My fuel mileage on my '11 4.0L has dropped significantly (30%+) in the last 2 weeks. There are 42000 easy California miles on the vehicle. 4 speed auto. 2 wd. xlt supercab.
prior mpg: 16 -18 current mpg: 12 -15
I can hear the catalytic converter "pop" quite frequently. (more than normal).
The K&N air filter is clean & fresh.
It seems as though it's burning overly rich air / fuel mixture.
Can this problem be identified by using an analytic diagnostic devise?
There are no warning lights illuminated.
Any other solutions?

 

Well you have all the signs of a true Rich mix, lower MPG and overheating Cats.

Yes, you could use a live scanner to watch fuel trims.

The system works like this
AFTER warm up
The computer gets the air flow data from the MAF sensor, this data sets the 0(14:1/air:fuel mix) for the fuel trim.
Computer opens the fuel injectors for a specific time(pulse width) to get the 14:1 mix
It then watches O2 sensor data(feedback loop)
If O2 sensor shows Lean then computer increases Pulse width, this is a +1 above 0
If O2 sensor shows Rich then computer decreases Pulse width, this is -1 below 0
The + and - number is the fuel trim
-19 to +19 is the limit for this before computer will set trouble code
In normal operation you would see constant cycling from -5 to +5 as computer trims fuel, at idle +3 to +7 is normal, idle should be rich.


O2 sensors only measure Oxygen, not fuel, if there is too much oxygen in exhaust(lean) then not enough was burned so computer adds more fuel +1
If there is too little oxygen(rich) then too much was burned so computer lowers fuel mix -1

A misfire means no oxygen was burned, so it all goes out to exhaust and computer sees this a Lean, so increase fuel +1

Vacuum leak means all the air is not coming in thru the MAF sensor so computer is using the wrong "0" for fuel trim, this usually shows up as Lean trim so higher + numbers, because computer is adding to little fuel for the actual available air.

The chance of both O2 sensors failing at the same time is pretty much 0.
But they should be reporting that rich of a mix, you should have hit the -20 preset for the rich code.

So that part is puzzling.

On the fuel rail is a Pulse Damper, it is at the end of the rail and will have a vacuum hose attached to it, remove this vacuum hose and check if there is gas or gas smell in there, if there is then raw fuel is being sucked in and that is the source of the Rich mix, replace pulse damper, but this is a long shot since I would expect O2 sensors/computer to pick this up and set the rich code.

You can also try cleaning the MAF sensor, never seen one go rich, only lean, but there is always a first time.

And the worst case is the computer has a problem, looking at a live scan may show that

 

Thank you for the suggestions.
I took the vehicle to a shop with the proper test equipment and very competent personnel.
They tested everything you suggested and all results were perfect and well within the established parameters.
The MAF looked as if it were brand new and again all voltage test results were perfect!
After I left I thought about two things. 1. the tests were done under no load, that is, the vehicle was in park and not being driven. Could the transmission be slipping? I know what a slipping transmission feels like but perhaps it's slipping just barely enough to affect my MPG's.
2. and I asked the mechanic about this... I had another problem with sulfur deposits on the contacts of the sensor on my fuel pump. At least I thought so because my fuel gauge was giving me no reading and the low fuel light was displayed from time to time, and once the check engine light was displayed. The solution for this problem was to re set the check engine light, use the existing fuel in the tank to about 1/4 full, drop in 2 pints of Sea Foam fuel system cleaner, & then fill the tank with "Premium" (93 octane here) fuel. He said that procedure should have no negative effect on my fuel mileage. What do you think?

 

Cleaning the fuel system/injectors with Seafoam is a good start, I put a can in all my vehicles once a year.

Yes, transmission or brake dragging does effect MPG, you could feel the hubs/wheels after the next long drive and see if one feels warmer than the others.
But that wouldn't explain the rich fuel mix and popping Cats, just lower MPG.

Did they do a fuel pressure test and did it hold pressure after key was off?
Long shot, but a small leak may not be noticed by smell, and the 65psi pressure does have alot of "push" behind it to find a small gap.

But a leak wouldn't explain the Cat over heating or rich exhaust, unless an injector was the leak.
That would explain the rich mix and popping Cats but not why the O2 sensors fail to see it.

Did the shop have any suggestions?

 

One thing you mentioned was the brakes dragging.

At first I dismissed this as I would think that I would "feel' that symptom in the vehicle pulling in one direction or the other, but I'm running out of logical reasons, so I purchased a Ryobi non contact infrared thermometer and took a reading after a 4.2 mile trip.

I was traveling at 65 mph for about 2.2 miles of the trip, the rest at around 30 - 35 mph. I applied the brakes 18 times during this trip. 95% decelerating from a speed of 30 - 35 mph. I applied the brakes in a gradual, orderly, normal, fashion so as to not create excessive force that would create higher heat values.

The readings were taken directly off of the rotors on all 4 wheels. The ambient temperature was 62 deg. F.

The reading are as follows:
Drivers side front: 168.8 f.
Passengers side front: 158.3 f.
Passengers side rear: 144.9 f.
drivers side rear: 119.5 f.

Do these readings fit within normal parameters for this short trip?
Is there a way to test if the brakes are causing excessive drag where it would affect my mpg's?

You've been so helpful so far I would appreciate your input.

Thanks.
Joe

 

I have never tested the brake temps so couldn't say specifically.

But they do not look hot, I would expect to see 150-200degF on the front, and under 150 degF on the back
Front of course generates more heat during braking.

I might have a look at the Passenger rear it seems hot if drivers rear is correct.

Generally front should be withing 10% of each other, which yours are.
And rear should be the same, which yours are not.
E-brake cable may not be releasing enough or just an adjustment
Or drivers side is not adjusted and isn't braking like it should so is cooler

 

You may want to reclean the MAF and put a regular filter back in. K&N lets a lot of dirt in. Plus if it was reoiled the oil may be coating the MAF