PCM EFI GT40 Surges above 2500 RPM

[RealDeahl]

Need some help diagnosing a new issue with my PCM GT40 EFI. Boat ran fine all weekend until I was on the way to the boat ramp to put it on the trailer. I took it to WOT and the boat would surge as if it was not getting fuel when the RPMs were above 2500. When I throttled down and kept the RPMs below 2500 the engine purs like a kitten. Throttle up and the surging starts again.

My first thoughts are fuel filter and maybe fuel pump. Anyone have any other suggestions?

[DKJBama92Mariah]

I think you're on the right track. Replace the fuel filter first. They are cheap and should be replaced periodically anyway. If you have the old style canister filter, you might consider upgrading to a spin-on type fuel filter/water separator. I got one at my local dealer for about $25 for the mount and filter. It only took about 20 mins to install.

If that doesnt solve your problem, then you may have fuel delivery problems. You can get a fuel pressure guage at any auto parts store and attach it to a fuel rail. I'm not sure what the exact pressure should be but i'll hazard a rough guess. For throttle body injection, it will probably be in the neighborhood of 10-15 psi. Multiport injection requires higher pressure and i think they run anywhere between 30 and 50 psi.

If its not fuel, then about all that is left is spark. It is possible that your coil could be failing and not be able to deliver enough spark at higher RPM; it could also be as simple as a poor +12v connection to the coil that is starving the coil when it demands more current.

One more possibility that i can think of. I'm not familiar with the EFI system in your motor, but many modern systems include a limp-home mode which coincidentally limits max engine revs to 2500RPMs. This mode is engaged when the ECU detects a problem that could cause engine damage at higher RPM. It allows you to still get home but wont let you trash the engine if there is a problem. Typically, it will do this in the case of low oil pressure or high coolant temp. From what i've heard, the system is reset when you turn the ignition off however and the engine will resume normal operation unless the fault condition persists or recurs. I would imagine that when this mode is activated, and you advance the throttle to WOT, the fact that the throttle butterflies are wide open and the EFI is limiting the engine speed through cutting fuel and spark, it could quite possibly cause the exaxt symptoms you are describing.

Hope this helps. Good luck and keep us posted.

Cheers,
DKJ

[RealDeahl]

Thanks,

Actually I missed typed the boat is MPI. All of the gauges looked normal when I was running before the surging started. Where can I find more info on the Limp mode?

[DKJBama92Mariah]

Unfortunately, I don't have a source for a PDF manual for your particular manual. If you have it, the printed owners manual from PCM might shed some light on it. Otherwise, contact Vince at Discount Inboard Marine 803-345-0996. From everything i've heard, he really knows his stuff and could more than likely give you some info in the limp-home mode as well as other possible causes for your problem.

Cheers,
DKJ

[RealDeahl]

Talked to Vince. He thinks change the fuel filter and spark plugs(just because) and see if the problem is still there. He has seen issues with the oil pressure switch that put the boat into limp mode. I will keep you updated. Thanks for the contact info.

[RealDeahl]

Turns out it was the oil pressure switch connected to the computer. The part failed and sent a false signal to the computer that put the engine into SLOW or limp mode. $27.00 later I am back to full speed. This oil pressure switch notifies the computer when the oil pressure is below 5psi. This puts the engine into SLOW mode which only allows the engine to get to around 2500rpms. The other switch that puts the motor into SLOW mode is the water temp switch. Overheating will cause slow mode. The water switch appearantly from the folks I have talked to is more reliable that the oil pressure switch.

The oil pressure switch is an easy DIY replacement. Oh by the way if your switch has failed and you don't have a replacement handy, you can with caution, disconnect the oil pressure switch. This allows you to run normally until you get a replacement. Be extra careful to watch your gauges until you install the new switch. The SLOW mode will be disengaged for low oil pressure. Make sure you disconnect the correct switch too. There are 2. A larger sending unit for the gauge and a smaller on for the computer. Both are Bell shaped located above the oil filter on the port side of a GT40 engine. The switch I replaced had a tan wire with a black stripe on it connected to it.

Special thanks to Vince and the Whole Crew at Discount Inboard Marine. They are wonderful.