Testing Common OMC fuel pumps

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This topic contains 12 replies, has 8 voices, and was last updated by Avatar johnyrude200 4 years, 2 months ago.

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    johnyrude200
    US Member - 2 Years
    Replies: 751
    Topics: 172
    #1222

    I’ve been dealing with a rash of failed fuel pumps on a number of motors I’ve been working on. I have not detected any pattern, other than the fact that the pumps are all probably aged. They are all the common small square pumps widely used on OMC motors 3-40hp for decades.

    As part of my motor analysis/diagnostics, I have been really considering making testing fuel line pressure as part of my standard procedure. The reason why is I had about 7 motors in January all have working pumps the first time I ran them (after doing all the usual stuff to get the motor going again for 15-25 mins), went to dinner for a half hour break, come back, and have them fail. I originally wrote this off to just coincidence back in January, but have now observed this happening with motors which I let run for 1-2 hours before giving them the go:no-go approval for sale and storing them for sale as of recent.

    I am a big fan of details and pattern recognition. I have been running all of my motors out of gas before putting them away (at this point in New Hampshire, I’m not fogging them anymore because we are finally staying above freezing both day and night consistently). However, I had a topic I posted some time back about my 2014 30hp Etec, where it was stated that I shouldn’t run it out of gas completely because it can potentially damage the pump.

    I am wondering if this is the case with the older pumps too. I mean..do the pulses from the intake need fuel on the other side of the diaphragm to keep that thing from breaking up if it’s dry? I would think a dry piece of thin plastic is a lot more fragile than a moist one, say nothing of one that has been sitting for 5-10-30 years since it was last used. I have observed that when pulling powerheads that if I soak down the exhaust to powerhead gasket with WD-40 or PB-Blaster, usually that gasket softens up and is fine to be reused. Try pulling it dry and it just plain disintegrates.

    More food for thought – even after the motor runs, I put them up to 1/2 throttle, pull the choke, and they fire up again. As long as I put the choke in and leave them at 1/2 throttle I can usually keep them running for another 2-3 mins. They start to die off, I pull the choke, rev back up in a pulsation, choke open, repeat. Then I know the motor is totally dry with nothing left in the carb bowl. But is this destroying the fuel pump diaphragm. I haven’t read much about ‘best practices for winterization’ or ‘long-term motor storage’ which describe this process I came up with from just screwing around with them.

    I have seen some explanation in the various literature out there, but was wondering what some of the veteran mechanics here could recommend? Just want to make sure I do a good job for my customers.

    I am going to start a 2nd thread to about joint longevity (would REALLY appreciate your recommendations in this department – thank you thank you thank you in advance!).

    I want to just come out and say, I’m sure there are many people who have worked on, literally, thousands of small motors (and big ones) who will read this. I am only on about 125-150, but hope to get the call into the big leagues some day. So thanks again for all your help. Really.

    frankr
    frankr
    US Member - 1 Year
    Replies: 4154
    Topics: 43
    #14052

    I’ve been running motors dry ever since I’ve been running motors. Without damage. It is true that when they came out with multiple carburetors, OMC said not to run them dry because one would run out of fuel before the others. OK those guys made a lot more money that I did but still I say BS. If running out of gas at idle is going to damage a motor, what is going to happen when you run that 6 gallon tank dry at 5000 RPM?

    Opinions will vary, so expect to hear the other side soon.

    BTW, some of the early square pumps were junk from the factory. Maybe some still are, I don’t know because I don’t take them apart any more. The ones that I call junk had flapper tabs on the diaphragm to act as check valves. "Real" fuel pumps should have real check valves.

    PS. Aslo, some of those pumps wouldn’t lift gas very high. So, if you are running a motor in a test tank, and the gas tank is on the floor, that might have something to do with it.

    Avatar
    johnyrude200
    US Member - 2 Years
    Replies: 751
    Topics: 172
    #14055

    Good to know about tank height; mine is about 2 feet below the pump height.

    Id like to construct a simple fuel pressure assembly to place downstream of the pump. Is there a ‘good vs bad’ range for PSI for viable pumps?

    Avatar
    fleetwin
    US Member - 2 Years
    Replies: 2614
    Topics: 31
    #14066

    Well, there is some merit to all the view points I guess. I agree with Frank though, running an engine out of fuel at idle speeds will not damage the powerhead. But, like you say, the diaphragm movement surely does accelerate quite a bit just before the engine runs out of fuel, which may damage the diaphragm. I remember hearing the VROs clack up a storm when I used to run them out of fuel while fogging, I DID break a few air motor diaphragms so I stopped that practice.
    I still run the small engines out of gas, and feather the choke just like you do, in an attempt to get most of the fuel out of the bowl. But, there is always a little bit of fuel left in the bowl in spite of these efforts. I always pull the carb drain plugs (if equipped) after winterization, especially if the engine will be in storage for more than just the winter months.
    I don’t know if your rash of fuel pump failures is coincidental, or somehow related to the fuel you are using. If all the pumps had failed diaphragms, then I might be inclined to suspect running out of fuel might have something to do with the failures. Keep in mind that an engine that sneezes/coughs a lot is more likely to damage the fuel pump diaphragm as well. As fair as a fuel pressure gage, well you could sure set up a gage to check the engines, but you will need a gage that reads low pressures well, like a zero to five PSI gage. Keep in mind that checking just fuel pump output doesn’t mean a whole lot unless you rig up a clear hose and vacuum gage to the fuel pump inlet as well. Restriction/inlet air leaks are often misdiagnosed as fuel pump problems. So, I don’t know if checking pressure/vacuum is a realistic procedure for every engine you work on, rigging up these things can be a real pain due to stiff hoses and flimsy plastic connectors. Needless to say, doing these checks is not that big a deal if you are replacing the engine fuel lines anyway.
    Many OMC fuel pump failures show up as engines that run out of fuel at lower speeds, but may run OK at higher RPMs. And, just like Frank says, sometimes we introduce problems by running the engines in test tanks with the fuel pump mounted 2-3 feet below the engine.

    Avatar
    johnyrude200
    US Member - 2 Years
    Replies: 751
    Topics: 172
    #14067

    All valuable info. Maybe I’ll give it another couple of months before installing another checkpoint on my list of engine analysis. I figured disturbing older fuel lines is only going to create more work in replacing them (albeit, not something I do normally if it’s still working properly), but that’s something

    Avatar
    kerry
    US Member - 1 Year
    Replies: 323
    Topics: 14
    #14079

    First, like Frank, I have been running motors dry at the ramp since my father did the same when I was a wee lad. Never had any pump problems or carb issues even after winter storage. As a teen and young adult, had two ski boats fed by 6 gallon tanks. Ran them til the tank ran dry and switched tanks. Again, no problems with pumps. I just wonder if the newer kits use a cheaper construction that fails easier. (Planned obsolescense?)

    Avatar
    johnyrude200
    US Member - 2 Years
    Replies: 751
    Topics: 172
    #14081

    I’m leaning towards gas or just a bad string of luck. Out of the 6-8 motors in a row that this happened, most of them were not run dry. They just failed after being used for about 5 mins during an initial run. I do put sea foam into all my gas in a diluted mixture (not the dose prescribed on the can) as a preventative in general; perhaps if these pumps are really old that’s all it takes is a bit of solvent to do them in.

    I’ll report back if I find more or figure out a pattern.

    Avatar
    melugin
    US Member - 1 Year
    Replies: 250
    Topics: 20
    #14191

    I wonder how long the fuel pump will last, whether used or not. The diaphragms were made of flexible material 50-60 years ago, in some cases. This material was not alcohol resistant. I reseal my floats, and use alcohol resistant gaskets, factory/or my own production.
    Like the check valve below the carb on pressure systems, the diaphragm material may be the next to go. My most common trouble is on the check valves next to the primer bulb. The new parts seem to give the most trouble.

    Avatar
    billw
    US Member - 1 Year
    Replies: 1019
    Topics: 33
    #14205

    Okay, I have to ask one stupid question….You’re barrel running all of these motors….on the same fuel tank and fuel line? Could you actually have a small air leak in one of the fuel connection o-rings? This problem is only made worse by having the tank sitting much lower than the pump….Fuel doesn’t have to leak out of these fitting for air to leak in.

    If this turns out to be the actual problem, don’t worry. You wouldn’t be the first to be fooled and you won’t be the last, either.

    Avatar
    rmhxruss

    Replies: 99
    Topics: 4
    #14224

    In our shop we have not had a problem with fuel pumps in the test tank. The top of the mounting board or transom is three feet above the floor on this tank. With 45 years of use works fine on all brands of motors. The tank bottom is two feet below floor level.

    Avatar
    johnyrude200
    US Member - 2 Years
    Replies: 751
    Topics: 172
    #14277

    Hi Bill,

    The tank appears to be OK; I have run a number of motors in the past week without fuel pump issues. The o-ring air leaks are pretty easy to detect on the 2-line systems; the newer single line ones seem to have more leeway in my experience.

    I’ll report back if I figure out anything new.

    Thanks,

    J

    Avatar
    mr-asa
    US Member - 2 Years
    Replies: 754
    Topics: 128
    #14280

    Silly question, but I didn’t see it addressed. Ethanol free gas?

    Avatar
    johnyrude200
    US Member - 2 Years
    Replies: 751
    Topics: 172
    #14288

    haven’t used ethanol free gas – not sure even where to find it locally. From a work standpoint, it would be impractical for me to work on the motors with fuels different than what the customers would be using anyway. If there is a better alternative I’m all ears though.

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