Engine dies after 40 minutes

[Scott Danforth]

since you are not the new owner of a new boat, wiring may have been patched in the past.

 

[Mrproffitt]

since you are not the new owner of a new boat, wiring may have been patched in the past.

No it’s original wire, it’s molded in a rubber connector with the power and ground wire attached like a plug into the tach.

 

[FunInDuhSun]

In no mechanic BUT I owned a boat roughly the same year with a Merc. 5.7 and spent an entire summer chasing the same problem. It was so infuriating that I would only run it for 30 minutes, then docked or anchored for a few hours. After doing that, it started and ran fine.
Finally figured it out- the coil was overheating. Replaced it and all was good.

 

[Mrproffitt]

In no mechanic BUT I owned a boat roughly the same year with a Merc. 5.7 and spent an entire summer chasing the same problem. It was so infuriating that I would only run it for 30 minutes, then docked or anchored for a few hours. After doing that, it started and ran fine.
Finally figured it out- the coil was overheating. Replaced it and all was good.

Yea I had read that about the Thunderbolt IV and V ignition systems, that’s why I decided to replace the entire ignition system with a newer one. Problem is, the issue followed.

 

[Mrproffitt]

Update: This morning I got the boat on the muffs and let it run for 30 mins. While it was running I started a “wiggle test” on the wire harness starting at the dash with the tach. I noticed that the tach would “move” as I wiggled the wires, so I cleaned the connections and continued the wiggle test. As soon as I grabbed the cannon plug at the engine, it would stumble, I did it several times. I took the plug apart and found it was lightly corroded in all the pins. I took my drill bit index and found a bit that fit the female pin sockets perfectly and cleaned them out, I used sand paper on the male pins and spread them a little bit with a small screwdriver. Applied some dielectric grease to the socket and assembled it. No more engine stumbles after another wiggle test. I can only assume at this point that I have found the issue with the boat at the power wire connections in the plug. I will know more when we take it out here in the next week. I truly hope it was that simple after everything that’s been done.

 

[Mrproffitt]

Update 2: was able to take the boat out on the river this morning and run it. I ran the boat for over an hour at 3000-3200 and no issue. I did have it cut off once while I was at WOT for a few mins and I believe it’s because I out ran my fuel pump. Replacement pumps are no longer available, so I put a 4 psi @ 50 gal per hour marine pump on it. So I’m thinking I may need to get the 6 psi @ 72 gal per hour pump. But the boat fired right back up and I was able to get right back to 3000 rpms without issue and go for another 15 mins to the dock. The tach no longer jumps back a forth as well.

 

[tank1949]

Hello, I’m having an issue with the boat dyeing after about 40 mins of use. Can not duplicate the issue on muffs. I’m the second owner of this boat and it’s new to me, the previous owner was chasing this same issue. It acts like the key is cut off while on plane at 3000 rpms, drops to an idle and will not take any throttle like it’s out of fuel. I am certain it’s not a fuel issue.
The boat is a 1996 Bayliner Capri 2050 with the 5.7 Mercruiser with two barrel carb. Here’s a list of things done to the boat in the last month.
New fuel system, marine pump, hoses, bulb, carb rebuilt, tank drained and checked for obstructions, dip tube checked, new filters, checked tank vent, new EST distributor, new ignition switch, new alternator, new battery, new starter, checked all grounds, new heads and all gaskets, checked shift interrupter, checked cannon plug at engine for corrosion. All this has been done chasing this issue.
I’m about out of options on this one. The only thing I can think of is something is getting hot and failing after about 40 mins. I’m thinking that the tach might be the issue grounding out, but it seems to be working fine, or possibly the master circuit breaker. I don’t know if I’m losing power at the distributor when this happens. But it’s next on the list this weekend.
The issue is persistent, and like clockwork every time we’ve taken it out. The engine runs like a champ for about 40 mins, then cuts out. I’ve chased everything in the manuals I can find to this point. Are there any hidden relays that I’m not aware of. Thanks for any ideas on where to look.

just for fun. Connect cheap auto electric fuel pump to tank and cycle gas. I still believe something is being sucked up.

 

[Mrproffitt]

just for fun. Connect cheap auto electric fuel pump to tank and cycle gas. I still believe something is being sucked up.

Well I did drain the fuel tank and put a camera in the tank, it’s clean. I’m just really hoping I found the problem at the cannon plug. But if not, I will get it figured out one way or another.