Mercruiser 1994 5.0LX stalls at high RPM after warm

ihcfan

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Joined
Jun 2, 2008
Messages
5
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.
 

DaveM

Petty Officer 1st Class
Joined
Feb 27, 2002
Messages
308
Re: Mercruiser 1994 5.0LX stalls at high RPM after warm

It looks like you have done all the expensive, time consuming stuff. I would look at replacing the anti-siphon valve itself. They are inexpensive and can fail. Some say the brass valves are more reliable than aluminum.

Look into using a portable tank with fresh gas if that does not solve the problem.
 

ihcfan

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Joined
Jun 2, 2008
Messages
5
Re: Mercruiser 1994 5.0LX stalls at high RPM after warm

Thanks, I'll try both this weekend and post a followup.
 

ihcfan

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Joined
Jun 2, 2008
Messages
5
Re: Mercruiser 1994 5.0LX stalls at high RPM after warm

... I would look at replacing the anti-siphon valve itself. ...Look into using a portable tank with fresh gas if that does not solve the problem.

I tried two anti-siphon valve tests. I punched the check valve ball and spring out of the old one so it was just a fitting, and the problem continued. Replaced it with a new one, and same results.

After heating the engine up and experiencing the problem, I switched to a 6 gallon tank full of new fuel. It ran the whole tank. The next day I pumped six gallons of fuel into the external tank from my internal fuel tank. I ran on the internal tank until it failed, then switched to the external tank, and it too failed.

I did both tests because the 'location' for the external tank in my engine compartment was higher than my built-in fuel tank, and I wanted to make sure it wasn't related to the 'head' the pump has to pull. It certainly seems like it's not.

So, I pumped the tank empty today, and next week I will refill with fuel and hopefully be done with it. I am still bothered because I can't explain parts of 'why' it would behave that way. For example, I would not expect bad fuel to cause a problem only after warm-up, or get better for a short period of time after I changed the fuel filter (though it could be a 'cool down' issue in the second case) I did notice the fuel in the tank smells funny compared to regular gas...more pungent. When I lived in CO our winter fuel seemed to smell different, and this has a similar odor, and I wonder if that has anything to do with it.

If new fuel makes the problem go away, then I will certainly be happy, but I'd like to at least understand why bad fuel would behave the way it does...works for 20 minutes, well after the engine's warm, and then die.
 

scschafer

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Joined
Aug 15, 2008
Messages
1
Re: Mercruiser 1994 5.0LX stalls at high RPM after warm

ihcfan -

Just curious, did you ever resolve your problem by pumping all old gas. I am experiencing exact same problem with my Mercruiser 3.0L(140hp). Talk about frustrating. I thought it had something to do with ignition so ending up replacing plugs, wires, coil, and even put in new distributor with new points, condensor, rotor, and cap. Ran good for about ten minutes until warm and then starting sputtering at high rpm. Any advise would be greatly appreciated :)
 

ihcfan

Recruit
Joined
Jun 2, 2008
Messages
5
Problem was that the fuel/water separator was not tightened down tight enough. I use a canister type fuel separator and when it would warm up, I believe the rubber gasket would get soft and allow air to leak into the fuel line, ultimately starving the engine of fuel.
 
Joined
Jul 27, 2014
Messages
9
Must be a mercruiser thing. My 3.0 has been doing the same thing for two weeks. Finally discovered yesterday my pickup tube in the tank has a screen that was full of gunk cleaned it out and problem solved
 
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