I just bought a 98 jeep cherokee 4.0 I6 with 140k...i got a good deal on it because i have to replace one of the freeze plug under the Exhaust/Intake manifold (which all has to be removed). As it is leaking do to a hole....The coolant will be flushed after the plugs are installed. Im planing to do a nice tune up with new plugs/wires/cap and oil change also adding a can of BG-44K into the fuel tank. i drive about 100 miles a day so im trying to get it tip top condition so it doesnt eat me alive on gas milage. Im also planing to change the Transfer case/Tranny/Rear Diff with synthetic fluid. Many people have told me its worth 1-1.5 MPG. Also was looking into a Intake tube(to help fuel milage) but something with a Dry synthetic air filter. Any suggestions? Also the tranny gives a little "buck" when you put it in gear and and pulls a little not much(i know it should)...but it shifts awesome! any ideas on what may cause that? Maybe U-joints? I also plan to do the Air Charge Temp sensor relocation into the air box (sensor will be replaced) while the manifold is off (manifold will be hot-tanked at a machine shop). So can any of you guys help me out here and offer any suggestions?? for anything i dont know or have missed? Thanks!!!

-J

Not sure about the 1-1.5 for sythetic fluid in the diffs. Recommend just using dextron 3 in the t-case. Same fluis you would use for the tranny. I always use Lucas heavy duty in my Diifs. You are not gonna be able to squeeze much mileage out of the bricks on wheels but if you are driving Hwy most of the time I don't see why you couldn't net 20mpg.

Why do yo have to replace the freeze plug? Better yet why is it leaking, not real common on the 4.0L for that.

I would not recommend putting synthetic in the transmission. I've never heard any good coming of it on a Jeep.

What are you think your going to get for fuel mileage? It's a Jeep and there not built for mileage..

I also suggest, checking, and maybe changing the O2 sensor, it helped me out..on MPG I get about 17.55 on highway with some lil trail ridding ...

I also suggest not to switch to syntetic, on a old/ high milage vehicle... they kinda dont like it..

I know the freeze plug issue isnt Too common but ihave seen others out there when i was shoping for mine that mentioned they were leaking. I was able to get a light gown there and it is the second one in from the front of the motor. Looks like it rusted from the outside in.......As for MPG i would be happy geting the 20-22MPG i have seen alot of people saying they get. It is BONE stock and i dont plan to mod it suspension wise at the moment. I bought it for the 4.0 I6's Die hard reputation and the good 4x4 system.

Exactly!! this is on my list although i didnt mention it ill change it. I guess they only have one o2?

I had some Mobil one 5W30 oil i was gonna run.

also for milage, I sugget to take a look at this thread https://www.cherokeeforum.com/f2/fuel-saving-tips-3510/

yeap ony one O2 sensor, I got it at 63 bucks at Pepboys, Bosch

I asked about the freeze, cause most I've seen with them bad had been over heated very badly and most were cracked blocks... Just something to keep your eyes out on..

i had a leaky freeze plug when i got my jeep. didn't notice it at all till i put my header on. it didn't overheat or anything, just was a hole in one of the plugs

Don't forget the one freeze plug on the backside of the head. That can be a bear to get to without removing the head or pulling the engine. Can't remember if there are freeze plugs behind the flexplate or not. Get the copper coated ones and you will never have to replace them again.

from what i can tell...as i got my drop light under there. it is 100% the freeze plug and it looks like it rusted from the outside in. there is alot of rust water that is in the system that i will flush out once i get it all back together again.

no advise on the tranny issue? ill check the U-joints when i get a chance?

I think we all over looked the trany do to the "freeze plug" issue.

With the trany try a filter and fluid change. Sounds like it might just be "gummy" in here..

Try what Nick suggested but if that doesn't help then post back up with just that question and maybe explain what you mean by "pulls to hard" Can't quite figure what you mean about that.

Blue I think he means more of a "surge" than a pull