Truck running weird

[Meads98]

I have a 1991 f350 with a 7.3 idi that I'm currently road tripping. Truck had been ran intermittently for several years.

About 4 hours into my drive the fuel filter light started coming in and out. Usually comes in at higher RPMs and drops out when I go below 2k. I might just be paranoid but it also feels like it's losing power for a second then comes back for a second then drops again. The power drop isn't consistent but frequent enough to be worrying. I've switched tanks multiple times and it doesn't go away so it make me want to rule out a bad pickup unit.
My main concern is that it's my lift pump going out. Any thoughts?

Edits as the come up: it has a brand new fuel filter but the diesel in the tanks has been sitting for a while.

 

[Jesus Freak]

It does sound like "mild fuel starvation"..... either the lift pump is slightly compromised or the fuel filter needs replacing.......im betting on the fuel filter......

 

[Meads98]

It does sound like "mild fuel starvation"..... either the lift pump is slightly compromised or the fuel filter needs replacing.......im betting on the fuel filter......

Ill add this to my original post but it does have a brand new fuel filter. The diesel in it is years old so maybe it's a little gunked up already?

 

[Nero]

Ill add this to my original post but it does have a brand new fuel filter. The diesel in it is years old so maybe it's a little gunked up already?

Entirely possible. Fill er up with fresh diesel and some sort of biocide, replace the filter again and go drive er some more.
Fuel filter light means theres a vacuum detected at the filter, which means its getting clogged up.

 

[Meads98]

Entirely possible. Fill er up with fresh diesel and some sort of biocide, replace the filter again and go drive er some more.
Fuel filter light means theres a vacuum detected at the filter, which means its getting clogged up.

Okay I'll give that a go. Anyways to rule out the lift pump before I get back on the road? My biggest fear is that the lift pump completely goes out and then I can't get it up on a trailer easily and I've still got a long way to get home

 

[Jesus Freak]

Okay I'll give that a go. Anyways to rule out the lift pump before I get back on the road? My biggest fear is that the lift pump completely goes out and then I can't get it up on a trailer easily and I've still got a long way to get home

Not really, if its pumping.... its pumping.... if you had a fuel pressure gauge, you might could tell something...... but ultimately it sounds like your "out running your fuel", so do what Nero said with some biocide and just slow down a bit..... put on some tunes(or preaching if your like me) and just cruise..... take it easy, dont let the sound of your own wheels drive you crazy......lighten up while you still can......lol...

 

[Meads98]

Not really, if its pumping.... its pumping.... if you had a fuel pressure gauge, you might could tell something...... but ultimately it sounds like your "out running your fuel", so do what Nero said with some biocide and just slow down a bit..... put on some tunes(or preaching if your like me) and just cruise..... take it easy, dont let the sound of your own wheels drive you crazy......lighten up while you still can......lol.

 

[Meads98]

Yeah I'll give that a go I guess. The drive to AutoZone was fine. No light or fuel cutting out but I also didn't really get above 2k. My only fear is this is the last big town I can get a U-Haul for the next couple hours and I still have 2,000 miles to drive

 

[IDIBRONCO]

Just in case, it might not hurt to pick up an extra lift pump. Most of them don't seem to last very long these days, but it might come in handy. So would an extra fuel filter.
Some questions to get more info. Did you just buy this truck? Did you fly/get a ride to buy it? How much fuel was in it when you left? In four hours, you should have used up most if not all of a stock tank of fuel. It wouldn't hurt to top off the emptier tank like Nero said and then run on it for a while.
Also, you need to be aware that if the lift pump is going bad, it can and probably will take out the IP if you keep driving the truck.

 

[Meads98]

Just in case, it might not hurt to pick up an extra lift pump. Most of them don't seem to last very long these days, but it might come in handy. So would an extra fuel filter.
Some questions to get more info. Did you just buy this truck? Did you fly/get a ride to buy it? How much fuel was in it when you left? In four hours, you should have used up most if not all of a stock tank of fuel. It wouldn't hurt to top off the emptier tank like Nero said and then run on it for a while.
Also, you need to be aware that if the lift pump is going bad, it can and probably will take out the IP if you keep driving the truck.

Yes I flew out to buy it. I both tanks were about 3/4 full and have been topped off since. I have an electric lift pump at the house I was going to put on as soon as I get home. Replaced the fuel filter and now I can not get the lines to bleed.

 

[Nero]

Okay I'll give that a go. Anyways to rule out the lift pump before I get back on the road? My biggest fear is that the lift pump completely goes out and then I can't get it up on a trailer easily and I've still got a long way to get home

If you get a psi gauge that you can clip onto the Schrader you can measure lift pump pressure. At idle you want about 4psi, and about 5-6 at wide open throttle.

This is what I use for diagnosing. The Schrader measures post filter, if I remember right.

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[Jesus Freak]

Yes I flew out to buy it. I both tanks were about 3/4 full and have been topped off since. I have an electric lift pump at the house I was going to put on as soon as I get home. Replaced the fuel filter and now I can not get the lines to bleed.

If its a manuel transmission, pull starting is easiest.......but you might not have a helper in your situation......

 

[Olds64]

You can also get a manual push button for your starter from the auto parts store. Hook it up to the starter relay on your passenger's side fender and you can manually crank the engine with the push button with the key in the RUN position.

If the electric fuel pump isn't installed yet you want to:

1) Install the manual push button across your starter relay.
2) Bleed the fuel filter by depressing the Schraeder valve and cranking the engine until a solid stream of fuel comes out.
3) Bleed each of the cylinders individually by cracking the injectors loose and cranking the engine. I like to go by the firing order 1, 2, 7, 3, 4, 5, 6, 8. My truck will usually cough by 7 and start on 3.

When bleeding the engine make sure you let the starter rest. I'd crank no more than 30 seconds and then give it 2 minutes to rest. That might be overly conservative but cranking against 21.5:1 compression will heat up that starter fast. Also, make sure you wear safety glasses. The injectors will spit fuel when bleeding them.

Once you get her home it'd probably be a good idea to do the ol' ATF trick.