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Many of you run across this, so I am pointing out the issue for mainly visitors and the novice DIY types out there.

If the engine crankcase is flooded full of fuel, it seldom means the carburetor is the problem. Most modern small engines especially those used on rider mowers have a pulse operated fuel pump. The diaphragms fail in these pumps and the fuel is leaked into the crankcase through the pulse line that often connects to a valve cover or the crankcase itself.

For models with a mechanical fuel pump on the side of the engine it can also happen to them but not as frequent. Drain the oil/fuel mix out and then the pump can be loosened up, fill the fuel tank and watch for fuel weeping out of the pump itself where it bolts to the engine.

Of course, engine with nothing but gravity feed fuel to the carburetor, the float and needle are the likely suspect for this problem

How many pulse pump leaking into the crankcase have you seen. Since most pulse pumps are above the fuel level in the tank the only time they can weep into the crankcase is when the engine is running. How many pulse pumps have you seen actually fail in any way.

How many small engines still use a mechanical pump. Most people on this forum outside of the older techs have probably have never seen seen a mechanical pump on a small engine.

Did any manufacture use a mechanical pump but Kohler? Have not seen many pulse pumps fail. We sell a lot of them to novices that don't know how to diagnose a problem.

ILENGINE said:

How many pulse pump leaking into the crankcase have you seen. Since most pulse pumps are above the fuel level in the tank the only time they can weep into the crankcase is when the engine is running. How many pulse pumps have you seen actually fail in any way.

How many small engines still use a mechanical pump. Most people on this forum outside of the older techs have probably have never seen seen a mechanical pump on a small engine.

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I usually see 3-4 every mowing season. I think it has a lot to do with the Texas heat where a I'm at, combined with modern Ethanol fuel.

Regardless of the tank height, a leaking pulse pump will create a siphon affect. It's funny to find a tank practically empty, but the crankcase is full of fuel.

hlw49 said:

Did any manufacture use a mechanical pump but Kohler? Have not seen many pulse pumps fail. We sell a lot of them to novices that don't know how to diagnose a problem.

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Some of the Kawasaki engines use mechanical. Located below and to the right of the carb. I see them on the older 345 JD mowers but the more reason engines use the same pulse style pump as everybody else.. And most fuel pumps sold are due to people not knowing how to test or assume it is the fuel pump when the mower doesn't start when in reality it was the fuel anti afterfire solenoid not working.

Auto Doc's said:

I usually see 3-4 every mowing season. I think it has a lot to do with the Texas heat where a I'm at, combined with modern Ethanol fuel.

Regardless of the tank height, a leaking pulse pump will create a siphon affect. It's funny to find a tank practically empty, but the crankcase is full of fuel.

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Are those pumps actually testing bad, or assuming due to other factors. And regardless of pump leaking or not, liquid won't siphon if the outlet is higher than the liquid being siphoned. If gas siphoned up hill then you wouldn't need the fuel pump to start with. So if the tank is lower than the pump and the pump is leaking in the crankcase then you have two problems. The other problem is the tank isn't venting causing it to pressurize.

Are you also factoring that an average gallon of gas weighs 6 pounds and that weight (per gallon) is applying pressure to the exit nipple to get out of the tank if it is a bottom feed style? If it is a siphon feed style like many zero turns are, the fuel is siphoned/ sucked "uphill" by the pulse pump. The pulse pump diaphragms do tend to fail in my area of the country mainly from high heat and direct sunlight.

Of course, the fuel cap vents have been known to clog up with dirt/dust, but that often creates a fuel starvation issue due to the tank suction that slowly develops with the engine running. When left sitting and the fuel begins to build vapor pressure and that will force fuel out of the tank and into the pulse pump.

95% of the time I see fuel leaking into the crankcase, it will be on a fuel system where the fuel tank is located above the carb, with NO fuel pump. In this case the most common cause is the float needle is not sealing the carb bowl. Most common remedy is to replace the float seat, and/or float needle.

Auto Doc's said:

Are you also factoring that an average gallon of gas weighs 6 pounds and that weight (per gallon) is applying pressure to the exit nipple to get out of the tank if it is a bottom feed style? If it is a siphon feed style like many zero turns are, the fuel is siphoned/ sucked "uphill" by the pulse pump. The pulse pump diaphragms do tend to fail in my area of the country mainly from high heat and direct sunlight.

Of course, the fuel cap vents have been known to clog up with dirt/dust, but that often creates a fuel starvation issue due to the tank suction that slowly develops with the engine running. When left sitting and the fuel begins to build vapor pressure and that will force fuel out of the tank and into the pulse pump.

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You still don't get it Gas or any liquid will not run flow uphill unless an outside force is acting on it and will not siphon if the exit is higher than the fuel level in the tank. Try it yourself. For a siphon to work the discharge has to be lower than the level of liquid that you are trying to siphon. What you are saying defies the law of fluid dynamics. And whatever gas is in the tank no matter what the weight will not push fuel uphill. Pulse pumps use vacuum to create negative pressure therefore atmospheric pressure acting on the fuel in the tank tried to fill the vacuum void. No vacuum no uphill fuel flow. And you shouldn't be pressure in the tank since the tank is vented to either the engine intake, or in the case of pre-2012 to the outside air. If the tank is building pressure than you still have a problem.