Hi, I have a 1994 Grand Cherokee with a 4.0L It has 93,600 miles on it. A couple of months ago it was getting about 17-18 MPG (city+hwy average). Now it is getting about 11.7 (city+hwy) and seems to have less power than it did before, especially accelerating uphill on the highway. I just replced oil and air filter. Any suggestions?? THANKS

When is the last time you changed your spark plugs? There cheap to replace & it could possibly solve your issue.

I have a 97 Grand Gherokee (bought a couple of months ago). Next time you start the vehical let it run a min. or two then wipe your finger at the tail pipe interior. If it's black (like mine was) you could need an o2 sensor but o2 sensors usually don't fail quickly but degrade over time. (spark plugs first though). If your mpg & performance dumped all at once like you said I would say your thermostat could be stuck wide open making the vehical run rich to heat up the motor. Is your temp gauge coming up to operation temp (mine is 210)

Where is the O2 sensor? And how hard is it to change? $$$?

u got 1 O2 sensor, it is located on the passengerside int he exhaust pipe in front of the Catalytic Converter.

97GRANDCHEROKEE
Sounds like your overheating, I dunno what the normal operating tempature of a 4.0L engine is, but my 5.2L was running at 210 last summer and it would get up to around 235 sometimes, I didn't think it was overheating untill a trip home from Pittburgh (80 miles from home) left me stranded on the side of the highway with a bent crankshaft, shattered timing chain cover, hole in the block, etc. and a $235 tow bill. then after replacing the engine about 2 weeks later blew my radiator (10" long crack in the side tank). well it wasn't til after replacing the radiator tha I found out that I was overheating the whole time. I've only owned this jeep for a year now, and last winter my tempature was at/around 180 degrees, summertime is was between 210 and 235, well after replacing the raditor my tempature in the summer time was around 170 degrees, and this winter my temp was only at 130-140.

Like I said maybe the 4.0L run hotter then the V8's but it might be worth looking into before you have any major problem, just some 'Words of Wisdom'..

i was also getting poor mpg untill i replaced my spark plugs. it took all of 20.00 to change my plugs and glad i did. like everyone said change plugs first that is key. then try the o2 sensor if it does not work. my 94 jgcl jeep has 193,000 miles on it and ive never had to replace my o2 sensor so i would not think thats it but i could be wrong also if you have a oil loss problem that could also be it. check the crank case vent tube and see if its cloged with oil if it is you need that fixed. oil kept on getting into my chambers and fould pretty much all of my plugs. also a quick way to check is to open the air box and take out the filter if its black and there is a puddle in the bottom of the box you have an oil problem. i dont know what its is thats causing it but it is a problem if you have then get it checked out asap. that also is a coulpret when you get poor mpg you engine is not buning the mixture all the way becaue the oil messed them up. oil does not like to be burned and is really hard to burn without messing it up. hope this helps

Spark plugs and possibly new plug wires (had a power problem similar at 100K, changed plugs, and no help, changed plug wires, even though they looked good, and problem went away). Also, cracked catalytic converter (internal) blocking exhaust flow (had that one on an 87 Cherokee) collapsed muffler, fuel filter partially blocked, what octane fuel are you using? Tire pressures good, brake dragging, pulling left or right? O2 sensor failure will give you a code. I have two on my 97 4.0L. One on the exhaust manifold, and one next to the cat. conv. Couldn't tell which was bad, shotgunned it and changed the downstream one and lucked out. They're expensive little buggers. Good luck.