Mercrusier 454 Bogs down @ 2500 RPM

[H20Dog]

I purchased a 1994 Mariah Z230 with a Mercrusier 454 EFI Mag. engine. I has run like a champ for the three years I have had it, until this summer. It would run fine but when the throttle was brought back to idle the engine would die or generally run rough then even out. I decided a tune-up was in order so I replaced the dist. cap, rotor, spark plug wires, spark plugs and fuel filter. On the next outing the motor ran great at idle (fixed the rough idle stuff) but when I went to run up the throttle to go on plane the engine appeared to lean out (at 3/4 throttle the engine suddenly just started increasing the RPMs on its own). I backed off to idle again. Now when I run up the throttle the engine bogs and won't go above 2500 RPM. Otherwise I runs great up to that point. I have a codemate for the EFI but it's not tossing any codes. Any suggestions?

 

[Don S]

Re: Mercrusier 454 Bogs down @ 2500 RPM

Other than getting an OEM service manual, the proper tools and equipment for systematic troubleshooting of the EFI systems, about the best bet is to check the basics. Do you have spark to ALL spark plugs? Compression test? I would also look at the fuel system of the boat, (not the engine) sounds like a fuel delivery problem to me.

http://forums.iboats.com/showthread.php?t=167561

 

[H20Dog]

Re: Mercrusier 454 Bogs down @ 2500 RPM

Thanks for the suggestions. I've checked fuel delivery from the tank to the filter pickup and it is clear. Compression good. Spark is getting to all cylinders correctly. Timing is correct. I was thinking of bypassing the mechanical fuel pump with an electronic but think at this point that I am drawing at straws. Since there isn't an easy place to put a fuel pressure gauge to test the system I think I'll just take it in for a pro to give it a look over. I know it's probably something small that I've missed. I'll post the results....

Thanks

 

[H20Dog]

Re: Mercrusier 454 Bogs down @ 2500 RPM

After much troubleshooting we found that the ECM module had failed and had defaulted back to a factory default of 2500 RPM (Big thanks to Mercury Marine for all of the help on this one). Unfortunately finding a new one was impossible (No kudos to Mercury Marine, more product support of legacy systems would be great). After four months of searching I decided to remove the EFI top end and install the factory carb setup for this engine. BEST decision I have ever made. The engine is FAR more responsive and quick out of the hole. Twice the cost of a new ECM but well worth it.

Thanks...