated. Any ideas? Its getting expensive swapping parts to see what the deal is, especially with fuel prices high and mpg, actually gpm low. Additionally, the port tach flickers a while before this happens. I'm not sure these are related because it's flickered for years.

 

[DaFish]

Experienced similar symptoms this past weekend on same port motor. Very interested in the expertice of the rest of the forum.

 

[CliffA]

maybe you an find some things to look at in this thread i started.....i had similar symptoms with my engine that you describe....

cliff

http://clubsearay.com/showthread.php/62806-TBV-ignition-electronics-troubleshooting

 

[Jimmy Buoy]

This might sound weird, but when this happens use a handheld IR Temp. gun and check the transmission temps of both transmissions and compare. Had a fellow boater with similar issues and it took 2yrs for him to nail it down to a bad hydraulic pump in his transmission. The flow of fluid was slow enough to allow the tranny to overheat which was putting his 454 in "limp mode", but it would only overheat after running on plane for an hour or so.

Good luck!

 

[DaFish]

Thank you for the lead Jimmy Buoy - based on how the engine responds when this happens, that make sense. We will be running along on plane, having run for an hour+ and the port motor rpm will slow. I can take the throttle and bring the rpm back up and back into sync, but the port throttle is clearly 1/2 to 3/4 inch higher than the starboard. So it does not go into limp mode, but does loose rpm on its own. The engine temps stay right were they belong, so I will look at tran temps.