3.0 Mercruiser

Tavinthor06

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Dec 11, 2023
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I have a 3.0 Mercruiser 140 hp runs fantastic on muffs without issue, once I get it in the water and under load it runs great for about 40 minutes then stalls out, I can get it to restart after about 29 minutes can run it wot for 40 minutes then stalls out. All new electrical, carb rebuild earlier this year. I've check all compressions and spark all are great. I'm stumped anyone have any idea what could be going on?
 

Lou C

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Nov 10, 2002
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What’s the gas in the fuel filter look like?
How about the anti siphon valve? Fuel pick up? Tank vent?
 

Tavinthor06

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Dec 11, 2023
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Gas in the fuel filter looks fine, I haven't checked the ani siphon valve to be honest not exactly sure where it is located on this boat (1979 glasplay 19' runabout). When I filled with ethanol free fuel like I always do, I got fuel spill out of the tank vent.
 

Grub54891

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Jun 17, 2012
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The anti syphon valve is located at the fuel line, on the tank pickup. The fuel line hooks right to it. If you remove it, and its clear/clean you can re-use it. I'd replace it myself.
Did you check for spark right after it quits?
 

Tavinthor06

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Dec 11, 2023
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Grub thank you,
Everything is brand new, plugs, wires, rotor, cap, ignition module etc. It only fails and stalls under load and it takes 40 minutes for it to do it. Wait 20 minutes fires right up can get another 40 minute run then does it again.
 

airshot

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Jul 22, 2008
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I have a 3.0 Mercruiser 140 hp runs fantastic on muffs without issue, once I get it in the water and under load it runs great for about 40 minutes then stalls out, I can get it to restart after about 29 minutes can run it wot for 40 minutes then stalls out. All new electrical, carb rebuild earlier this year. I've check all compressions and spark all are great. I'm stumped anyone have any idea what could be going on?
Look carefully at your fuel. Had a similiar issue years back with water in the fuel. If you don't already have one, add a water seperator filter. The vent line should have an upward loop before it exits the hull to prevent water spray from going thru the vent line back into the tank. That was my issue, drained and cleaned the tank, added the upward loop and problem was solved. Water spray off the hull would go into the vent and down the line into the tank. At one time after an engine stall, we removed the seperator filter and poured the remnants into a metal bowl, tried lighting it....all water, would not burn ! Todays fuels can be a crapshoot, often contaminated so adding filters can really help.
 

Grub54891

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Agreed with the above, also if there is debris in the tank it can plug the anti siphon valve, and some have a screen on the end of the pickup tube. Doesn’t hurt to check those out while you are there.
Edit to add, do you actually check for spark when it quits? New components can be bad out of the box also.
 

Tavinthor06

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Dec 11, 2023
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Checked for spark, we're good there, changed the water fuel separator today was full of water, now it won't start at all not getting any fuel pressure.
 

airshot

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Jul 22, 2008
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Checked for spark, we're good there, changed the water fuel separator today was full of water, now it won't start at all not getting any fuel pressure.
Clean out entire fuel system !! Water may have damaged the fuel pump.
 

Tavinthor06

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Dec 11, 2023
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Clean out entire fuel system !! Water may have damaged the fuel pump.
Looks like mine has a mechanical pump located on the starboard side of the motor, I do have electric wires running to the tank, could there very well be a fuel pump in the tank as well?
 

Scott06

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Apr 20, 2014
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Looks like mine has a mechanical pump located on the starboard side of the motor, I do have electric wires running to the tank, could there very well be a fuel pump in the tank as well?
No that is the sending unit for tank level.
when it stalls you still have spark ?
when it stalls and you pump the throttle do you see gas squirted in by accelerator pump?

if when it stalls dribble some gas down carb. Does it run a bit then?try to determine if you are missing gas or spark when it dies and will not restart. Open gas cap when it dies see if fuel tank is under a vacuum from plugged vent
 

Tavinthor06

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Dec 11, 2023
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Yes got plenty of spark, changed out the fuel filter and water separator today, not getting any fuel now, wondering if the mechanical pump failed, any suggestions on how to test before I buy a new one.
 

matt167

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Sep 27, 2012
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Hook up an external fuel tank and see if it’ll run.

My parts boat quit on the previous owner and he thought it needed a fuel pump. It would run out of fuel after a short while. I hooked it up to my portable tank, and it fired up and ran great. I looked into the fuel situation quickly as it’s only a parts boat to me, but the line or the pickup was obstructed. It was a good thing as the boat was structurally compromised and ready to break apart.
 

Scott06

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Yes got plenty of spark, changed out the fuel filter and water separator today, not getting any fuel now, wondering if the mechanical pump failed, any suggestions on how to test before I buy a new one.
If you are not getting fuel likely the pick up (some have a screen inside of pickup tube) , or anti-siphon valve are not working or there is a blockage in the line. I think some guys have t'ed in a vacuum gauge on suction side to verify. As mentioned put a remote tank on temporarily to isolate fuel tank and supply system. If it runs on a remote tank then you know where to look
 

Tavinthor06

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Dec 11, 2023
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Took the boat out today after changing the water fuel separator and the fuel filter, cleaned tank and ask the lines, ran amazing for about an hour and a half no issues could idle throttle down and back up with ease and then it started not responding to throttle and would die however would for back up in neutral with no issue and could give it throttle no problem in neutral but then put it in gear and half throttle would die then fire right back up.
 

Scott06

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Took the boat out today after changing the water fuel separator and the fuel filter, cleaned tank and ask the lines, ran amazing for about an hour and a half no issues could idle throttle down and back up with ease and then it started not responding to throttle and would die however would for back up in neutral with no issue and could give it throttle no problem in neutral but then put it in gear and half throttle would die then fire right back up.
Reving in neutral is meaningless it take no power to rev up with no load on it. when it dies have you checked spark strength with a cheap spark gap tester? If spark is good when it dies you are still dealing with a fuel supply issue .
 

Tavinthor06

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Dec 11, 2023
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Spark is on point, no issue there, I know it's a fuel issue all lines are cleared no clogs no pressure build up in the tank. Just not figuring out why it'll run so amazing for over an hour then just suddenly die. Aside from taking the carburetor apart or the fuel pump I'm not seeming to figure it out
 

sinlessearth

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Oct 28, 2023
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Just a thought. Can you remove the gas cap when it has trouble running and see if that works? Maybe it's a venting issue and opening the gas cap would allow the tank to vent and the carb to suck in more fuel.
 
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