Bad gas mileage
 

I have a 1988 Ranger 4x4 with a 2.9 and a 5 speed manual. I've had the truck for almost 20 years and normally I'd always average around 16-18 miles per gallon with it. About a month ago I noticed the check engine light was coming on every time I drove it. Around the same time I also noticed my gas mileage had dropped dramatically. I checked the codes and the only one showing was 41 which is lean exhaust. So, I replaced the oxygen sensor and wiped the keep alive memory. Now the check engine light stays out most of the time but still comes on maybe once or twice a week with a code 41. Before replacing the sensor it was coming on every time I drove it. No change in my mileage either. The last several tankfuls I've only been getting an average of 7 or 8 miles per gallon. The truck runs great otherwise. No black smoke, runs and idles smooth with no surging or hesitation.

Things I've done so far:

Complete tuneup with new cap and rotor, plugs and wires, new fuel filter.
New oxygen sensor.
New air filter.
I've swapped coolant and intake air temperature sensors from another parts truck with a 2.9. No change.
I've pulled the vacuum line off the fuel pressure regulator and it shows no signs of leakage.
I've sprayed carb cleaner all around the intake and throttle body with the engine running, searching for a vacuum leak but it never stumbled and I cannot hear any hissing like it has a leak.
Brakes are not sticking and there is no excessive weight or anything that should be causing the engine to work harder than normal.
The engine runs as smooth as ever, no lack of power.

I'm out of ideas. I can't afford to keep driving it like this. Right now I'm putting almost $50 worth of gas in it every week and my commute is less than 30 miles a day round trip.

Given the age of the vehicle I'd suspect the PCM itself is failing. It could have suffered water intrusion but another common problem is leaking from the electrolytic caps that causes corrosion of components which then leads to strange behavour.

make sure the ground is ok to that sensor,,,,a bad ground can cause your problem

and hot side is 0.10 volts i think,,,,but not 100 % sure on that voltage

Any idea where that sensor ground is located?

I do have two Bronco IIs with 2.9 engines that I'm parting out so I have a couple extra PCMs I could try as well, but I'd like to eliminate all other possibilities first.

on a 1989 it goes to pin 49 on PCM,,,,not sure for 1988

The sensor grounds through the PCM then?

I went through all of the vacuum lines and rechecked for vacuum leaks again this morning but nothing. So, I decided to try swapping the PCM. I pulled the kick panel off and found a large mouse nest between the kick panel and PCM. After cleaning it out and pulling the PCM out I found one wire had the insulation chewed off and was touching against the metal floor. Pin number 20, black with green wire. Looks like that is a ground wire anyway so it shouldn't have had any effect even if it was shorting to the floor. I swapped the PCM with one I already had from a Bronco II. Fingers crossed, we'll see what happens.

I drove it about 50 miles this afternoon, making several stops along the way. The engine runs as smooth as ever with no chugging, hesitation or lack of power but the check engine light came on during every single ignition cycle. With the original PCM it was coming on about once a week lately. I topped off the gas before returning home and checked my mileage again: 7.5 mpg. I checked the codes when I got home and its still throwing code 41.

I give up. I've been battling this problem since early December and nothing I've tried has made any difference. I've been wrenching for almost 23 years and I've never seen anything like it. I love the truck but I'm beyond frustrated and I cannot afford to keep driving it like this.

you should contact pawpaw on this forum,,,he is one of the smartest guys here,,,he has worked on these old rangers forever,,,,and knows way more than i ever will,,,,,just wondering did you replace sensor with a motorcraft one,,,,some after market parts are not that great and these old rangers are part picky sometimes,,,i always try and use motorcraft parts if available,,,,even relays,,,,,a member here used a new relay not motorcraft and it was worse than the faulty one,,,,just trying to tell you how picky these old rangers can be,,,,,they sometimes hate after market parts

I bought the sensor from Napa but it was a NGK brand (which is supposed to be the OEM manufacturer), so it wasn't some off brand I've never heard of. Also forgot to mention I swapped the MAP sensor today too but that didn't make any difference.

Tomorrow I'll pull the PCM again and start pinning out the wiring between it and the oxygen sensor. If that checks out then I have no choice but to park it.

Hold on, you've done some good trouble shooting but don't give up, lets think about this some more. Since this is a speed density controlled fuel trim V6, But only has One O2 sensor, there is no way for us to know if you have corrupt fuel trim in A cyl bank, as with only 1 O2 sensor, its reading is an average of Both cyl banks fuel trim.
SO, if you have a faulty/dirty/runny nose fuel injector, that's dumping fuel into a cylinder in A cyl bank, you don't have a O2 sensor for Each cyl bank to tattle on how fuel trim is doing in Each bank.

SO, maybe listen to each fuel injector with a stethoscope, to determine if one or two sound different than the others, then go to that cylinder & remove the spark plug for a deposit read on how that cyl fuel trim & combustion process is doing & let us know what you find.
Seeing as how your a pro wrench turner, if you want to get really crazy, pull all of the pugs for a read, to see how All cylinders are doing.

Or hook up a scan tool that can monitor fuel injector squirt time & see how it looks.

Also have a look at fuel pressure, to make sure it's ok,

Some more thoughts for consideration, let us know what you find.

Also, I show the code 41 to also indicate loss of O2 sensor switching, so since this is a new heated O2 sensor, how did the wiring harness wiring & electrical connector contacts look while you had it disconnected? Did you ever perform a continuity check on its ground lead?

If the computer senses/thinks the fuel trim is Lean, it'll Lengthen fuel injector squirt time to try & enrich the fuel trim, to correct for what it thinks is a lean mixture.
SO, does the exhaust smell, or look Rich?

I've been battling this thing all morning and I'm even more frustrated with it now.

I disconnected the PCM and tested between pin 49 and the oxygen sensor connector. It showed 15 ohms. Traced the orange wire from pin 49 to the back of the left cylinder head. I pulled the wire off and cleaned that connection, then tightened it down again and rechecked. Same reading. So, I tested between the pin and the engine block and it zeroed out. I also found pins 40 and 60 (which are both grounds) were reading around 25 ohms when I tested between the pin and the body. Reconnected my meter between 49 and the oxygen sensor connector, still got 15 ohms. I finally located a splice between the pigtail coming off the ground cable at the battery and the inner fender. I wiggled it slightly and the ohm reading dropped to 5. I cut the splice out and connected that wire directly to the battery cable bolt and all of my ohm readings zeroed out. I thought finally...I fixed the problem. Nope. Took it out for a short drive and within five minutes the check engine light came back on. I brought it back in and its STILL throwing code 41.

If one of the injectors was dumping excessive fuel, wouldn't it show code 42 for rich exhaust instead? The exhaust smells normal, no black smoke or rich smell to it.

Ok, good find on the splice anomaly.
If the computer thinks the air/fuel ratio is lean, it'll increase the fuel injector squirt time to enrich the air/fuel ratio, to try & get it back into balance, so what does your scan tool show the injector squirt time to be?
What does your fuel pressure gauge show the KOEO & KOER fuel pressure numbers to be?