Sudden drop in gas milage

[fsram]

I have looked through this forum and have not found any help so I am starting new thread. I am towing a 5000 lbs camper with my 2004 5.7. For the first 12 to 15 trips, I got 10 - 12 MPG with all kinds of power to cruise at 70 MPH or more. The problem is about 4 months ago the mileage dropped to 5-6 MPG. There are no engine codes showing, I have done a tune up plus,wires and air cleaner, I also had the trans serviced and fuel system cleaned, replaced the cats and new straight through muffler. None of this was done any good. I have also checked fuel pressure and the compression both are good. I have run out of things to try.

Does anyone have any ideas?

 

[fastf]

Let me ask you did you loose gas mileage when you replaced your cats?The reason I have asked a this is your engine needs back pressure and believe me if you got the thing completely opened up this might be your problem .People do this stuff with no back pressure it kills the gas mieage this is just a wild guess and some of you people may say naw that ain't it but it has happen.One more when have the last time you run a injector cleaner through the engine like Seafoam is good but Berrymans B-12 will break carbon up faster if you don't believe me go to YouTube and there is a comparison between the two.Berryman B-12 beat out Seafom I have used both and there not much complain but B-12 does it quicker.

 

[07MegaCabRam]

What plugs did you use?
You may have used a plug that is too hot for the operating range of the Hemi.

 

[fsram]

All the work done was after the gas mileage loose trying to recover all parts were replaced with factory parts and champion plugs. the trans was serviced at my local dodge dealership alone with a fuel system cleaning. Everything I tried helped some but best MPG I got was 8 and it did not last very long then back to 5-6.

 

[SYKRAMMAN]

Those dam sensors screw everything up.
Guarantee its a stupid sensor.
Like map or iac, iat, etc etc.
They do go bad without throwing a code.

 

[fsram]

That's what I am thinking however if I have to replace all the sensors, I might as well by a new truck. I took it to a local speed shop and had them drive it hooked to a PC they say it it showing that it is running 3 times rich under a load. But had no idea why. They tried to program it out put it did not work.

 

[MADDOG]

Sounds like a bad injector or two if it's running too rich.

 

[SYKRAMMAN]

If it's running rich then I would try the map, iac, tps, first. Those are the sensors that control most of the mixing of air and fuel. And they are pretty cheap at 50-60$

 

[fsram]

Changed the map and iac sensors we will see how it does next weekend