Oil & Fuel MX5100 only runs with air vent open

mbsledge · Sep 8, 2013

My MX 5100 (about 200 hours on it) quit after about 30 minutes of bush hogging. The only other time it quit, I cleaned the fuel filter element, bled the fuel system (as described in the Operator's Manual), and it ran fine. Followed the same procedure yesterday, it ran O.K. with the air vent cock open, but ran rough & stalled with the cock closed. I then replaced the fuel filter with a new one, filled the fuel tank (it was down to about one gallon), and ran the engine with air vent cock open for 30 seconds, cut the engine off and closed the cock. Tried to start the engine, but it sputtered, then quit. Repeated the cocke open/cock closed cycle 5 times, but engine only runs with the air vent cock open.
Questions: Has air gotten into the injector lines, preventing normal operation? How could air enter, with fuel in the tank? Should I try loosening the injector lines at the head to force air out? Why is it bad to operate the engine with the air vent cock open?

atgreene · Sep 8, 2013

You still have air in the system from changing the filter. Does it have two filters? Did you change both? Did you fill the fuel filter (preferably with power service or some other additive) before spinning it on?

If you put a dry fuel filter on you'll be a while getting the air out. If your other filter is plugged, you need to clean that as well. Did the filter look bad, slimy of brown?

mbsledge · Sep 9, 2013

The problem started before I replaced the filter - engine sputtered and quit while bushhogging; there was at least a gallon of fuel in the tank. I then opened the air vent cock & it ran fine, closed it, cranked the engine - it ran for a few seconds and quit. I then replaced the fuel filter (it didn't look too bad, but had 200 hours and had been cleaned once) with a new one (I prefilled the bowl with fuel) and continued to have the same problem - runs with cock open, stalls with cock closed.
-MX5100 has only one fuel filter.

Tx Jim · Sep 9, 2013

Have you checked that fuel tank vent is open? On a M4900 that I previously purchased used I found a tiny white screen in the water separater/filter that was plugged that would make it stop running.

mbsledge · Sep 10, 2013

Thanks for the suggestion. Where are the fuel tank vent and water separater/filter located?

Tx Jim · Sep 10, 2013

Fuel tank vent on my M4900 that I did own was on the top of the fuel tank. You can check if vent is the problem by loosening or removing fuel cap. Here's a parts photo of a water separator similar to what Kubota uses.

mbsledge · Jun 13, 2014

Problem solved by removing gas tank (not a simple task - get lhelp from a mechanically-inclined friend or neighbor), then rinsing it thoroughly with fuel several times. Fom then on, take extreme care to prevent trash from getting in the tank. I always filter the fuel running into the tank with a fine screen.