4.3l MerCruiser problems

Orion4x4

Active Member
I got a new to me bayliner trophy with a low hour 4.3l MerCruiser swapped in for the original 3.0l. It starts perfect, runs awesome, idles great but always running in at the end of fishing it stumbles and tries to die. It will come up to cruising speed and run awesome for 5 minutes but there it cuts out. It never completely dies but almost then it will try to come back but drops off again. If you throttle back it idles fine but when you throttle back up it will run good for 30 seconds then crap out again. Doesn’t seem to matter if I’m cruising at 2000rpm or 3000rpm. I think it happened on the run out in the morning one time too. I’ve changed filters and cleaned the carb screen too. It has an electric fuel pump on it. I checked when it happened last time and the fuel pump wasn’t hot. Any help would be really appreciated guys.
 
could be any number of things but my 5.7 mercy did the same sort of thing , turned out it was the electric fuel pump, to trouble shoot we connected a 20 liter tank to the engine but kept it sitting fairly high, that allowed a bit of gravity feed to help the electric pump, boat ran fine when we did this, so changed out the pump, fine after that.
 
I got a new to me bayliner trophy with a low hour 4.3l MerCruiser swapped in for the original 3.0l. It starts perfect, runs awesome, idles great but always running in at the end of fishing it stumbles and tries to die. It will come up to cruising speed and run awesome for 5 minutes but there it cuts out. It never completely dies but almost then it will try to come back but drops off again. If you throttle back it idles fine but when you throttle back up it will run good for 30 seconds then crap out again. Doesn’t seem to matter if I’m cruising at 2000rpm or 3000rpm. I think it happened on the run out in the morning one time too. I’ve changed filters and cleaned the carb screen too. It has an electric fuel pump on it. I checked when it happened last time and the fuel pump wasn’t hot. Any help would be really appreciated guys.

Sounds like a fuel pump.
 
My 4.3 has done that twice over my 16 years of ownership... both times it was the fuel pump.
 
My 4.3 has a high and low pressure fuel pump, did you check both? One is almost impossible to reach, believe it’s the HP.
 
Thanks guys. That’s what I figured it was but figured a second opinion before spending the money is always helpful.
 
Sometimes a plugged tank vent will do the same. I can't recall who but someone from this forum had this problem with your symptoms. The thru hull piece had a nest of some sort inside. Sounds like fuel starvation.
 
Are you still having trouble? Perhaps if you have electronic ignition, it's the Pertronics module. Go back to points/ condenser and check if it's better.
 
I would think coil also...... But could be fuel related. Are you getting any condensation on the coil when running in? Cleaning your deck and water getting into the hatch? doesn't take much... keep some wd 40 on board and spray the coil when it happens to see if it will help...
My thought was coil and spring fever beat me to it...
Happened quite often on an earlier boat I owned that ran a 5.7L ... water would get in around the engine hatch after rinsing our fish and drip onto the coil...

maybe check to see if you have any vacuum line leaks also. Tighten any fuel fitting hoses that may be drawing in a bit of air when the engine is demanding fuel...
 
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if your 4.3 has the carter electric fuel pump. then go rock auto .com

the merc or serria one which was the same cost wise was 2/3rds cheaper including delivery...
think it was like $65
 
If it does have a Carter while I have one on my ski boat I have had three others on other boats that would at times not regulate pressure and blow fuel past the carburetor needle and seat flooding the engine at idle and allmost or stalling the engine, excessive cranking to restart. I installed return to tank fuel pressure regulators to prettymuch eliminate the possibleity. Good pump but not the best internal pressure regulator.
 
Could be your fuel pickup, something blocking it and your building a restriction over time. Had this happen to me, thought it was my fuel pump changed it out and didn't make a difference. Mine use to run great for an hour maybe then start to stumble and die. Trying running it on a day tank and see if that makes a difference. Check the screen in your pickup and see if something is blocking it.
 
It sounds to me like its time for a carb rebuild. The 4.3L has the TKS system so when it's cold, there is a solenoid that provides extra fuel at idle (sort of like a half-choke). When warm, the TKS shuts down (you can hear the hiss sound go away). If it won't idle after the TKS is off, it likely means you have a dirty carb. If you can't rebuild the carb (not terribly difficult), try running a bottle or two or more of Seafoam in your fuel tank -- go with a heavy concentrate -- I do this with my lawnmower and weed wacker -- 1 can of Seafoam to 1 gallon of fuel. Run the boat hard. Let it sit for 30 mins, start it again -- might smoke like hell when it starts but cleans all the crap out of the carb and jets....
 
So I have a fuel pump on the way. A carb rebuilt is on the to do list in the next little while. I’m going to go through all my grounds and check the coil this weekend and hopefully I can track down the problem with one of these things. Thanks for all the advise guys. It’s greatly appreciated.
 
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