My fuel economy has been gradually declining on this truck. I've had it since new, and used to get 12-13 mpg ( I have a large contractor cap, and tools which total about 2000lbs). Now I'm getting around 7.5mpg on the highway, and 8-9 around town.

I've replaced plugs, wires, coils, and air filter. Nothing has made any improvements. I'm at a loss on what to look at next. I have zero DTCs showing.

Any ideas?

Thanks

 

maybe the O2 sensors?

 

I was thinking about that, but wouldn't i see some codes if that were the case?

 

I would expect codes but it's worth looking into it. Maybe some one has a OBDII reader and you can look at the different parameters real time.

What is odd is that you are getting worse milage on the hwy then in town. Maybe the torque converter is not locking up?

 

O2 sensors don't throw codes unless they are suddenly bad, or dead altogether, in my experience. If they gradually get worse and worse, the computer "learns" to deal with them (to a point). Tolerances and all that jazz.

Whenever you replace O2 sensors, they recommend you "reset" the computer with a battery disconnect (for a while, make sure everything is fully discharged) and reconnect for this reason. Forces the computer to reset to factory defaults, so to speak, including forcing it to relearn the O2 sensors from scratch (no memory).

Also, have you had your alignment checked recently? A bad alignment that is slowly getting worse, especially if caused by defective suspension parts, can contribute to bad MPG.

 

I assume you've checked obvious things such as low tire pressure or a frozen caliper.

Also, that this is "real" MPG (i.e. measured by human) rather than the computer.

 

I figured that with 4 O2 sensors, the system could tell if one them was out of line. Anyway, when you changed the spark plugs, did any of them look darker than the others?

 

Thanks for the input guys

Alignment has been done recently, and it just got new pads & rotors all around.

I had thought about the torque converter not locking up, but my RPM seems normal on the highway.

This is dashboard MPG, but it works out about right. I'm getting less than 400km per tank. I used to get closer to 600.

Also, I didn't replace the plugs myself. I had my mechanic do it as this is my work truck, and I don't have the time to work on it. But he didn't mention anything was abnormal at the time.

I have an OBDII reader. What live data should I be looking at?

I might go ahead and order new O2 sensors to rule then out. I have over 180,000km on the truck and they are still original.

 

I thought you could see the O2 sensor voltages with the reader. Best bet is to just change them out if they are not too expensive.