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95 f350 7.3 fuel filter light comes on when rpms goes up to 2000. changed filter housing, lift pump. still comes on. put gauge on at 2000rpms goes from 60 to 0 back and forth. what else should i look for.
 

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Do you have a electric fuel pump or are you still running the stock mechanical?

Either way I would say that it sound like a bad pump.

Some info about your truck would really help.
 

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Do you have a electric fuel pump or are you still running the stock mechanical?

Either way I would say that it sound like a bad pump.

Some info about your truck would really help.
i replaced the mechanical pump. still doing the same thing. needle on gauge was flickering 0 to 60. engine will runs good but don't push it to long once the light goes on. back off throttle light goes out. starts up good. don't know what to try next. thanks
 

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The thing I would look for is a restriction in the fuel system from the new fuel pump back to the tanks.

That fuel filter light sensor monitors the amount of vacuum, or suction, being created by the fuel pump while it is sucking fuel from the tank. I've been told that the light activates when the vacuum rises to 7 inches of mercury. This high vacuum level can be caused by such things as the fuel pickup in the tank being clogged, an obstruction in the tank selection valve, or an obstruction anywhere in the fuel line.
 

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thanks for your reply, took tank off this week end found screen and foot valve in pieces. installed new one no light runs great. t-rex auto part was perfect and good price.
 

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Good for you finding that problem.

A lot of folks recommend to just replace the fuel filter sensor with a plug if it starts leaking at the fuel bowl.

However, Navistar knew what it was doing when it included that alarm light as part of its trouble warning system. It could save a lot of time and money knowing that when the light activates, there is excessive vacuum in the tank-to-engine fuel system that should be addressed before it leaves you on the road in the middle of nowhere.
 

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It would be great if they would have found a sensor that can take the punishment from the fuel pump! The sensor failed on both of my trucks (and they both got plugs). Nothing like having the 5th wheel hooked up, truck loaded, family ready to go and seeing fuel dripping from the truck while idling in the driveway. Luckily it was the 97, where the sensor is easily accessible (15 minutes later and we are off on our trip). If it was the 94, I would have been pulling the fuel bowl. Cheers!
 

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I change my fuel filters every oil change or other depending on what it did and where it went in the past 5k. I have a plug in the 300k truck (was leaking and thought it was my problem, it was and still is the pump). I do agree with MP that it is a good warning alarm trouble shooting time saver of why the truck could just die. If the replacements are good I may pull the plug (keep it in the glove box) and go back to stock. At this age and mileage (not to mention junk fuel) tank foot failure is probably coming.
Also THANK YOU Divefree for getting back and following up on your find. As our trucks get older and more miles are put on them it is good to keep the information coming in on what might happen.
 

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Patrick, I'm not sure that fuel light sensor is subject to the fuel pressure pulsations from the mechanical pump. The fuel light sensor reads the level of vacuum on the tank-to-pump side vs the pressure that is on the pump-to-fuel-bowl side. No?
 

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You could be right, but if it was under vacuum I would think it would suck air on failure rather than leak fuel. At any rate, they are a common failure point. Cheers!
 

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Patrick, that's a good point that I've wondered about for years.

I'm not sure exactly how the vacuum switch is plumbed into the fuel bowl. It apparently is located in a place that has fuel being drawn into the fuel bowl by the diaphragm "suction" side of the fuel pump, which obviously is under vacuum conditions. So, if the fuel filter becomes clogged with debris which disallows fuel from freely passing through it on the "pressure" side, and/or if the fuel line supply system from the tank has an obstruction which disallows fuel from freely flowing to the fuel bowl, this can cause the fuel filter light to alarm because of an abnormally high vacuum condition created by the fuel pump diaphragm diligently trying to draw fuel from the tank.

Now, if the vacuum sensor's vacuum-sensing plunger or piston gets worn to the point that its internal seal no longer holds, this likely will allow fuel from that portion of the fuel bowl to seep out of the worn seal. By the same token, that same worn and leaking spot logically would also allow a little air to be sucked into the fuel system under the vacuum condition that the sensor is reading.

If all this is correct, it shows how well the original 7.3 Navistar design can handle air in the fuel system compared to some other designs that would likely shut the engine down. This is evidenced by the fact that although a worn fuel filter sensor might leak a little fuel and allow in a little air, it doesn't harm proper operation of the engine. Otherwise, we'd see complaints about a faulty fuel filter sensor causing engine operation problems rather than a simple leak.

Please take all this for what it is, speculation on my part, because I have long wondered about but never have read exactly how the suction side of the OBS fuel system works, at least in such detail.

In any case, I am glad to say that, to date, my '96 still has its original 24-year-old fuel filter sensor in place without leaking. Obviously, not everybody is so lucky. But if mine were to begin leaking today, I still think a $50 replacement is worth the cost versus being stranded on the road some day from an unforeseen fuel line obstruction that can do me in.

As in so many things in life, you pays your money and takes your choice or, in this case, chance :=)
 

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Here is a diagram of the fuel flow on our OBS trucks. I believe that the fuel filter restriction light is on the FPR which is on the high pressure side.
 

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Thanks Bugman.

How would it detect vacuum if it is on the high-pressure side with what, 50 psi or so of pressure?

I can't tell where that sensor is on the diagram.
 

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I don't think that it would be on the fuel line side from the tank area, I know that the one time that I ran my rear tank dry there were no lights as I was coasting down the freeway waiting for fuel to get to the filter from the front tank.

It's been a while since I have looked at my fuel filter since I changed it last. But the location of the restrictor sensor/vacuum switch near the FPR would make me think that it may be on the high pressure side. Since even on the high pressure side if there is no fuel or a blockage it would trigger a vacuum switch.

But I am just guessing.
 

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I don't see how the vacuum switch could read any vacuum from running out of fuel, because without fuel, there would be no vacuum created in the system.

IOW, the diaphragm part of the fuel pump can only pull a vacuum if there is liquid fuel for it to "grab" with which to create a vacuum, or negative pressure. This fuel pump could not create vacuum without fuel.

Looking at it another way, when the tank runs empty and there is no fuel in the lines, the pressure side of the fuel pump has no fuel with which to create pressure, so the fuel pressure falls to zero.

The vacuum side should work the same way.
 

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But is it on when there is a vacuum or is it off when there is a vacuum?

If it is on when the pump is pulling from the tank then it would be on when there is a restriction.

If it is off when the pump is pullin from the tank and only turns on when there is a restriction somewhere along the line then I don't believe that it could be on the feed side of the filter

Sent from my SM-J737V using Tapatalk
 

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This is the way I envision the OBS fuel system flow works:

A vacuum is created in the fuel lines that connect the fuel tanks to the engine by the diaphragm side of 7.3L engine's mechanical fuel pump . This vacuum sucks fuel from the fuel tanks all the way to the fuel pump's diaphragm, which then pressurizes the fuel in the fuel bowl to a nominal 5 psi. This low-pressure fuel is sent through the fuel filter, and then the filtered fuel flows to the high-pressure side of the fuel pump. This high-pressure side of the fuel pump has a piston to pressurize the fuel to a nominal 50 psi, at which point the fuel flows to the fuel rails and then to the fuel injectors. Any unused fuel is sent back to the fuel tanks. A fuel pressure regulator keeps the fuel at a nominal 50psi.

In this scheme, there is only vacuum on the feed side of the system; all the other parts are pressurized. So, it has to be on the feed side that the fuel light sensor is located since it is, by definition, a vacuum switch that turns on the fuel filter light to inform the driver when the vacuum is too high.
 
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