Low MPG lately

A couple months ago I did a COMPLETE exhaust replacement. On my wifes 1998 xj 4.0/aw4/231 I replaced the exhaust header with an APN, downpipe, new cat, new muffler, new 02 sensors, basically everything new. Her MPG was around 16-18MPG before this and is now down to 11-13MPG after everything new was put on.
I also have a new CSF radiator, water pump, 195 thermostat, and hoses installed around the same time. I just don't get as to why the MPG went down so much with everything being new. There is no codes and the only thing I see is some black around the exit of the tailpipe. That tells me it is running a little rich. It does have a bit of a hickup at idle but does not affect the performance of the motor at all.

Any thoughts?

coolant temp sensor? I hear they can also affect your MPGs.

For 96-01 OBD-II, a sudden drop in MPG's by 30-40% points to an O2 sensor issue. Test you O2 sensors for proper functioning and inspect the sensor wires for short circuits, broken wires, and for chafed insulation. Also, since you recently had the manifold removed, check for intake/exhaust leaks. Inspect other sensor wire harnesses that may have been damaged by recent repair work, or may have been moved and came into contact with hot exhaust parts.

Do not forget about winter fuel, should be going away soon.

As has been stated, check your o2 sensor.

Besides that, I see you're in WI. Both my '96 and '95 xj's got 12-14 in the TN winter which is mild compared to WI. I normally got 16-18 in the summer with them.... something to think about....

X2 on the prior O2 sensor post. Also check for the wrong O2 sensors.

Go here and learn how to test your O2 sensors live.

http://www.naxja.org/forum/showthread.php?t=1012701&highlight=testing+oxygen

And check the engine temp to see if the T-stat is bad, and letting the engine run too cold. Also test the CTS for accuracy.

This likely has nothing to do with the new exhaust.

O2 sensor, map sensor, tuneup hardware, etc.....Something else is causing the drop in gas mileage.

Be sure that you get at least three gas mileage samples before coming to any conclusions. And gotta do the math. Miles driven divided by gallons consumed.

I don't think that the winter fuel is making a difference for me. The past two winters with the same setup, except for the new exhaust and cooling system, we were averaging around 18 MPG during the winter months. The drop in MPG came once I installed the new parts this past summer.

The engine temp stays at 195 with the new thermostat. I was wondering if that is to low for the 4.0 to run right, but when we are wheeling, it will stay around 200 and that is it unless we are really giving it hard then it will go to 210 and sit there. It has never gotten over that temperature and it runs the same at all temps.

I will see about testing the brand new Bosch 02 sensors today once I am done with the brakes. I just have a hard time believing these would go bad, but you never know.

No leaks on the exhaust header that I can detect. I sure hope this is something simple as the MPG we are getting is keeping this jeep at home. Glad I have a few jeeps to pick from but the wife wants her baby back!

Read the link I posted on testing the O2 sensors, wires, relay that supplys power to the internal O2 sensor, wrong O2 sensor (needs to be the 3 (87-90) or 4(91-01) wire sensor, not a universal to work). I have read a number of dropped MPG stories here over the years that seemed to be caused by after market exhaust headers....????

195 F is fine, so not the T-stat. Also check the MAP, IAT and CTS, but my money is on a problem with the O2 sensor or wires, relays that support it.

i did the same thing to my '99 ex. mani, no crush in the down pipe, cat, muff and took a similar hit in mpg ( in town). all my stuff checked out just fine. i have a better middle range but my mpg is f'ed now. my highway mile still around 19-20 though. if on cruise the whole way i can get 21.