Hi all,
I bought a 91 F-250 a few weeks ago and have been having fun with the learning curve as it is my first diesel.

Yesterday, I noticed that the fuel filter light came on under high load and RPM. Today, I was about 10 minutes into my drive to work when the fuel filter light came on again, but this time, I was traveling at a constant speed of 60 mph down the highway at a slight decline. It stayed on the whole way back home.

Any help in diagnosing the problem would be great. I'd be happy to provide any more information I can to help. Thank you in advance!

 

welcome to FTE.
first thing to do is change the fuel filter. if the light still comes on after changing the filter, i would check fuel pressure as the lift pump may be going bad.

 

Thanks for the reply. I'm replacing the filter later today and will then take it for a test drive. If the light is still on, then I guess the next step is to check fuel pressure.

I read that there may also be a blockage of some sort in the fuel system somewhere that could also be causing the light to come on. I hope it doesn't come down to replacing the entire system.

 

usually if there is a blockage, the engine will starve to the point it will not run.
i change fuel filter every other oil change.

 

And be sure to fill the filter with CLEAN diesel before installing. When in doubt, pour the fuel in the outer ring of holes in the filter.

 

What Tom said. The fuel filter light comes on when there is a vacuum pulled on the filter housing. If the filter plugs, the injection pump starves for fuel pulling a vacuum on the filter and causing the light to turn on. Note the water in fuel light is a different condition caused by water shorting out the contacts of the water in fuel switch on the bottom of the factory fuel filter bottom housing, or switch in some one piece style filters.

I had this problem with my truck a few years ago. Same symptoms, no light when going slow, but when towing, or on highway the light came on. It was a real subtle thing, light watching a analog gauge I could watch the light fade on and off as the load changed when accelerating versus coasting. I too thought it was a fuel filter and I believe had ordered some, but not yet had a chance to replace it.
Then one morning I was out to breakfast with friends and on the way home stopped at a red light, started up again and made it 100 ft past the intersection before the truck stalled and would not restart. After getting towed out of the roadway by a nice passerby I did some troubleshooting and found out the lift pump crapped out. Note that in this case, a weak lift pump doesn't put out enough fuel to keep up with the injection pump, and that causes the vacuum and the light comes on.

Also note, if you are in a cold climate, the fuel filter light can act the same way when your fuel is gelling up on you! I found that out a few winters back during a particular cold spell (and now treat my fuel!) Fun fact on that one, I happened to pull into a gas station to fuel up the diesel work truck and had to wait for a service guy to swap out the fuel filter on the pump. He said the weather so cold and windy that even though the fuel is treated, it was still gelling in the filters and stopping the pumps. All he had been doing the past few days was running around swapping filters on pumps. Cold fuel gelling up clogs filters much like crud does and causes the light to turn on.

Fuel filter is quicker and easier to swap, and regular maintenance if you don't know the history of the vehicle. Lift pump isn't too bad to change, but a bit cramped to get at.

 

Thanks for your response. I never got a notification for it like everyone else's, so I'm lucky that I saw it at all.
As I'm new to these, I don't quite have to terminology down. Is the lift pump the same thing as the mechanical fuel pump or is it a different piece?

 

yes same thing. gas is called a fuel pump. diesel is called a lift pump.