O2 Sensor Replacement Question

[Rearanger]

I have been monitoring my average fuel consumption since I purchased this truck - there are a lot of data points. The mileage peaked at about 23K and held steady until about 36K when it started a slow decline. My driving habits are fairly constant and the ethanol content has always been about 10%.

At about 11K I did a hot engine compression test and got numbers from 185 to 195. I just did another at 69K and got the same numbers - so lost compression is not the cause. The plug condition showed perfect fuel burning.

It appears that it's time to try replacing the O2 sensors to get my mileage back. This truck has one on each bank and a third downstream of the CAT. The downstream sensor is called a CAT efficiency monitor.

My question is, does the after the CAT sensor contribute to fuel trim? My FSM did not indicate this and searches here were fruitless. There was some Goggle discussion both ways - but not about the Ranger.

Any techs out there that can tell me if it's worth replacing the downstream for better mileage?

Thanks

 

[adsm08]

The downstream sensor does not contribute at all to fuel correction. The upstream sensors are there to see the amount of oxygen left over after combustion, and help the computer correct the fuel calculation. Since this is a result of trying to prevent the problem after it happens a normal O2 signal will bounce up and down between about .2 and .8 V.

The catalyst material is there to take all the byproducts of the combustion process (NOx, HC, CO, and O2) and chemically rearrange those byproducts into the less harmful compounds CO2 and H2O (this is why you shouldn't worry about water dripping from the tail pipe). As part of this reaction the cats need to absorb nitrogen and oxygen.

The downstream sensor is there to look for oxygen in the exhaust after the cats. Free molecular oxygen in the exhaust stream after the cat is a sign that they are no longer able to do their job because they can no longer absorb oxygen. Since the cats should allow no oxygen through a normal downstream signal should be flat-lined near .5V (the upstream and downstream run in different voltage ranges. Put a downstream in an upstream position and watch what happens to the way the engine runs).

The computer compares these two signals to monitor the cats' ability to function. If the upstream switches and the down stream stays flat everything is OK. If the downstream sensor starts to produce a waveform similar to the upstreams then the computer decides the cats are fried and sets an efficiency code. This is the ONLY purpose of the downstream sensor.

 

[Rearanger]

Thanks adms08 for that clear & detailed explanation.

 

[Sasquatch_Ryda]

Would not hurt to throw a thermostat into it as well...the faster it gets to operating temp the more efficient it will run.

 

[Rearanger]

Would not hurt to throw a thermostat into it as well...the faster it gets to operating temp the more efficient it will run.

I'm trying to understand that advice, even though it's a cheap replacement. The stat is closed at cold start-up, so unless it's leaking or opening too soon I don't get how the engine will heat up faster. Do they bypass?

Temp gauge is always at same temp.

 

[adsm08]

Thermostats are designed to fail in the open position so that they don't cause an overheat if they go bad. If the stat is getting weak it will open sooner and not let the engine heat up as fast.

The temp gauge is an idiot light. It only knows cold, good, and too hot, and won't indicate a weakening thermostat.

Changing the trans fluid may help as well, if it hasn't been done already. The unfortunate thing about it is that most engine do "peak" for MPG, somewhere between 5 and 50K miles and you can't usually get it back up to peak once it comes down. You can get it back up, but they rarely hit the peak again.

 

[Rearanger]

The unfortunate thing about it is that most engine do "peak" for MPG, somewhere between 5 and 50K miles and you can't usually get it back up to peak once it comes down.

This may be the case. Several tanks and no real improvement in MPG after O2s and stat. Today I checked my ECT sensor to see if was in range.

FSM states 194F = 2.80Kohm, 212F = 2.07Kohm. Laser temp gun showed 193F just under sensor and the reading was 2.63Kohm after a good run to get to hot engine. So it appears sensor is doing fine.