99 3.0 gets poor mileage

Hi, I bought the truck last spring cheap, but totaled. It was running good, so I didn't look it over as good as I should. When I got it home, I checked the codes and noticed an EGR related code, so I cleared them and noticed that the O2 system was immediately "OK", not "INC" like usual. I then looked under the vehicle and found that the cat was missing and there were no O2s present. Obviously someone tweaked the PCM to ignore the O2s. Mileage on the truck is usually around 15MPG. I've had a couple of these 3.0L Rangers before and they were in the 21MPG range (almost all freeway miles).

This truck also was converted to the later model plastic plenum and larger throttle body, hence no EGR. I've found a way to get around that for emission testing, but I just want to improve the mileage. Is it likely that a cat, O2s, and an unmodified PCM will resolve the issue?

Thanks in advance for any help you guys can give me.

Byron

The O2s are essential feedback to the PCM so that fuel trims around the 14.7 ratio is maintained. As well, the PCM has no way to tell if you're running rich or lean. If you wanted to diagnose poor running with scanner you'd have no way to do this.

Not sure why the PO removed all that - maybe just ignorant about emissions sensors and their value.

Best to go back to stock, including CAT - I think.

I agree that having no O2s can't be helping your situation, but 15 is probably the low end of what is common for 3.0 fuel mileage. 17-19mpg is probably most common. 21 would be the high end.

Any details on the truck? It probably goes without saying, but truck configuration can play a role here too. A 4wd, auto trans, extended cab truck is going to be much heavier and struggle more with fuel economy than a 2wd, manual, regular cab, etc.

Cat doesn't matter for MPG one way or the other, but I and my kids and grandkids will thank you if you put it back on

Yes, O2 sensor feedback is needed or computer will run engine rich as a safety precaution.
Lean mix WILL melt valves and pistons, rich mix just costs driver a bit more in fuel

EGR system allows for a more aggressive spark timing without pinging/knocking, so a benefit to performance.

I am averaging 21.13 mpg with a average distance of 264 miles. My data is secude a little due to tracking a partial fill up. Realistically is more around 22 mpg. I am 100% stock in a 99 3.slow ext cab auto 2wd.

hoosier1104 said:

I am averaging 21.13 mpg with a average distance of 264 miles. My data is secude a little due to tracking a partial fill up. Realistically is more around 22 mpg. I am 100% stock in a 99 3.slow ext cab auto 2wd.

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That's in a Ranger??
A FORD Ranger not a Polaris Ranger, lol, or a Fiat 500 engine swap

Thats is pretty good

It only has 58k miles on it as well. About 1100 of that is me.



Sent from the garage of the never ending projects.

stmitch said:

I agree that having no O2s can't be helping your situation, but 15 is probably the low end of what is common for 3.0 fuel mileage. 17-19mpg is probably most common. 21 would be the high end.

Any details on the truck? It probably goes without saying, but truck configuration can play a role here too. A 4wd, auto trans, extended cab truck is going to be much heavier and struggle more with fuel economy than a 2wd, manual, regular cab, etc.

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It's a 2WD, standard cab, 5 spd. I've always gotten around 20-21 out of the 3.0L Rangers. I've had 4 of them over the last 15 years. Like I said, 90% of my miles are freeway around 70MPH, so right in the max range for economy. It is lifted and has larger wheels and tires, but my old 2000 Trailhead truck had almost the same size tires and it still got 21 as long as I wasn't hauling anything. Right around 2400 RPMs on the freeway.

hoosier1104, you may want to check those Fuel Buddy figures with actual fill-up/mileage math. My Scan Gauge gives me optimistic MPGs which don't bear out with math method.

I only have about 83k, but at 58k did get better mileage.

Rangstang said:

It's a 2WD, standard cab, 5 spd. I've always gotten around 20-21 out of the 3.0L Rangers. I've had 4 of them over the last 15 years. Like I said, 90% of my miles are freeway around 70MPH, so right in the max range for economy. It is lifted and has larger wheels and tires, but my old 2000 Trailhead truck had almost the same size tires and it still got 21 as long as I wasn't hauling anything. Right around 2400 RPMs on the freeway.

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Was the speedo/odometer corrected for the larger tires? If not, you may be traveling farther than what you're recording, which would obviously impact your mpg calcs.

For a mostly uninterrupted, high-speed drive cycle, aerodynamics will really affect fuel economy. The lift and larger tires aren't helping you. Some subtle aero mods like a partial grille block or smooth bellypan would probably show decent improvement in mpgs.

stmitch said:

Was the speedo/odometer corrected for the larger tires? If not, you may be traveling farther than what you're recording, which would obviously impact your mpg calcs.

For a mostly uninterrupted, high-speed drive cycle, aerodynamics will really affect fuel economy. The lift and larger tires aren't helping you. Some subtle aero mods like a partial grille block or smooth bellypan would probably show decent improvement in mpgs.

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Actually, the speedo cal is off as it is 15% less than actual, but I take that into account when I calculate my mileage. I've used a speedo app on my phone to verify the difference and it's right at 15%. Besides, I use the truck for the same trip day after day to/from work so I know the miles.

As for the height of the truck, it's only maybe an inch or so taller than my last one (2000 Ranger Trailhead) that got 20-21MPG.

I'd say the best place to start is what I figured with the O2s and PCM and go from there. Thanks!

Rearanger said:

hoosier1104, you may want to check those Fuel Buddy figures with actual fill-up/mileage math. My Scan Gauge gives me optimistic MPGs which don't bear out with math method.

I only have about 83k, but at 58k did get better mileage.

Click to expand...

I do the math every time I fill up and the app is dead on to what I get.