One of the most problems happens in the Middle East specially Saudi Arabia the the older car is the more ignition system damage , so here is one of the most common problem happens when the cherokee not been used for long period

Lot of white smoke coming out the exhaust pipe, the RPM is less then normal sounds like the engine with shut down and fuel smell coming out from the exhaust system

So how to solve this slow fuel burning in the engine. Is changing the spark plugs and ignition wires enough ?

 

Pull the spark plugs and inspect. If one is consistently not firing it will be black. Start by replacing the black plugs. Also inspect the distributor cap and rotor, plus the plug wire. Any of those could be the problem.

The black plug(s) will point you to where to look next.

 

Is the jeep in Arabia?

 

The plugs are giving a very low power need to be changed

And yes in Arabia we have a turible vendor that don't give any maintenance

 

did you change the cap, rotor, plugs and wires yet?

 

Always a good idea to start with basic things first so that means a full tuneup, which for your 1991 consists of fresh Champion copper plugs (gapped to .035) and quality spark plug wires, distributor cap, rotor, fuel filter AND air filter. Then see how your Jeep performs after the tuneup.

How many miles on your XJ? Never a bad idea to perform a compression test on all cylinders. A good "snapshot in time" of the internal condition of the engine. The compression spec for the 4.0 engine is 120-150 psi, with no more than a 30 psi variation between cylinders.

Have you ever seen the "check engine light"? If so, pull codes using the following technique and post the exact number(s) here. Good luck!
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Retrieving OBD codes on the Jeep Cherokee (XJ) OBD1 Models: 1991-1995

1) Start engine (if possible). Move transmission shift lever
through all positions, ending in Park. Turn A/C switch on and then off
(if equipped).

2) Turn engine off. Without starting engine again, turn
ignition on, off, on, off and on within 5 seconds. Record 2-digit
fault codes as displayed by flashing MIL.

3) For example, fault code 23 is displayed as flash, flash,
4-second pause, flash, flash, flash. After a slightly longer pause,
other codes stored are displayed in numerical order. When MIL begins
to flash fault codes, it cannot be stopped. Start over if count is
lost.

NOTE: A "5-5" will always occur to signal the end of the sequence. Some will flash the "1-2" code indicating a recent battery disconnect even if you haven't done it. Ignore these two codes, and mark down all others.