OK . Got a 99 Cherokee 4.0 4x4 auto .100.700 miles. Just bought 4 weeks ago . No mods . I work at Chrysler powertrain . Drove with a DRB connected . No codes . Runs great , idles smooth, but 15.5 mpg combined . Worst jeep mpg I ever had ! New wires,plugs,cap,rotor,map sensor. I do have a crack in exh. manifold in the the front . Cat is clean , new rear exhaust . I will check tomorrow the front 02 sensor on freeway . Good at idle . Rear is good and only checks cat . Other than exhaust leak not sure what could cause issue . That is 02 seeing lean . Thoughts ? Thanks ! This my 4th 4.0 liter and the worse MPG .

 

Sounds about normal...
Fix the crack and maybe youll see 1 more MPG.

 

My 99 is getting about the same, if not a little worse. No big tires or anything too. Makes me wonder what I'll get when I put 33s on her.

 

I get about 20 floored often with 31s on stock gears with CAI and cat back dumped thrush glass pack.... and I have a pin hole in the gas tank. I'd put some intake and exhaust stuff on that thing

 

Replace front O2 sensor (I like NTK) test your coolant temperature sensor.

Your mileage really isn't that low; I get around 16-18 with my bone stock 99 (meticulously maintained) with combined driving in the winter.

 

Yeah I'd kill for 15. Replace the cracked manifold. I've heard of people getting up to a 5 mpg gain after putting in a new one

 

Think i will try exhaust manifold repair and a front 02 sensor !

 

I have a 99 with a 4.0 and an automatic transmission. I get about 17mpg. I have a 98 with a 4.0 and a manual transmission and was getting about 21 mpg. In all fairness though I was driving the 98 before they started to use 10% ethanol in the gasoline. My 99 has new champion copper plugs, new wires, cap, rotor, APN header, custom head pipe that is not crushed like the stock one, 2 new O2 sensors, new magnaflow high flow cat, flowmaster 44 muffler, a K&N air filter and I am running 235/75/15 tires.

 

I agree that is the best way to proceed.

 

I too have a stock 99 Cherokee 4.0 4x4 with auto. I've been averaging between 14-17 in town depending how I drive, and almost 20 on the highway. Lately I've been trying to keep it around 2000 rpm, with no jack rabbit starts in an attempt to improve city mileage. Been told that is the mileage these beasts get. Prior to owning, had always driven Toyota's or Subaru's, and I won't even mention the great gas mileage I got with them. Last Subaru (lease) was an 08 Outback Wagon, and was great to drive, but could only dream about going where the Jeep has taken me. My hobbies are gold prospecting, and fly fishing.

 

Great to buy american !

 

in my 97 i was getting 20 mpg over the summer and as soon as it got cold i am not getting between 16 and 17. 235/75/15 tires motor is stock except for the 703 neon injectors

 

This is turning into another "what is your MPG?" thread...
Winter gas does make a big difference... I got 18 pretty consistently this summer, as soon as winter gas came around I dropped to 16. With 4wd and some wheel slip in snow, I get around 14.

 

I'm telling y'all, hit up ebay and get a CAI then go to advance and buy a thrush glass pack and weld in on after the cat. 20 mpg out of any non Renix XJ

 

A well maintained stock renix should get the same OR BETTER mpg than a comparable 91+ jeep...