4.3L Thunderbolt IV starts and runs for between 20 sec and 2 mins then dies.

peerpr3sur77

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Jul 19, 2020
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The boat had been running fine until this weekend. Took it out two weeks ago and it was running fine. Took it out this weekend and ran for about 20 mins. I shut her down to get the kids ready to tube and when I started her back up she fired right up and then I started pulling the kids and she died, like the key was shut off. I did that several times without towing anything and on a couple of occasion I would throttle her way up to around 4-5K rpm and then put her in gear and she would go for about 2 mins. That wasn't consistent though.
Since I got her home, I've replaced the fuel filter, pulled the fuel line from the carb and pointed it to a small gas can and ran the boat.. she dumped out fuel just fine while idling. I've pulled and cleaned the ignition sensor inside the distributor. I'm waiting for a replacement to come in the mail.
Any other things I should be looking at?
Thanks,
 

Furbird

Seaman
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Jul 8, 2020
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62
Electric fuel pump or mechanical? Look at the other 4.3 threads on the first couple of pages and you'll see where I'm going with this question.
 

peerpr3sur77

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Jul 19, 2020
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Electric, but I pulled the hose and ran the motor on what was in the carb and it pumped solid with no interruption or hiccups.
 

tank1949

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Apr 4, 2013
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Have you checked for crap blocking gas tank vent? Are you able to easily pull fuel pickup tubes to examine? Many have screens inside them and they get clogged over time. Get empty 5 gal gas can and let your electric fuel pump fill up can, if it can? Motor's demand for fuel at idle is not as much 3000 rpms. You may have crap floating in tank that has built up over the years and now is causing havoc.
 

Furbird

Seaman
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Jul 8, 2020
Messages
62
The 4.3 with electric fuel pump is powered by the oil pressure switch. It is NOT the same one that goes to the gauge. If the switch is failing, it cuts the fuel pump off thinking the oil pressure is non-existent. You said this only happens when the engine is warm, which means the oil is hot and thinner than cold and at idle. Just because you have fuel at idle on a cold motor does not mean you have fuel at higher RPM's on a hot engine. Also, just because a fuel pump works when it's cold doesn't mean it works when it's hot. If the pump is on it's way out, the hotter it gets, the less it pumps. I've experience this myself (in the auto world.)

Tank's theory is also worth looking into as well. Something is cutting off your fuel supply, whether it's supply or pump.
 

peerpr3sur77

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Joined
Jul 19, 2020
Messages
5
Have you checked for crap blocking gas tank vent? Are you able to easily pull fuel pickup tubes to examine? Many have screens inside them and they get clogged over time. Get empty 5 gal gas can and let your electric fuel pump fill up can, if it can? Motor's demand for fuel at idle is not as much 3000 rpms. You may have crap floating in tank that has built up over the years and now is causing havoc.

I haven't done that yet. I was going to pull the hose from the take and drop in a 5 gallon and let it run from that to see if I got the same problem. That was my plan for today. I'm thinking it either has to do with the antisiphon or crap in the tank. I'll let you know later today if that is the cause.
 

peerpr3sur77

Recruit
Joined
Jul 19, 2020
Messages
5
The 4.3 with electric fuel pump is powered by the oil pressure switch. It is NOT the same one that goes to the gauge. If the switch is failing, it cuts the fuel pump off thinking the oil pressure is non-existent. You said this only happens when the engine is warm, which means the oil is hot and thinner than cold and at idle. Just because you have fuel at idle on a cold motor does not mean you have fuel at higher RPM's on a hot engine. Also, just because a fuel pump works when it's cold doesn't mean it works when it's hot. If the pump is on it's way out, the hotter it gets, the less it pumps. I've experience this myself (in the auto world.)

Tank's theory is also worth looking into as well. Something is cutting off your fuel supply, whether it's supply or pump.

It's funny, I thought the same thing a few weeks ago when I couldn't get it to run at all. I replaced the whole assembly, switch and sensor. Both are new and it still wouldn't run. Turned out that my daughter dropped the kill switch to off and didn't think to tell me while we were being towed back to the ramp. ugh.. love kids!! :). Since then I've learned out to bypass that switch, and I tried that as well to make sure it wasn't an oil/oil switch problem.
 

peerpr3sur77

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Jul 19, 2020
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Okay, after more testing, the fuel line is clear and the antisiphon valve is working just fine. Turns out that my original jumper I made for the oil pressure switch wasn't making a solid connection. I made a new one and bypassed the switch and the boat runs fine.. oil pressure sits at 4 BAR and just bounces right on that line. I did pull the ignition sensor from the distributor and cleaned off all the rust and put it back in.. it idles closer to 650 now and doesn't stumble, and starts right up. Turns out the new oil pressure switch I bought just a few weeks back is faulty. I'm gonna have to see if the place I bought it from will warranty it. Thanks for the help guys!!
 
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