1989 Cherokee getting 11mpg, why??

Hi there, I don't post here often as I usually just spend my time browsing, but I am coming to the experts for some help on this one. I have a 1989 Cherokee Limited with the 4.0 Renix engine with around 187k miles on it. It is seemingly running pretty good, but I am getting absolutely TERRIBLE gas mileage out of it. I'm usually getting around 12mpg at best. I am looking at doing a tune up to it, but I'm not sure what all I should be looking at. I don't have an infinite amount of money to throw at it so I am looking at specifics for what I should replace.

I am planning on (obviously) an oil change and new air filter, as I know it's been a while since I've done one of those. I pulled one of my spark plugs today, and this is what it looked like.


They look dirty, but not too bad. Should I change them and new plug wires as well? How about the distributor cap and rotor, could those affect my gas mileage? It has a new fuel filter as of about a year ago, and has a new transmission filter and fluid as of 2 months ago.

What other things should I be looking at to help with my fuel economy? With these gas prices going up, anything I can do to make it better would really help me out. I'm not expecting to get 40mpg or anything, but even just the 17mpg that many other people get from these would be great. 11mpg is just way too low.

plugs, wires, cap rotor, air filter, oil, are all good starting points. even some injector cleaner such as sea foam would be wise.

Do not forget about the O2 sensor, map sensor, air temp sensor, even the tps can all effect the fuel mileage....
check out this site for how to check the Renix sensors
http://www.lunghd.com/Tech_Articles/Engine/Basic_Sensors_Diagnostics.htm

Get your DVM/OHM meter out and start checking sensors. CTS, O2, TPS(adjust), MAP and the MAT will cause poor mileage if any one of them is out of range.

This craptastic gas they are giving us now does not help either.

All good suggestions here. Put all new tune up hardware in first. Use cap and rotor with brass, not aluminum contacts.Wires, need to be replaced too.

Then test the sensors.

cruiser54 said:

All good suggestions here. Put all new tune up hardware in first. Use cap and rotor with brass, not aluminum contacts.Wires, need to be replaced too. Then test the sensors.

Click to expand...

I agree.

Fresh Champion copper plugs gapped to .035, new plug wires, cap, rotor.

The importance of fresh tuneup hardware cannot be over emphasized for engine efficiency!

The above suggestions are all worthy of doing anyway, but in some driving scenarios/styles 12mpg may be all she's gonna do. I've run about 10 different XJs over the last few years, and all had darned good highway mileage, and pretty crappy street mileage despite my tuning and tinkering. My current street XJ is a 96 4x4 that has gotten as good as 22-23 humming on the interstate 60-70mph, but 10-12mpg on the two lane hilly road I commute to work back and forth. I drive like a grandpa, lowest rpms possible without lugging. I tell you this in case the small mods you do have little to no effect on your mpg - don't let it drive you crazy.

X one million for the O2 sensor. They last between 60-100k miles, and fail softly, so you don't notice. Should be considered a routine maintenance item, along with testing all the aforementioned sensors.

I replace my O2 sensor every 2-3 years just for maintenance. I also make sure ALL, every single one, of my vacuum hoses and connections are tight and in good shape. So many things run off vacuum on the RENIX era Jeeps that it's just good business.

In CA running good smog numbers is (to me) a good indication of engine efficiency, which is the whole point anyway. I get appx. 18-20 mpg overall. Long road trips - maybe 22, round town (and I emphasize town, not city or traffic) and hilly stuff maybe 16-18.

You simply have to spend some money on it from time to time if you want it to run right.

P.S. That spark plug isn't at all OK, it's just shy of fouled. The ceramic should be TAN, the rest should be clean.

didnt read anyone mention the fuel filter. probably one of the biggest causes of bad fuel economy and poor running condition. id say do your normal tune up AND change the fuel filter. see how she runs,then start with the more expensive stuff like sensors. as mentioned though, check that O2 sensor if it has been on there a while.

A clogged fuel filter will cause it to run lean and won't yield a black plug.

In initial post (bottom) fuel filter changed a year ago.

The lighting on the photo isn't the best, but that plug looks like the engine is running with a rich fuel/air ratio.

For clarification: Factory maintenance schedule was that at every 30,000 miles you replaced plugs, wires, cap, rotor and fuel filter.

daman, youre worse than me....
i have 4.88s , 35s and lockers,a dn lots of exta weight...
renix 4.0 195K aw4 etc etc in my sig...

i have :
new 02 sensor MAP TPS(adjsuted properly) cap and rotor, plugs, coolant temp sensor, crankshaft sensor, fuel pump relay, cleaned MAT sensor, newer water pump and radiator air filter, oil change, new vaccuum lines, ford 19lb injectors, 2 guage ground wires , EGR is is great shape and solenoid is in good shape, new battery and 2 guage alternator cables, newer exhaust/intake manifold gasket
tons more to list


and i get around 13.5 MPG on the freeway and way worse in town.
i used to get around 17mpg with 33s and 3.55s...


any one getting better MPG with a similiar setup?

Check your air charge temp sensor.

Yea, just got my jeep running correctly right after I just got it going again and had a bad o2 and coolent temp sensor out of no where cause the last time I dove it before the breakdown I was getting 20mpg lugging it. It was such a lost in mpg compared to normal that I was checking for gas leaks and such cause it still ran great.

The whole milage thing is very subjective, I get about 9-10 miles per gallon but my jeep is usually sitting in NYC traffic (0 mpg), so city driving for one person may be 35 miles per hour with a few lights or sitting in gridlock for an hour.

The single most significant improvement in MPG I ever got with replacing bad parts on my 87 was from replacing the O2 sensor. It immediately increased by 5 mpg city and highway.

I had literally replaced everything except the knock sensor and O2 sensor and was still getting 8-9 mpg local and 12 mpg highway. New O2 is now 14.5 mpg local and 17 mpg highway.

Yep. When they stay in "open loop" because of a bad O2 sensor, mileage really suffers.
Know what else can affect mileage adversely? Low running temp, like not having a 195* thermostat in your engine.

I've paid for a new O2 sensor in a single roadtrip before. Mass to central Pennsylvania and back, stuffed the new sensor in right before leaving and my mileage went from under 10 to around 20 (remember, this was all highway driving, I hit 23 driving like a granny on the highway in the summer but winter gas limits it to around 19-21 highway) instantly.