My 1995 XJ Cherokee Sport with 350,000 kilometres has had a random misfire for the last 18 months or so. The CEL sometimes lights when the misfire is severe. It does not come on again after I switch off for a few seconds and restart. The only fault codes it shows are 12, 13, 14 and sometimes 15 (battery disconnected, MAP sensor vacuum and voltage too high or low, and no speed sensor signal detected). 55 (end of code output) does not show up. When misfiring under power, it backfires in the exhaust system. The backfiring seems to occur in the catalytic converter. Fuel consumption seems to be higher than when it is running well.

Over time, I have tested and swapped sensors that proved to be not faulty after all, checked for vacuum leaks and dealt with them apart from the manifold gasket which may be all right. I tested most of the circuits where they are more easily accessible. Continuity, resistance and voltages are correct. I have replaced fuel pump and strainer, fuel filter, oxygen sensor, ignition coil, distributor rotor and cap, spark plugs and all high tension leads, but not all at once. I had the injectors cleaned ultrasonically. The mechanic also found filings or some other rubbish in the fuel rail. That gave more power that I did not know it had. I would check the effect of each "fix" before trying something else. Compression was good when tested some time ago. Oil consumption is good and due partly to leakage. When not misfiring or about to misfire, it does not lack power. It has never refused to start with a good battery. Earlier on, it would idle well but now the misfire sometimes occurs when idling. It does not stall. The automatic transmission gives no trouble. The misfiring becomes less after 15 or 20 miles of continuous running and when the engine is well warmed up. Coasting in neutral when it misfires seems to help. The misfire seems to affect all cylinders and all six sometimes cut out for a moment. There are a few more things I could check but not till the weather here improves.

I have to solve this problem myself because I can't find a mechanic with experience of the 90's models, not even at the Jeep dealer seventy five miles away. It seems as though something is gradually deteriorating with time. Could it be the capacitors in the computer? The CEL has never failed to light for a few seconds before start up.
each fix seemed to improve matters for a while. Then the problem would come back.

 

Fix the vacuum leak, and clean the engine grounds. Check the fuel pressure. That "rubbish" in the fuel rail could be an issue if its restricting the flow, whatever that rubbish was. Did you happen to look inside the tank when replacing the pump? Its not uncommon for the tanks to need a good cleaning.