TB 454XLI limp mode issue

Having an issue with my port engine when bringing boat up on plane it gets to around 1800rpm and then falls off to idle. Bring the throttle down and back up and it runs back up to 1800 then back to idle again. It has been going on for a couple of months and every now and then when it gets up on plane she will run fine for miles. Fuel is clean as is all filters. Pressure checked fuel while problem occurs and pressure stays at #19lbs and doesn’t flinch. All gauges are reading good temps as they should. Replaced oil pressure sensor, coolant temp sensor, trans temp sensor, ignition module, rotor and cap. Also swapped ecm and tps with starboard engine and still the same. Engine runs fine at any rpm in neutral. Not sure where to go next and any help would be appreciated. Maybe knock sensor,map sensor???

Mike

antisiphon valve? Remove and clean or just replace.

Does it still have the MSTS box on it? Does the timing advance?

Fuel pressure remaining steady is a good sign...the factory manual will provide the details - there was an early (lower) TBI pressure spec and a larger value for the 'later' applications....17 psi and 30 psi +/- 2 going on memory... so, if you have the earlier setup, you should be good...

I had that problem last year and it was the Vac tube for the MAP switch. It was laying between the rocker cover and hot coolant lines. It was flattened about 3" and pinching itself closed. I replaced and relocated it above the lines so it would not get pinched. Never had that problem since. The other engine had the Vac line run the same way, so I corrected it too.

I had same problem. It was the vacuum rubber hose which was pinched under the coolant lines which run to heat exchanger. New lines installed boat run fine.