Why would new 3FE fuel pump improve gas milage (1 Viewer)

[paulzak]

Wondering if anyone knows why a new fuel pump might improve gas milage... by a lot?

My '92 FJ80 3FE has been getting 8 mpg for a long time. I replaced the aging fuel pump and sock and now I get 12mpg. I'm not complaining but I can't see how this could logically happen. The truck has 260k miles so it was time. The truck accelerates better now so either the sock or the pump was due. Pump and sock replacement is the only work done.

Anyone know why milage would improve? PFM?

 

[Irish Reiver]

I don't know the answer but i had the exact same thing happen to me about 10 years ago with an old Cherokee. Similar mileage to yours. Fuel pump gave out and when i fitted the replacement my MPG was considerably improved. I never understood why...

 

[leonard_nemoy]

Damn, I think it is time for a new fuel pump.

 

[RoaringFork]

The fuel injection mapping assumes fuel pressure is adequate. Based on air meter and o2 sensor inputs the ECM adjusts fuel injector “on time”(duty cycle). If the fuel pressure is weak, the air meter/o2 reports a lean condition and the on time is increased. The injectors are on longer, and the combustion is less efficient(think piston movement vs injection timing/length)... you could say the ECM was running you fuel injectors at the equivalent of wide open throttle while your foot was at half throttle. Hence the better economy and power...

 

[BILT4ME]

Think of it like taking a Viagra so you no longer pee on your shoes.

You need a thick full stream at the injectors to provide an adequate spray pattern.

More pressure equals more atomization and more thorough combustion. That equals more power and more efficiency.

 

[paulzak]

The fuel injection mapping assumes fuel pressure is adequate. Based on air meter and o2 sensor inputs the ECM adjusts fuel injector “on time”(duty cycle). If the fuel pressure is weak, the air meter/o2 reports a lean condition and the on time is increased. The injectors are on longer, and the combustion is less efficient(think piston movement vs injection timing/length)... you could say the ECM was running you fuel injectors at the equivalent of wide open throttle while your foot was at half throttle. Hence the better economy and power...

Thank you. I imagined that the system was reporting a lean condition but that didn't immediately equate to more fuel in my mind. But the goal of the system was to compensate and keep the engine running and that's exactly what it did, at a cost.

 

[paulzak]

Think of it like taking a Viagra so you no longer pee on your shoes.

You need a thick full stream at the injectors to provide an adequate spray pattern.

More pressure equals more atomization and more thorough combustion. That equals more power and more efficiency.

This means that the fuel injector cleaning I did when I rebuilt the head 30k ago now is only now benefiting me! If I'd know the impact of pressure on fuel efficiency, I'd have replaced the pump years ago. Thanks!

 

[paulzak]

Damn, I think it is time for a new fuel pump.

That's the lession I take. I will stay on top of fuel pressure from now on.

 

[ariff]

Interesting this is one thing I have not tried since my MPG has been declining over the years.. I have tried just about everything to get above the 9-10 mpg in the city. Is there a way to test the fuel pump to see if it is having this issue?

 

[jonheld]

Interesting this is one thing I have not tried since my MPG has been declining over the years.. I have tried just about everything to get above the 9-10 mpg in the city. Is there a way to test the fuel pump to see if it is having this issue?

There is a fuel pressure test in the FSM.

 

[ariff]

There is a fuel pressure test in the FSM.

And looks like you need a special tool (SST 09268-45012) to test fuel pressure. I am looking at the testers available for rental from the parts store and they don't appear to have any adapters to fit our fuel delivery pipe.. Anyone have a workaround to test fuel pressure?

Edit: Nevermind I think I just figured it out.. Seems like a good project now that the weather here is starting to warm up..