I have a Mercruiser 1994 5.0LX problem that has been plaguing me now for several weeks. After a cold start the boat runs 10 to 15 minutes at all throttle ranges, then sputters when at high RPM. Dropping down to <1000 RPM it will continue to run, but above that it will run for a few seconds then cough and sometimes die, forcing me to transit home at near idle.
I have read many posts from people with similar experience, and most boil down to a carb, pump, or filter problem. I am pretty sure it is fuel related, but will let you know what I've done thus far (to eliminate different 'potential' problems).
Having just purchased the boat, and experienced this problem the first time out on the water (she did fine for her sea trial...figures!), I began with the basics.
1) I did a general tune up, replacing the coil, plug wires, cap, and rotor. (Thunderbolt IV ignition). No improvement.
2) I pulled the anti-siphon valve, tank pickup tube, and in-carburetor filter. Found a bit of grunge in the pickup tube that I thought was the smoking gun. Cleaned it, tried again, same results. Reinspected the tank pickup tube, figuring the screen could have clogged again, and it was clean.
3) Replaced the fuel filter...it ran well for another 10 minutes, then failed again.
4) Replaced the fuel pump, and there was no improvement.
5) Added fuel stabilizer to the fuel tank, same results.
6) Removed and completely rebuilt the carburetor (Quadrajet). The accelerator pump piston had detached from the shaft, causing me relief since I figured it HAD to be the culprit. Ran it again, and the problem returned.
7) Replaced another fuel filter...again it ran well for 10 minutes, then died.
8) Checked my tank vent, and opened my gas tank fill cap to make sure I wasn't causing a vacuum...no improvement.
So, needless to say I'm a bit frustrated. During one of the trials, where the engine was sputtering, I had my wife steer the boat, while I pulled the flame arrestor, and looked down into the carb. As it was sputtering, I tapped the top of the accelerator pump repeatedly to pump a bit of extra fuel into it. The engine perked right up, but in a few seconds, I had pumped the accelerator reservoir dry. I don't think it can be pumped dry when fuel is properly flowing, so I'm convinced this is a fuel delivery problem. Also, since it always goes away temporarily when I replace the fuel filter, I expect is something related to the filter and fuel.
I took one of the spent filters and hack sawed it in half, only to find a nearly pristine paper filter inside. The filters are Sierra's 21 micron filters, and are on the suction side of the pump (between the gas tank and the pump). This is the factory fuel line plumbing for the boat.
I know much has been written about Ethanol mixed fuels, and I did refuel the boat at a gas station when I initially took it down to launch it. The 70 gallon tank had sat for a year 1/2 full. Most fuel problems I read about related to ethanol, however, seem to be phase separation or water in the fuel. I have never gotten water from a replaced fuel filter. If it's phase separation, I would not expect the accelerator pump to 'run dry' during my test. Fill with ethanol, yes, but not run dry. Instead, it seems as if my filter is 'clogging', and that if I leave the boat sit and cool off, the 'clogging' is reset, because from a cold start the boat will always run for 10 minutes or more before sputtering at high rpm.
Suggestions? Ideas? Anything? I'm running out of ideas, short of completely bypassing the fuel filter to see if it makes the problem go away.
I have read many posts from people with similar experience, and most boil down to a carb, pump, or filter problem. I am pretty sure it is fuel related, but will let you know what I've done thus far (to eliminate different 'potential' problems).
Having just purchased the boat, and experienced this problem the first time out on the water (she did fine for her sea trial...figures!), I began with the basics.
1) I did a general tune up, replacing the coil, plug wires, cap, and rotor. (Thunderbolt IV ignition). No improvement.
2) I pulled the anti-siphon valve, tank pickup tube, and in-carburetor filter. Found a bit of grunge in the pickup tube that I thought was the smoking gun. Cleaned it, tried again, same results. Reinspected the tank pickup tube, figuring the screen could have clogged again, and it was clean.
3) Replaced the fuel filter...it ran well for another 10 minutes, then failed again.
4) Replaced the fuel pump, and there was no improvement.
5) Added fuel stabilizer to the fuel tank, same results.
6) Removed and completely rebuilt the carburetor (Quadrajet). The accelerator pump piston had detached from the shaft, causing me relief since I figured it HAD to be the culprit. Ran it again, and the problem returned.
7) Replaced another fuel filter...again it ran well for 10 minutes, then died.
8) Checked my tank vent, and opened my gas tank fill cap to make sure I wasn't causing a vacuum...no improvement.
So, needless to say I'm a bit frustrated. During one of the trials, where the engine was sputtering, I had my wife steer the boat, while I pulled the flame arrestor, and looked down into the carb. As it was sputtering, I tapped the top of the accelerator pump repeatedly to pump a bit of extra fuel into it. The engine perked right up, but in a few seconds, I had pumped the accelerator reservoir dry. I don't think it can be pumped dry when fuel is properly flowing, so I'm convinced this is a fuel delivery problem. Also, since it always goes away temporarily when I replace the fuel filter, I expect is something related to the filter and fuel.
I took one of the spent filters and hack sawed it in half, only to find a nearly pristine paper filter inside. The filters are Sierra's 21 micron filters, and are on the suction side of the pump (between the gas tank and the pump). This is the factory fuel line plumbing for the boat.
I know much has been written about Ethanol mixed fuels, and I did refuel the boat at a gas station when I initially took it down to launch it. The 70 gallon tank had sat for a year 1/2 full. Most fuel problems I read about related to ethanol, however, seem to be phase separation or water in the fuel. I have never gotten water from a replaced fuel filter. If it's phase separation, I would not expect the accelerator pump to 'run dry' during my test. Fill with ethanol, yes, but not run dry. Instead, it seems as if my filter is 'clogging', and that if I leave the boat sit and cool off, the 'clogging' is reset, because from a cold start the boat will always run for 10 minutes or more before sputtering at high rpm.
Suggestions? Ideas? Anything? I'm running out of ideas, short of completely bypassing the fuel filter to see if it makes the problem go away.