ODB-1 code p0137 o2 sensor solved with fuel pump, but why?

[hokies_86]

1993 Ranger 4.0 with 170k or 270k miles. ODB1, so not much diagnostics. P0137, Oxygen sensor not switching/system rich Left or Front HO2S code throws everyday. So of course i did the wrong thing and replaced o2 sensor, still throws code. Replaced fuel filter, nothing. cleaned MAP and IAC , checked voltages, operation, nothing. Changed plugs and checked wires/coil, no change. Had almost convinced myself to replace the caps on the computer, but kept putting it off. Local mechanic wanted $600 to change the computer, I was not convinced they knew, either. Then twice, it stalled at idle - fuel issue, maybe? I had to pull the bed to change the fuel filler neck hose anyway, so i also changed the fuel pump. Now, no code with the new fuel pump! SO i guess it was starving for fuel, and somehow the computer kept trying to make it richer, to the point it threw a code? I would have thought the injectors would compensate.. and how does that result in O2 reading too rich? Any thoughts?

 

[RonD]

Welcome to TRS

1993 Ranger can't have a Code like P0137, that's an OBD2 code, not used until 1995 in Rangers

1993 would use 3 digit OBD1 but similar, list here: https://www.therangerstation.com/tech_library/3digitcodes.shtml

137 (R) Oxygen sensor not switching/system rich Left or Front HO2S - Fuel control

1993 4.0l would have 2 O2 sensors, drivers side(left) and passenger side(right)

In any case "Rich" or "Lean" codes are in regard to Computers calculated fuel mix, doesn't mean engine is actually running Rich or Lean
If tailpipe blows Smoke its running Rich, if engine pings/knocks its running Lean

In the case of Rich code driver's side, it means computer has limited the open times on injectors and O2 on that side is still showing high Voltage
O2 sensors can only see Oxygen not fuel, O2 voltage works like this:
0.1volt is high oxygen, or "lean"
0.9volt is low oxygen, or "rich"

So O2 on that side was showing high voltage
Computer tries to keep O2 voltage at 0.4v by adjusting fuel injector open times

Yes, changing the fuel pump wouldn't explain why this code went away, 136 yes, but not 137

And 1993 4.0l usually ran Batch Fire injection, so 3 injectors opened at the same time
Calif 4.0l could have had sequential injection, would have a Cam sensor
Point of mentioning this is that BOTH O2 sensors(left and right) should show high voltage on a Batch Fire unless an injector was leaking on Drivers side
Codes 137 and 173

O2 sensors are the only sensors that wear out, 100k miles or 12 years and they need to be changed, they just start costing you money after that as MPH slowly drops
So O2 needed to be changed most likely

 

[hokies_86]

Thanks for the explanation on the O2 voltages. I wish there were an easy way to read them live. And you're correct, i should have said 137, not p0137.. and i agree changing the fuel pump should not have affected the code. I'm trying to figure out in hindsight how to troubleshoot this code to know for sure what the issue was.

The local shop swapped left and right sensors at the ECM and got the same code, that's why they said the ECM was bad, but maybe the batch fuel injection makes that test irrelevant. I changed the left O2 sensor 3 months ago and it still threw the same code (checked wiring, etc). And no vacuum leaks i could find. My mpg was about 15; haven't checked it since the code went away. That will be interesting to see if it changed. So still a mystery/coincidence it went away when it did, and just hoping it doesn't come back.

You mentioned leaking injectors, if the code comes back, should i replace them?

 

[rumblecloud]

There is a way to check for leaks prior to replacing, but someone else will have to explain that process.

 

[RonD]

Yes, you can use Clear Flooded Engine mode to test for leaks of fuel into intake

All fuel injection computers have this mode, not a Ford thing

Turn key on
Press gas pedal down to the floor and hold it down
(if RPMs are 0, and throttle sensor shows WOT, wide open throttle, then computer will not open fuel injectors, thats Clear Flooded Engine mode)

Try to start engine
It should just crank but not Fire or start because there is no fuel
If so no leaks

For this to work you must have WOT, so if engine starts check WOT
Put a stick on gas pedal so its pressed all the way down
Go to engine end and see if you can open throttle a bit more, if so then you have a stretched throttle cable, no WOT
Easy to fix, Google: Ranger throttle cable mod