The engine is programmed to run with the slight restriction from the cats and can use more fuel. Have the codes checked at Autozone to see what you have. The mileage should be a lot better. Why were the cats removed?
I had a Scanner and before my girlfriend decided to break it, it was throwing P0420 (Catalyst System Efficiency Below Threshold (Bank 1)) and the other code that follows for Bank 2, also P0402 (Excessive EGR flow) which i have yet to figure out why also.
I would go to autozone to get it check but in CA there is no free Diagnostic checks anymore =(.
Other than those codes i would highly doubt there would be anything else since the truck has ran like this since i got it about 8 months ago from the Mech.
They were removed becuase the cats got so clogged up that it burned all the valves in the heads, and the mech said it would have been about $900 to replace the CATS.
What happend to the OX sensors with the cats removed? Are they still hooked up?
The 402 code is a leaking EGR or a hole in the pipeing at some point, or a faulty DPFE, stuck EVR etc..
The EGR system fault will have an effect on fuel milege.
If the OX sensors are not hooked up, the engine runs on fixed 'rich' fuel tables.
Trying to get around them is now showing you you cannot to that with these modern computer controlled systems.
Without rear OX sensors the 420 code will be set, you cannot get rid of it.
You have a SES/CEL lamp and codes set.
Only way is to repair for those codes instead of letting them go and comlpaing about how the thrck runs.
Sorry to sound harsh but you need to come to reality.
o2's are still in place and hooked up, i have been wanting to do the DPFE for awhile and see what happens, maybe next check.
Before the truck ever broke down almost 3 years ago it had the 401 and 402 codes and was still getting around 16 - 17 and had some power. maybe 15 at the very lowest. The last owner did buy the DPFE but it was the duralast one from autozone and even when he put it on it still would throw the same code.
I've check the piping to the EGR and thats good, EGR valve, Vacuum Solenoids, Throttle body holes, and everything else in the EGR system is good and new but the DPFE
I can tell you that the EGR codes set has dissabled the EGR function.
This function saves gas when it operates.
Why: the EGR is supposed to open at highway speeds over about 45 mph and reltively light throttle openings.
When this happens the ignition is advanced and fuel injection levels cut back.
This is where the savings take place.
Put the truck exhaust back as stock and repair for the codes and you will get better mileage.
You cannot get around this and expect good results.
The systems were not designed to be bypassed and ignored.
It's cheaper over the long term to have everything correct.
Now you are paying for it all the time.
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