Thunderbolt 5.0 keeps dying

[amplay]

I have a 1998 Mercruiser Model 5.0L (2-BBL) 1998-95 GM 5.0L - 305ci - 8cyl; S/N OL045552

Rebuilt carburetor in the spring, and engine ran well. Now about 50 running hours later having a few problems. First the engine died going from 2500 RPM to Idle. Then the engine died on throttle up, and throttle down. Then I would have trouble starting, then no starting. I jumped the Oil Pressure Sensor, and engine starts every time, but dies after about 10 seconds. I have replaced the Coil, Distributor Cap, Rotor (all were original, so ready to replace) but no change. I disconnected the gas line going to the carburetor, and gas immediately leaked out, so I don’t suspect a fuel issue. Not sure if this is related, but I have a (slightly) wondering tach.

 

[alldodge]

Disconnect gray wire off - side of coil, it goes to the Tach. If that fixes then its the Tach

Put a meter on the + side of coil and see if your loosing 12V

 

[amplay]

Thanks for your reply. I have an old fashion timing light. Glows bright when the engine is running, then starts to flicker, then goes black (and sometimes the engine continues to run for a second or 2.



I’ll put a meter on the + and see when happens.



I tried taking off the grey wire and the engine would not start, and the timing light never lit up.

 

[alldodge]

How many wires are on the - side?
It might have 2 Gray wires and one goes direct to the Tach. Pic of your motor

 

[amplay]

I only have 1 wire on each side
See attached video
View attachment VID_20220927_123528368-A.mp4

 

[alldodge]

Sounds like its running out of gas. Have to crank a bit to get more gas back in the carb.

Disconnect fuel line from carb and put in a container to catch fuel. Might use a piece of hose so it can be put further away from the motor. Crank the motor to see how well fuel is pumped.

If good, by pass oil pressure switch and turn key ON to see if pump keeps pumping without starting motor.

 

[amplay]

Thanks! I'll try tomorrow

 

[Bondo]

Sounds like its running out of gas. Have to crank a bit to get more gas back in the carb.

Disconnect fuel line from carb and put in a container to catch fuel. Might use a piece of hose so it can be put further away from the motor. Crank the motor to see how well fuel is pumped.

If good, by pass oil pressure switch and turn key ON to see if pump keeps pumping without starting motor.

Ayuh,...... Agreed,...... The timin' light was blinkin' as t died too,....

 

[amplay]

Thanks for your concurrence. If I disconnect the fuel line, and pour into a cup, approximately how much fuel per minute should I see?

 

[alldodge]

Unable to say but there should be a steady stream. A 90 gal per hour pump will pump about a gallon a minute

 

[Bondo]

Thanks for your concurrence. If I disconnect the fuel line, and pour into a cup, approximately how much fuel per minute should I see?

Ayuh,..... I'd go with a length of hose, jambed onto the fuel line, into a catchcan, myself,.....

You'll see a quart, quickly,....

 

[amplay]

I replaced the fuel filter, no change. Then I disconnected the fuel line at the carburetor. The engine started, and I got a 2 second squirt of gas, then the gas stopped a few seconds later, then the engine died. I was able to start 3 times.
What do you think?

 

[amplay]

Let me clarify, 2 second of gas, 2-3 seconds of no gas, then the engine died. I was able to restart twice.

 

[alldodge]

Either a fuel pump problem or power is not staying connected to fuel pump.

When oil pressure switch is bypassed will the pump run and continue to run with the key ON and motor not running?

 

[amplay]

I have the oil pressure switch bypassed. Fuel pump does not run with key on but motor not running

 

[amplay]

Is there a sensor that tells the fuel pump to stop?

 

[amplay]

I put meter on the fuel pump leads. I get nothing with just the key on. When I try to start, I get a momentary pulse, then nothing

 

[nola mike]

Is there a sensor that tells the fuel pump to stop?

That's the job of the OP switch. If you're jumping it correctly, you should have 12v at the fuel pump terminal.
Sounds like you aren't jumping it correctly or you need to jump to the fuel pump directly.

 

[alldodge]

Have a break in connection. There should be a purple wire feeding power, that's where the break is. Might be the connector end

 

[Bondo]

I put meter on the fuel pump leads. I get nothing with just the key on. When I try to start, I get a momentary pulse, then nothing

Ayuh,..... That power comes from the key switch, when set to On,....

I'd be lookin' at the key switch itself, 'n then the cannon plug, where to boat meets the motor,......