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Discussion Starter · #1 ·
1991 Trooper 2.6 / 5spd 155k mi

So Ive had this truck for about 40k miles. Never once a problem

ran out of gas this morning. Not completely though, it would stumble on full throttle. I made it to the gas station and threw some gas in there and it wouldnt start.

I messed with it for a while and used some starting fluid, etc. I can get it to fire and if I pump it... it will stay running. I cannot hold it at any RPM as it will steadily slow down and die. But if I pump that effer, it will stay going.

I replaced the fuel filter just now assuming that I had sucked some crap out of the tank and clogged it. Nothing. Still runs about the same. I cant drive it.

I dont think it could be injectors because it will run and it sounds good when I rev it... but when the revving stops, so does the engine.
w.t.f.??????

any ideas?
 

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My first thought would have been the alternator.

But if it keeps running, if you 'pump' it, that would rule the alternator circuit out.

Ok, just think of the system, then of it's individual components.

What exactly, is happening when you 'pump' it ?

ie, the throttle body gets opened. Fuel is released by the computer. I have no idea how much of that is affected by the movement of the throttle. Etc. Just keep disecting what happens, into smaller and smaller pieces. Then determine how each of those pieces affect the whole.

Thinking in that way, I'd start with air. Open a vacuum port or two, and see if it keeps running.
 

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The pump might still function though at a diminished capacity. The gas cools the pump and if the gas is low enough, the pump can either quickly die or go on life support. I'd do a pressure check.
 

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Discussion Starter · #4 ·
Where do I test pressure at?

Im pretty sure its the pump. I cant imagine any other system affected by running out of fuel. As mentioned, it had no trouble running until after I shut it off at the gas station.
 

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You most likely fried your pump(if thats what the early troopers have). Take the fuel line off right before it goes into the fuel filter, put the ignition in the on position and see if any gas comes out.
 

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jwood1016 said:
You most likely fried your pump(if thats what the early troopers have). Take the fuel line off right before it goes into the fuel filter, put the ignition in the on position and see if any gas comes out.
It's a 91. He's going to have to jumper the fuel pump relay, to make that test.

I'm with shooter, the fiorst thing to do is to check the fuel pressure. I would also open a vacuum port or two, to see if it keeps running.
 

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Discussion Starter · #7 ·
Okay, so if I pull a vac line off (@ manifold?) and that keeps the truck running, what does that indicate?

Also, the pump is pumping the fuel because it runs when I rev it.

Is it all or nothing with these pumps?
 

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shibbershabber said:
Okay, so if I pull a vac line off (@ manifold?) and that keeps the truck running, what does that indicate?

Also, the pump is pumping the fuel because it runs when I rev it.

Is it all or nothing with these pumps?
I don't think it's all or nothing with the pumps.

If you pull a vac line or two, and it runs, that would mean it is not getting enough air. Or is getting too much gas.
 

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When you run an EFI engine out of fuel, you will cavitate the pump, when that happens the heat causes the rotor and gear to expand. When you're dealing with tight tolerances any thermal expansion within the pump assy. will take up the already tight operating clearances, from there it's all downhill as the gear rotor assy. eats itself up. It will still pump gas, but it can't move the volume or create the pressure needed to start the engine.
By pumping the throttle you are making the ECU increase the pulse width of the injectors, this is allowing just enough fuel delivery that the engine will start and somewhat run as long as you keep the ECM tricked into increasing the injector pulse width.
If you check fuel pressure, you need a minimum of 32psi.
 

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LeeH said:
When you run an EFI engine out of fuel, you will cavitate the pump, when that happens the heat causes the rotor and gear to expand. When you're dealing with tight tolerances any thermal expansion within the pump assy. will take up the already tight operating clearances, from there it's all downhill as the gear rotor assy. eats itself up. It will still pump gas, but it can't move the volume or create the pressure needed to start the engine.
By pumping the throttle you are making the ECU increase the pulse width of the injectors, this is allowing just enough fuel delivery that the engine will start and somewhat run as long as you keep the ECM tricked into increasing the injector pulse width.
If you check fuel pressure, you need a minimum of 32psi.
And there you go.

Exactly what I meant. What happens when you pump it, and why it keeps the engine running only while you are pumping.

Just wish I was that good at diagnostics !

I had a hunch that was where the answer would be, but you had the actual answer.
 

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Selador said:
And there you go.

Exactly what I meant. What happens when you pump it, and why it keeps the engine running only while you are pumping.

Just wish I was that good at diagnostics !

I had a hunch that was where the answer would be, but you had the actual answer.
Thanks, that's years of working at dealerships coupled with me currently working on an M.E. degree. :D
 

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LeeH said:
There should be a schrader valve on one of the fuel rails.
Doesn't have one. Need to use a pressure gauge on the fuel rail side of the regulator. If reading low, run voltage directly from battery to pump and read again. Direct voltage / correct pressure indicates the probability of a defective pump relay.
 
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