Have a 05 dodge dakota with 140,000 miles. The gas mileage has really gone down hill. I replaced the plugs, old ones still looked decent. How often should the injectors be replaced>

There is no scheduled maintenance that I know of. It depends on fuel quality and use. I have run DD's 165 and 175k w/o injector service.

When they are serviced is is usually cleaning on or off the car. If done off the car, they are checked for flow rate and pattern. On the car is with a fuel additive or may be plumbed up to a machine to inject cleaning solution.

If one or more are not working well, it will normally throw a code. No harm in getting them cleaned but may not be your problem.

I wonder if there may be other issues like tires or alignment, plugged converter ect. Also, does the transmission shift smoothly through all gears and lock up at cruise? Any aero changes?

The only problem that I've ever had with high mileage injectors is leakage. In your case I'd recommend checking your codes. When I've had a sudden nosedive in mileage typically it has been a drifting O2 sensor, not injectors.

I see this every winter in Ohio,customers start complaining about poor fuel economy and longer crank times when starting. The thermostat stays closed longer not letting some of the sensors see the hot water like they did in warmer weather. The fuel trim stays rich longer and you will burn more fuel.
Now about that fuel in Ohio In most counties they add more alcohol to the fuel in winter and thus your fuel consumption goes up when your A/F sensor sees a lean condition and adds more fuel until it's happy your engine isn't going going lean.
That's just my guess

Gus

Well this is interesting, no codes show up and the truck does run good otherwise. Maybe I should change the O2 sensor just to be sure. Anyone know where its mounted? Alex

The internet is our friend. There are quite a few sites that cover this if you decide to do it.

Had exactly the same thing happen on wife's car and after a while of poor mpg the Check Engine Light came on. One upstream O2 sensor later, mileage returned and CEL went off.
This was about 140K miles.
I like NTK sensors.

R.

You need to watch the 02 sensors on the live data stream, a code may or may not be set by a contaminated 02 sensor. The upstream one is the only one that matters. It could dump up to %25 extra fuel before the code is triggered, more than about %5-%10 usually means something is getting skewed.

Avoid the snake oil at the parts store for cleaning injectors, just try a couple tanks of premium fuel from a tier 1 supplier, they have way more cleaner than normal gasoline and since the engineers supplied it already mixed they probably knew what they were doing better than someone bottling acetone in their basement.

I am not sure about the 05 but early 4.7 had a single squirt hole in the injector, a swap to some ferd 4.6 infectors usually nets a little better MPG as they atomize the fuel better, if your swapping injectors anyhow.

A car usually gets significantly worse MPG in the winter because fuel is not evaporated as good. Gear oil gets thick and hard to stir, oil gets thick, PS fluid gets thick, trans fluid gets thick...

Alcohol content usually goes down in winter because it don't evaporate as good and hinders cold start a hair, of course E-85 never really made my cars hard to start but that is what the fuel makers say.

So where is this upstream sensor located on the 05 4.7 engine

Gonna be real honest here, if you don't already know this then this job is probably not for you.

You have two upstream O2 sensors, one on each side of the engine, they are UPSTREAM of the cats. Think about how the exhaust flows and which direction is upstream.

Without a scan tool you'll be guess which one, if any, of the upstream sensors is bad. may as well replace both rather than one of them if you're going to do it.

Which, at this point, you are just throwing parts at a maybe problem.

As was mentioned above, in many cases the O2 sensor will drift. This is IMHO more because of age than anything else.

I'd say that after 100K miles the upstream O2 sensor(s) should be replaced even if it's nothing more than prevention.

Rockauto is your friend when you do this.

I used to be pretty open to using whatever sensor I could buy for less on Ebay, but late-model cars have enough differences in their O2 sensors that I would try to get one specifically for my car.

If I'm all wet on this last point somebody please correct me. I'm talking specifically about the wideband sensors.

R.