Engine wont start in the water. Then loses power at full throttle.

[woodguy81]

Hopefully someone can point me in a direction here. Ive done searches here and on several other sites but cant find a solution to my problems. First with the details. Ive got a 2003 mercruiser 5.0 Mpi with an alpha one gen2 outdrive.
First problem. The engine starts no problem on land but will not start in the water at all. So for i've upgraded to a 1000 mca battery, checked for the flappers in the exhaust, they are not there at all. Not in the pipe or drive. Just not there.

Second problem. After starting the boat and backing it into the water it runs great for about 1-2 miles and then gradually starts to lose power. Like a couple hundred rpm every couple minutes. In an effort to track down the issue ive removed and drained and washed out the fuel tank, New fuel filter, plugs, wires, cap, rotor, injectors, and fuel tank vent. Also shortened the vent line to remove the low spot in it so it cant collect and liquids in it. Last item that i replaced was the anti siphon valve with a bigger brass version.

Other items changed were the impeller and water pump on the motor.

Any help would be much appreciated. I just bought this boat last fall and its been nothing but trouble.

 

[QBhoy]

Strange one this. Interested though. I have the same engine and perhaps a few suggestions on faults I’ve had (had most of them, but now under control)...Perhaps I’ll start with asking

what you reckon has happened to the flappers ?
also under normal circumstances I’d maybe throw the old are the risers leaking water into the engine causing the no start...but if it’s like mine, you will likely have the safe risers ?

Any beeps from the buzzer on the dash ?
i know you’ve changed the diz cap...but I’d urge you to check it again. If it was changed last year. Change it again after winter causing the usual condensation issue. The famous diz cap issue on these are caused by mainly condensation build up, over tightening or not quite sitting correctly and tightening down. It looses insulation or cracks very easily and it’s game over. Also ridiculous but I changed mine for new last year and found out that a brand new cap was faulty straight out the box. Known issue apparently.

Few other things after that like fuel delivery, high pressure fuel pumps and sea water pressure sensors after that...could go on, but maybe check that diz again trust me. You can never rule it out on any situation.

 

[woodguy81]

Riser gaskets were replaced 2 days ago. only because i was looking for the flappers. no leaks on them. The boat does start on land, just not in the water. Also new mercury distributer cap installed a few weeks ago. What or where are the sea water pressure sensors and what is their purpose? Could a bad fuel pump cause the power loss at high rpm's?

 

[Bt Doctur]

whats the fuel pressure at the rail?

 

[woodguy81]

I’m going to try and get a gauge to check that today.

 

[woodguy81]

Got a good gauge set and measured about 42 pounds at the rail pre startup and idle. It drops to about 37-38 pounds at 3000-4000 rpm on the hose cold. I haven’t gotten to the lake but I want to see what it’s at when I experience the power loss after everything heats up good because after I shut it down and let it all cool down it’s like it’s fine til it’s at running temp again. I can’t get it that hot on the hose. Also changed the 10-30 oil that was in it to the quicksilver 25-40 oil and a new k&n oil filter.

 

[Bt Doctur]

Now that the pressure seems ok, If you still suspect a fuel issue "T" a vac gauge into the fuel line BEFORE the fuel pump.
The fuel line from tank to pump may be degrading internally and it will show up on a vac gauge