2001 Jeep Cherokee XJ - 4.0L, AW4, 4x4, ~239,500 miles…

So this all started about a week ago. I purchased this Jeep in December 2022 and it has been leaking coolant onto the ground with a visible green puddle since I got it. At some point, a previous owner installed an aftermarket aluminum radiator, which of course developed a couple cracks on the driver’s side.

A week ago, I had a mechanic install a basic OEM replacement radiator from an auto parts store. I didn’t want to do this myself as anytime I dive into cooling systems I make a huge mess and it takes me all day, and this mechanic was able to do it pretty quickly.

The very next day, the Jeep began throwing CEL’s for multiple cylinder misfires on cylinders 3 & 4. The Jeep has had a rough idle but otherwise drove fine. The CEL flashes when accelerating and stays solid during idle. I took the Jeep back to the same mechanic, and between the two of us, we figured it was likely either the coil rail, spark plugs, or leaky fuel injectors.

I took this opportunity to do a minor “upgrade”/tune-up. I purchased the Viper Coil mod kit from KSuspension along with 4-hole fuel injectors, also from KSuspension. This weekend I installed them both with no hiccups. I also installed the heat shield kit for the fuel rail and injectors from Design Engineering, as I figured I might as well do this at the same time. I also replaced the spark plugs with new NGK plugs.

On my test drive today, the flashing CEL returned. Another trip to an auto parts store revealed that it is P0303 - a misfire on cylinder 3. As before, the Jeep drives fine - perhaps even a little better with the tune-up. Idle is still a bit rough but not as bad as before.

Now I suspect what many of you may be thinking… Cylinder 3 misfires on 2000/2001 XJs can sometimes be a sign of a cracked head. I have reason to believe this is not the case though. The mechanic I took it to actually thought the head on my engine had already been replaced. He didn’t take the valve cover off to check the casting, so I’m not sure how he was able to tell, but he seemed pretty confident about it.
And while I have had to add a little coolant to the overflow bottle in the past week, I believe this is because of the recent radiator replacement as the Jeep purges air from the system. Also, the coolant that came out during the radiator swap was still very green. And I’m actually still chasing a small coolant leak as there is a noticeable green puddle on the ground where I park. I took the oil fill cap off after my test drive today and I did not notice any signs of coolant in the oil.

Some other codes that this Jeep has is a code for the ABS system, which I was already aware of. There is also a code for an evap system leak and a catalyst system efficiency code. I forgot the exact numbers as I reset everything when I did my “tune up” and the P0303 code was the only one that showed back up when the parts store person checked it today. My mechanic suspects that I may have a cracked exhaust manifold, which wouldn’t surprise me as they are so common on these Jeeps, but I have not verified this yet.
So, to review, I have ruled out the possible causes of my cylinder 3 misfire being the spark plugs, coil rail, fuel injectors, and heat soak. I don’t think it is a cracked head but I suppose anything is possible. I guess whoever replaced the head could have put another stock 0331 head on it, and that could have cracked.


Does anyone have any ideas on this? I didn’t mind doing the tune-up as the Jeep needed it anyway, but I don’t want to throw any more parts at it without a proper diagnosis.

Any and all help is appreciated…

 

Well my mechanic performed a radiator pressure test on my XJ today and it wasn't able to hold pressure. So he thinks it is a blown headgasket after all.

Edit:

Are Clearwater heads still everyone's recommendations?

https://www.cylinder-heads.com/produ...head-complete/

Or what do y'all think of this one from Cylinder Heads International?

https://www.shop.headsonly.com/JEEP-...-HEAD-TUPY.htm

Edit 2:

I ordered the one from Cylinder Heads International.

 

Well, to anyone potentially following this thread, I got my Jeep back from my mechanic today after he installed the TUPY head I bought. As he was tearing everything down, he discovered a bent pushrod and broken rocker arm, and these were likely the cause of my misfire. That doesn't explain why the Jeep failed the radiator pressure test though. Anyway, everything is back together and runs as it should.