2008 DF 140 fuel issues

[was997sailor]

I have a 2008 DF 140 on a 20 SEAARK. Last spring I noticed a sputtering and loss of power while operating at mid throttle range. Idle would also sputter. I replaced the fuel/water separator filter twice throughout the summer. I replaced the low pressure filter this last weekend and still have same problem. Watching the motor run I noticed that the motor will idle fine then start sputtering. The fuel level in the low pressure filter will drop below both of the fuel hose connections. Could the low pressure fuel pump be bad. I have removed it and plugged the inlet with me finger and pushed the plunger. It very slowly returned. any help will be greatly appreciated.

[redlowrey]

The low pressure fuel pump has a diaphragm and two check valves inside. When the camshaft rotates the lobe on it will push the diaphragm up and in doing so will draw in fuel through the inlet check valve and close the outlet check valve, as the lobe rotates it then lets the diaphragm go the other way closing the inlet check valve and open the outlet check valve, with fuel going to the vst. The reason the lever would not come back is because you created a vacuum inside the pump, when you stuck your finger over it not letting the lnlet check vale to open. The simplest thing you can do get a one litre container and fill it with fuel and hook up a hose to the inlet side of the pump put it inside the bottle and work the leaver in and out you will soon see if the pump is crook. Watch where you squirt the fuel. How much water are you getting out of the water separator,

[was997sailor]

I am getting very little if any water out of the separator. I can't figure out why I can't get a full filter of fuel in the low pressure filter. I have checked hose fittings to make sure I am not pulling air in. The low pressure pump pulls fuel from tank and then pumps to VST I think. that is why I was thinking low pressure pump.

[Harper]

The low pressure pump pulls fuel from tank and then pumps to VST I think. that is why I was thinking low pressure pump.

What you say here is true, but the low pressure pump PULLS fuel through the low pressure filter (the filter is between the tank and the pump). So how do you get air into the filter? By sucking in air, I would think, which would seem to indicate that the pump is fine, and you have an issue somewhere in the fuel line between tank and low pressure filter.

[was997sailor]

So with fuel only in half of the low pressure filter when it starts sputtering, i should concentrate on the hoses and connections between the low pressure filter and the tank. Does the high pressure side pull fuel from the low side or is it pumped to the high side. would like to think high side is ok if I can't keep a full filter on low side.

The SEAARK I have has a belly tank under the floor and it is going to be a bugger to get to the tank.

[Harper]

The low pressure pump pumps fuel into what you could compare to the fuel bowl of a carburetor (I realize that anyone younger than 50 or so isn't going to be very familiar with carburetors). This is the "bowl" in the VST. The level in that "bowl" is controlled by a float valve. The low pressure pump always pumps fuel (or attempts to), but it only flows into the bowl of the VST when the pressure created by the low pressure pump is enough to overcome the pressure exerted on the needle seat by the float in the VST. When the level of fuel drops sufficiently to drop the float and lift the needle off the seat, the fuel can flow into the bowl of the VST. The float keeps the fuel level in the VST relatively constant, and the high pressure pump is immersed in fuel in this bowl of the VST and draws it's fuel from there. The low pressure pump has to pump at least the same volume of fuel that the high pressure pump is pumping and the motor itself is consuming, because it has to pump enough to keep that VST bowl at the proper level. If the low pressure pump is sucking air from somewhere between the tank and the low pressure pump, it cannot keep the bowl in the VST full, so the high pressure pump starts to also cavitate and draw air and inject it into the fuel rail and the injectors.

A little air in the low pressure filter should not be a problem, however, unless it indeed causes the low pressure pump to fail to keep up with fuel demand. I believe that the filter is designed so that the pump draws from the bottom of the filter housing, even though the filter outlet is the one on top. Are you sure you have your filter hooked up correctly?

[was997sailor]

I replaced the filter as it was on the motor. I have owned it since it was new. Has ran great until this fuel problem. We are having thunderstorms here for the next couple of days. I will try and take some pictures of what it is doing while it is running when the weather breaks.

how does air from the filter changes get out of the system. Does it bleed out of the VST?

[Solarman]

With the motor hood off, try pumping the fuel bulb up firmly and look for any small leaks from bulb to the vst, if you see any drips of fuel anywhere, fix that. If nothing is dripping, the start from the bulb back to the tank. If you have that much air in the filter bowl, you must have a leak. Yes, the vst will vent any air until the float shuts off fuel flow. Check around filter bowl, I've seen more than a few leak from cracks in housing, and from orings not sealing properly.

Good luck, let us know if you find anything.

[was997sailor]

Ok I bypassed the water separator and now have a firm bulb with no leaks. Not sure if I actually had a leak though because the problem is still happening. At an idle it will start sputtering and spitting. when it is doing this I have squeezed the bulb and it is still firm, not flat or sucked in. LP filter is staying full now. Can I connect a fuel pressure gauge on the high side rail and make sure I am not losing fuel pressure on the high side. I am going to put a fresh set of plugs in it this week to rule that out. That can't hurt.