Engine dies after 40 minutes

[Mrproffitt]

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.

 

[Scott Danforth]

if you think its the tach, disconnect the grey wire at the coil

Have you removed the spark arrestor and looked down the carb throat to make sure you get two streams of fuel if you move the throttle?

have you tried to open the fuel tank filler cap? if it makes a hissing noise, your fuel vent is plugged and you probably are running out of fuel.

have you measured fuel pressure at the carb when this happens? you need the special fitting https://www.hardin-marine.com/p-62701-fuel-fitting-connector-91-18078.aspx

 

[Mrproffitt]

if you think its the tach, disconnect the grey wire at the coil

Have you removed the spark arrestor and looked down the carb throat to make sure you get two streams of fuel if you move the throttle?

have you tried to open the fuel tank filler cap? if it makes a hissing noise, your fuel vent is plugged and you probably are running out of fuel.

have you measured fuel pressure at the carb when this happens? you need the special fitting https://www.hardin-marine.com/p-62701-fuel-fitting-connector-91-18078.aspx

I have done these things and I’m getting 6 psi at the carb.

 

[Rick Stephens]

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.

I'm a little confused here. Does the motor die or does it stay running at idle speeds? Two very different scenarios.

 

[Mrproffitt]

I'm a little confused here. Does the motor die or does it stay running at idle speeds? Two very different scenarios.

It dies from 3000 rpm all the way down to an idle and runs fine, but will not take any throttle.

 

[Mrproffitt]

It dies from 3000 rpm all the way down to an idle and runs fine, but will not take any throttle.

It dies like the switch has been cut off. No spitting or sputtering like it’s out of fuel

 

[Rick Stephens]

I'd redo the carb (again). All it takes is a funky needle valve.

When done the last time, was an OEM kit for the carb or a chinesium copy?

 

[jimmbo]

So once it slows down, what exactly does it do when you try to give it more Throttle? Does it Die, or just keep Idling?
Does this only happen at 3000 rpm, or does it do it at other speeds?
I know you say you checked the tank vent, but is it actually connected to the Tank? Have you tried opening the Fuel Cap when this shutdown happens?

 

[Scott06]

It dies like the switch has been cut off. No spitting or sputtering like it’s out of fuel

When this happens have you stopped and pulled a couple plugs to see what they look like? I assume when it happens and you stab the throttle fuel is being squirted out of the accelerator pump nozzles?

 

[Mrproffitt]

So once it slows down, what exactly does it do when you try to give it more Throttle? Does it Die, or just keep Idling?
Does this only happen at 3000 rpm, or does it do it at other speeds?
I know you say you checked the tank vent, but is it actually connected to the Tank? Have you tried opening the Fuel Cap when this shutdown happens?

The tank vent is not an issue, the engine shuts down no mater what rpms after 30-40 mins. It shuts down from 3000 or 4000 rpms, engine complete dies until it’s at an idle. This is not fuel related at all. It has fuel at the pump (6 psi), fuel in the carb, but will not take any throttle

 

[Mrproffitt]

When this happens have you stopped and pulled a couple plugs to see what they look like? I assume when it happens and you stab the throttle fuel is being squirted out of the accelerator pump nozzles?

Yes it’s getting fuel as I’ve stated. Completely new fuel system and 6 psi at the carb. I have not pulled the plugs as they are new after the heads where replaced. Something in the electrical system is getting hot and either cutting power, or grounding out. I’ve done away with the Thunderbolt V ignition and all sensors to it. It acted the same way before I changed all the parts from my original post. I’ve been through both manuals, and can’t find what’s causing this issue.

 

[Mrproffitt]

I'd redo the carb (again). All it takes is a funky needle valve.

When done the last time, was an OEM kit for the carb or a chinesium copy?

The carb was rebuilt with name brand parts. I don’t buy China unless I have to. I rebuilt the carb myself, 35 years as a mechanic

 

[Scott06]

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.

Ok if you are confident it has gas would look for spark loss with a spark gap tester when this happens both off the coil ad at the plugs and definitely disconnect the tach wire at the coil.
Maybe you can rig a test light off the 12v supply to the coil so you can see when it happens if you have 12 v under a load vs measuring on a voltmeter. If you are indeed getting a voltage drop 40 min in on the purple wire, I would bet it is under the dash. If you trace out typical helm 12v ignition on flow from the key through the dash gauges it is typically daisy chained through gauges etc , really stupidly... plenty of potential for given age of boat and humidity that there could be loose or corroded connections.. you could run directly off the battery with a 12 v jumper to coil/purple wire to test, maybe simpler way to get a fence around the issue

 

[Mrproffitt]

I’m going to disconnect the tach and replace the circut breaker this week and try that. If that’s not it, I’ll have to start tracing wires as there’s nothing left to cause the issue. It’s funny that it’s the same issue after 4 years with previous owner, now that I have it with everything I’ve done.

 

[Mrproffitt]

Ok if you are confident it has gas would look for spark loss with a spark gap tester when this happens both off the coil ad at the plugs and definitely disconnect the tach wire at the coil.
Maybe you can rig a test light off the 12v supply to the coil so you can see when it happens if you have 12 v under a load vs measuring on a voltmeter. If you are indeed getting a voltage drop 40 min in on the purple wire, I would bet it is under the dash. If you trace out typical helm 12v ignition on flow from the key through the dash gauges it is typically daisy chained through gauges etc , really stupidly... plenty of potential for given age of boat and humidity that there could be loose or corroded connections.. you could run directly off the battery with a 12 v jumper to coil/purple wire to test, maybe simpler way to get a fence around the issue

I have made a jumper wire for the coil this morning with the same thinking. I have also found several loose connections in the engine bay with no affects on the issue. I have also studied the wiring diagram for the dash, and can only see that the tach might be the issue. I think it’s getting hot after about 30-40 mins, and not quite grounding out, but cutting the voltage at the coil from say 13.8 from the system, to say 4-6 volts. Enough to run at idle, but not enough under load. Hopefully that makes sense. The tach still works, but because it’s internally grounded, I have no way to test it besides unhooking it.

 

[Mrproffitt]

I like the test light idea, I could immediately see if I’ve lost power to the coil while sitting at the helm. I was going to use a voltmeter, but that’s a way better idea

 

[Mrproffitt]

After much debate and thinking on this today, I’ve narrowed it down to this. The only common thing between the old ignition system and the new one, is the tach wire and the power wire. So I will eliminate the tach as an item to start with and if it still happens again, then the power wire is grounding somewhere. I will report the findings soon.

 

[Scott06]

After much debate and thinking on this today, I’ve narrowed it down to this. The only common thing between the old ignition system and the new one, is the tach wire and the power wire. So I will eliminate the tach as an item to start with and if it still happens again, then the power wire is grounding somewhere. I will report the findings soon.

Not sure the history of the boat but had a similar issue with my brothers center console last year. Ran fine in the fall when put to bed, wouldn’t start in spring. Was able to jump the engine (yam outboard) at the cannon plug would run and start. Ran a jumper from boat harness at motor up to dash , to ohm out under floor harness. Turned out has several ohms resistance on each wire due to corrosion. water probably from previous salt water use in conduit below floor. Replaced harness from engine to helm problem solved

 

[Mrproffitt]

Ok if you are confident it has gas would look for spark loss with a spark gap tester when this happens both off the coil ad at the plugs and definitely disconnect the tach wire at the coil.
Maybe you can rig a test light off the 12v supply to the coil so you can see when it happens if you have 12 v under a load vs measuring on a voltmeter. If you are indeed getting a voltage drop 40 min in on the purple wire, I would bet it is under the dash. If you trace out typical helm 12v ignition on flow from the key through the dash gauges it is typically daisy chained through gauges etc , really stupidly... plenty of potential for given age of boat and humidity that there could be loose or corroded connections.. you could run directly off the battery with a 12 v jumper to coil/purple wire to test, maybe simpler way to get a fence around the issue

I got to looking around under the dash and found a wire that’s not on the factory wiring diagram that’s now confusing me. My tach wire at the tachometer is gray with a brown stripe. So now I’m wondering what’s between the tach and coil for them to change the color of the wire.

 

[Scott06]

I got to looking around under the dash and found a wire that’s not on the factory wiring diagram that’s now confusing me. My tach wire at the tachometer is gray with a brown stripe. So now I’m wondering what’s between the tach and coil for them to change the color of the wire.

Who knows you it is possible it changes color at the cannon plug, but if you disconnect at the coil it should be irrelevant for testing unless the tach is reducing the voltage on the 12 ignition on circuit