O2 sensor voltage vs ltft.

[Dirtman]

My truck has a long term fuel trim of about +13 throughout the throttle range. I know this is below the threshold for a DTC but it's still high and means I'm using more fuel. The truck has the maf cleaned, new plugs and wires, checked for vac leaks, new air filter... all the basic stuff.

The truck has almost 70,000 miles which again is young for a o2 sensor to be bad but my truck does idle for very long periods. I put the bank 1 sensor 1 o2 sensor on a graph and its peak voltage only hits about .7. Again this is obviously still under the threshold for a DTC but it should peak to .9 volts correct?

Is it worth replacing the upstream o2 sensor? Any other ideas how to bring the fuel trim down?

 

[adsm08]

You can certainly replace the O2 sensor, it can't hurt. By age you are there, by miles alone you are close, adding idle time, you are probably in the ideal window.

I have rarely seen the LTFT dwell much lower than 10, and wear and altitude both have effects on fuel trims.

 

[RonD]

Assuming 2009 2.3l
O2 sensors read Oxygen levels not fuel
.1v is Lean, high oxygen
.9v is Rich, low oxygen

O2 sensor voltage for upstream O2 isn't making sense, "+" fuel trims are from .1-.3v O2 voltage, Lean burn, so computer opens fuel injectors longer to add more fuel, "+" fuel trims, fuel trim = injector open time

If O2 was running .7 then you would see "-" fuel trims

Downstream O2(B1 S2) should run a steady .7 or so, since most of the oxygen is burned in the Cat converter

Small vacuum leaks would cause "+" fuel trims
Older fuel pumps with slightly lower pressure would cause "+" fuel trims

But it wouldn't effect MPG, the computer is just getting or using the wrong data.
In the case of a vacuum leak computer base calculation is off because its not getting correct air flow, so it adds more fuel to compensate for the extra air, if there was no vacuum leak the air would be coming in thru the MAF instead so same amount of fuel used

Computer is programmed for 55psi fuel pressure so bases open time for injectors on that amount of fuel flowing in when injectors is open for say 100ms, if pressure is lower, then slightly less fuel flows in, so Lean burn and computer adds a bit more, but it would be the same amount of fuel overall if pressure was 55psi

Idkle does require slightly richer mix to pervent over heating and to Feed Cat Converter fuel to burn to keep it HOT

 

[8thTon]

If the O2 sensor begins arbitrarily indicating a leaner mixture then the processor will add fuel to bring it back, making the mixture richer.

Remember that an O2 sensor is not a linear device, it is a threshold sensor. It switches from a low output to a high output at a particular point, but it doesn't tell you anything other than that. The ECU must hunt the mixture up and down to keep the O2 sensor switching states. So measuring the instantaneous voltage may not mean much.

From what I've read the failure mode is actually that they get slow and the response to the ECU hunting around gets delayed, with the result that the ECU is fooled into thinking the mixture is leaner than it actually is.

 

[Dirtman]

Good info. On both sides. For 30 bucks though I'm replacing it and that is with an oem Bosch sensor.

Bosch is oem right?

 

[adsm08]

Good info. On both sides. For 30 bucks though I'm replacing it and that is with an oem Bosch sensor.

Bosch is oem right?

Motorcraft is the OEM branding. Bosch is probably making them for Ford though.

 

[Dirtman]

Assuming 2009 2.3l
O2 sensors read Oxygen levels not fuel
.1v is Lean, high oxygen
.9v is Rich, low oxygen

O2 sensor voltage for upstream O2 isn't making sense, "+" fuel trims are from .1-.3v O2 voltage, Lean burn, so computer opens fuel injectors longer to add more fuel, "+" fuel trims, fuel trim = injector open time

If O2 was running .7 then you would see "-" fuel trims

I ment that it is hitting .7 at its peak during switching. A properly functioning sensor should constantly switch from .1 to .9 correct? Mine is switching from .1 to .7.

 

[Dirtman]

Well the results are in. Im no scientist but after installing the new o2 my ltft numbers dropped a noticeable amount. Im now around +8.6 at highway speed and +10.9 at idle after a couple days driving. Weather hasnt changed so Im calling it a win.

The original sensor was definitely made by bosch btw. It had a Ford logo and part number but below that was the same part number on the bosch i used to replace it.