Background. Purchased a 1998 cherokee classic with 136k miles at $500. It had a cracked head so I never saw it run. It came with all previous repair records. Nothing stating it had a misfire. I replaced the head with one off of a rebuilt engine. Had it machined. Shop said it was probably a new replacement. My father got the jeep running and pressure washed the engine and bay. He met we with it. In order to get home I had to drive up a mountain. In the course of that, the CEL started flashing and I could feel it missing. Initially it was just cylinder 3, then also said 5 and random.

So far we've tried:
New Distributor cap and distributor shaft.
New plug wire's
New plugs
Tried changing the ECM
New injector on cylinder 3.

I'm only getting about 12.5mpg.

Any other suggestions on what to check?

Thanks

 

In the following case, under similar conditions it turned out to be a failing/erroneous crankshaft position sensor. The author of the video (pretty experienced tech) even had a tough time narrowing it down, because only under certain load (higher RPM) conditions, did it present itself. No problems starting/idling, or even running most the time.

 

Several years ago a shop got similar codes (had other problems too) and the root of the problem was burned wires near the exhaust manifold (factory wiring and routing) that were shorting together. The shop replaced and rerouted the wires, that problem gone. Sadly, that shop is gone now too

 

Found the problem. Bad spark plug on #3. It was brand new.. My father said in his 50 years he's never seen a plug that didn't fire

 

In my many years, I have found several spark plugs with cracked insulators right out of the box and electrode gaps closed. This is generally caused by rough handling in shipment and not poor manufacture. I use NGK plugs in my jeeps and they come with cardboard tubes around the threads that protect the electrodes pretty well. Some people say that new plugs come pre-gapped for the application they are made for, but I ALWAYS check the gap before installing them, leaving nothing to chance.

There is one thing about the 4.0L that you have to watch out for. Even though it produces lots of torque below 2000 rpm, pulling a load at say 1500, will tend to burn exhaust valves. When it's working pulling uphill or towing a trailer or loaded with camping gear, downshift to keep it between 2000 and 3000 for best longevity. This is the main reason manual transmission jeeps tend to last longer than automatic trans ones. You have way more control over engine working parameters then relying on the auto to do it.

Just my opinion and not intended to be gospel. Personal experience seems to support this however. I drove my 2000 WJ 4.0L for 6 years with a converter that didn't lock up, so it was running around 1800 at 55 mph. I live in the hilly terrain of central upstate NY where to state speed limit is 55 and the load on the engine constantly changes. Then the converter decided to lock up and the engine would run at 1500 at 55 mph. 6 months later, engine power dropped off and I had 3 burnt exhaust valves. Now, after fixing it, I run those same roads in 3rd gear at around 2200 in lock-up, but the engine is happy with that.

 

I've been using Bosch Platinum +4 since my first plug/wire change. Hardly ever need replacing, but I am not an off roader or heavy use owner. 352,000 miles and maybe two sets, and the second set probably wasn't needed yet. Have had issues with the coil wire on a set of wires before, but not those plugs.

 

I am not a great changer of plug leads either & dont consider them a wear item, and only change them if they appear to be damaged, or failing (which is almost never)

The times I had a problem, was AFTER changing them, probably new rubbish made in who knows where

believe one of my XJ has factory original wires, over 210k miles

I do change spark plugs at factory recommended interval

 

Some advice I caught from Pat Goss on his radio show was to leave plugs and wires alone until a performance difference associated with plugs and wires comes up. Like drop in mileage, misfires, iirc. Then check them and replace if needed.

My commute is about seven miles each way these days, so a drop in mileage might be hard to notice. All of my stalls and engine stumbles lately have been due to wiring outside of the plugs zone.