Paralyzer

I'm seeing if anybody knows a common problem.
It's a 2005 Fountain with Mercury Racing 377 Scorpions motors (blue). I was cruising along last weekend at about 4000 rpm running great then one engine started loosing rpm. It was gradual until it got to where there was obviously something wrong. When I throttled back that engine died. It would restart if I hold the throttle wide open but not run quite right. After it sits a while it can be restarted just like normal and will idle and seem normal until I take it out and put a load on it. It will only wind up to 4000rpm at full throttle, then when you throttle it back down it will die.

I took it to a mechanic to get scanned and it has no error codes. I checked all electrical connections and cleaned them with contact cleaner but that did nothing. It is at a large marina getting worked on right now by their best guy (they say). He changed cap, rotor, plugs and wires. No effect. He said the plugs looked rich. He's checking fuel pressure next.

What do you think?

Katanna

iTrader: (1)

My first concern is he started changing parts without a diagnosis. So he scanned it and didnt get any codes. Next step would be fuel pressure. Then check the timing, and verify it is advancing. If that checks out I would do a compression and leak down test.

mrv8outboard

It sounds like the fuel supply is restricted. Testing the "pressure" may not show that. Place a brass tee in the supply line with a vacuum gauge and a section of (temporary) clear tubing and monitor it under load. You should have a solid clear stream of fuel and around 1'' of vacuum. If you are close to 4'' you have a restriction, a lot of bubbles is an air leak. I also agree with Katanna as you should find the cause of the consequence before replacing parts. Also the basics needed compression,spark,fuel and air.

Paralyzer

Thanks Guys. In defense of the mechanic changing parts, I have only owned the boat a year and don't have a maintenance history so a standard tune up was on my list for this spring but I didn't get to it. He popped the cap and said it didn't look good so I agreed we should eliminate that possibility.
Next on his list was fuel pressure so judging by your comments he's on the right track. I haven't heard back results yet. I will ask him about checking the timing advance, that one sounds very reasonable for the symptoms I have (low power when the rpm climbs). Checking compression would be a chore on a couple of the cylinders but he did say one of the plugs was wet so maybe at least check that one and a couple other easy to get to cylinders.
What controls the timing advance on these systems?

To be truthful I was hoping 5 people were going to chime in and say, "My boat did that exact thing and it was the Idle Air Control Valve" or some other very simple fix.
Thanks again
Jay

Paralyzer

Here's an update. The mechanic said he was on the phone with Mercury and came to the conclusion that it's definitely clogged or stuck injectors. At that point I brought the boat home.That didn't sound right to me but getting the injectors checked out is cheap so I pulled them and took them to Mr. Injector. They were fine. In the meantime I bought a Rinda Scanner. Yesterday following advice from here I hooked up the scanner and was able to see timing and fuel volume. I cross referenced to what the other engine did and they checked out the same.
I did a compression test and bingo, #3 cylinder is only 30lbs.
Where I messed up right from the start on this thing (so did the mechanic) is I didn't notice until yesterday it is only running on 7 cylinders. The computer compensates and adjusts the idle and on the water with the way the exhaust is it's hard to hear. Running it on the hose made it more noticeable.
Today I'll pull the valve cover. I'm hoping to find a broken spring that I can just replace to get through the summer and do a top end rebuild on both engines this winter. Probably wishful thinking. The engines have 315 hours.

1BIGJIM

iTrader: (1)

I doubt it's s broken spring if you have 30lbs of compression. Bring cylinder to TDC of compression stroke and put air in the cylinder and listen to where it's coming out.

Paralyzer

30 lbs is because both valves in that cylinder are stuck closed. It wasn't that air was leaking out, it was air couldn't get in. No push rods for either one. Pulled the valve cover. One push rod totally gone, one push rod not where it's supposed to be. One rocker arm stud broken off. Pulled the intake and found parts. One hydraulic roller lifter missing all the guts, lifter parts sitting around in the tray, plus the missing push rod all bent up in there. The only thing I didn't find is the snap ring for the lifter. I'm not sure what started all this but I have a suspicion. When I was pulling the rocker arm off that was missing the push rod the jam nut was loose. I suspect it wasn't tightened down and it backed off and let the push rod slap around in there which took out the lifter and then when it flew out it jammed in the rocker next to it and broke the stud... that's one theory of many. The good news is it's a couple hundred in parts and I'll be back on the water next weekend.

Black Baja

iTrader: (2)

Sounds like it bent a valve in the whole process.

mrv8outboard

I would do a leak down test on that cylinder before I did anything. Why? Because you are only addressing the consequence not the cause of the failure of parts. Also if you replace the broken parts and not fix the cause the next failure may be catastrophic. If you follow the path you suggest also check installed valve height, you may have tulip-ed a valve and it hit the piston. Harbor Freight has inspection cameras to look in the plug hole as well. Find the cause and fix it once.