I Have a 91 YJ 4L 6 cyl. My fuel consumption has dropped dramatically since about 2500km ago when I did a service. New oil and filter , fuel filter, serp belt, spark plugs, injector cleaner in tank, thermostat, K&N air filter, new temp sensor. I am getting about 80 to 100 km less per tank. I have also inspected the fuel lines and replaced the exhaust system and o2 sensor. Please help.

SOURCE: Won't accelerate past 40 miles per hour

these jeeps did have a small problem with the tp sensor. i would also have the back pressure of the exhaust checked to see if you have a catalytic converter choked up.

SOURCE: jeep wrangler yj 1987 2.5l problem????

check air flue meter may be sticking put matchstick in the hose to let more air in if this works change air flue meter

SOURCE: 96 Jeep Cherokee RPM Surge While Driving

If the problem began before you changed the plugs that they are likely OK...Some engines "like" certain brand plugs better than others. You may have a weak ignition coil. Poor spark will generally show up at higher rpm's. Spark should be blue and have good "snap" to it and should easily bridge a 1/4" gap or better. If fuel was contaminated, it should show up at lower rpm's as well. System cleaners sometimes work but often are added far too late to do any good. (tank can hold up to a gallon of water without noticing it.) I use 3oz of acetone and one oz marvel oil per 10 gallons of fuel about once a month...cheaper and more effective than most cleaners. (and does not "melt" gaskets and seals as legend has it) The vehicle speed sensor can also give you problems but needs testing to determine if it's got any problems.
It's difficult to diagnose by remote control, but I hope you benefit form something here!

SOURCE: Motor is a 4.0 in

Check your EGR valve; if this is gummed up, it could be the cause of your Jeep not wanting to run.

SOURCE: excessive fuel consumption, backfire on decelleration, and rough idle

check your pre-cat or even main cat-converter