You said stock tires, so I assume they are stock size... so perhaps a stupid question, but is your odometer calibrated correctly? Is a mile really a mile?

 

I'm having the same milage BUT I have 35s on 3.55 gears. And I know the mileage isn't accurate so does anyone know of approximately how much it could be off?

 

\o2 sensor may not set a code unless it is stuck either at full rich or full lean. I may be wrong, i could be, i am not a mechnic, but hook a volt meter up to your o2 sensor pigtail while it is connected and engine running, carefull! do not jump the heater wires to the signal wires or you risk frying the computer. Lol. I know. \\\\\\\if the values you observe, are constantly jumping between 0.1 and 0.9 volts, then you can rule out the front oxygen sensor being bad. If the values dont change and stay at one voltage then the sensor is bad. I know I will have the support of everybody here when I say check it out before going any further and it is always wise to check parts properly and find the problem before throwing money into buying new parts just guessing what the issue is.

 

Also check your coolient sensor it was making my jeep horrible on gas and messed up my heat and air

 

I also agree with checking the coolant temperature sensor, again witht he volt meter, not sure of the correct values, but just to specify, its the sensor on or near the thermostat housing, and not the one near the firewall located on/near the head.

 

I have done all of these things pretty much. I've changed oil and filter, transmission fluid and filter. New air filter. New O2 sensor after catay. I got new muffler put on. Put new spark plugs in. Changed coolant temp sensor. And added seafoam to a full tank I am still getting half the gas milage I use to get. @ 160k miles on 99 jeep Cherokee 4.0l 4wd
Does anyone have any other clues as I am running way to rich and pushing way to much gas. Thank you in advance.

 

If you pull your plugs out what do they look like?

Do you have access to a scan tool? It would be real easy to see what the fuel trims are doing and to test the O2's

Have you checked for vacuum leaks? Wait its not a MAF system but I would still check for vacuum leaks.

So:
Stock height
Stock gears
Stock tire size
Normal rig?

exhaust leak?
We really need to see those 02 sensors.

Did the new muffler affect it at all?

 

I'd check for an extremely hot rear drum after driving. Could be a dragging rear brake. Also could be a cracked manifold, letting air into the system and throwing the O2 sensors off.

 

I will have to check my drums next time I drive but. I did find a crack on my manifold. So saying its throwing off my code.


The plugs have some black on them. I do not have a scan tool. I did find two vaccum leaks I plug them but did not change to much. I did find a crack on my manifold. And yes it is a stock jeep.