• Welcome to the new NAXJA Forum! If your password does not work, please use "Forgot your password?" link on the log-in page. Please feel free to reach out to [email protected] if we can provide any assistance.

How to troubleshoot the XJ radiator explosion.


NAXJA Forum User
Northern CA
99 XJ, 230k

80F outside, AC on, was going slow up a long hill and stopped about 5min to check email. Check Gauges light came on sitting there. Drove about 300 yards further, gauges looked fine. Parked in my driveway.

As soon as I shut the engine off... I heard the coolant in the radiator start to boil into the overflow. Same thing happened 5 years ago due to a bad radiator cap. 30 seconds later... BOOM. Radiator explodes at the top right by the top hose.

I was VERY lucky. I was just starting to open the hood when it blew, a few seconds later and it would have exploded right into me.

Oil looks fine. So does coolant. Only thing I can think of is I don't recall hearing the electric fan on as I pulled in, which is usually very familliar. Fan blade doesn't spin free so I presume clutch is fine. It's the original T-stat/ water pump. Flushed my coolant 3k ago.

This story seems to match a lot of the other exploding XJ radiator stories where it blows up parked in the driveway and everyone calls BS.

Why does this happen? Does boiling on shutoff indicate a problem in a specific area?

What do I troubleshoot here? Just shotgun and do radiator/ pump/ and T stat now?

What's THE best radiator to get?

Well I think I found the problem. The electric fan had come unplugged. Probably my own fault not seating it properly several months ago.

Knowing that any reason to change the water pump and thermostat components? Should I just do it while I'm there or can they keep going?
You are going to be in there with the coolant drained, why would you not replace everything ? Preventative maintenance helps avoid breakdowns and tow trucks. When my 1998 blew a radiator it cost me $250 and 4 hours of time to get it home.
I'm with Tim. You already got the radiator out, might as well do the WP. I would definitely do the thermostat. Also check your hoses. Flush the heater core.
Does it make a difference who supplies the fan clutch? Looking at some of the duralast reviews it sounds like they are engaged all the time. OEM the best option?
I do NAPA clutch. They seam to work. Currently there is a ZJ version that is much larger but sounds louder and longer.
I would order a thermostat housing too. They often corrode over time. There is a high flow version too.
I think, would have to look but too lazy, there is a bleeder too.
So I replaced the entire cooling system. Everything went smoothly.

However after installing the Spectra radiator my hood no longer latch's well. On the driver side I have to push it down several times to try and get it to click and even then it doesn't seem like it's clicking well.

I feel like I'm missing something obvious.

Is it possible the radiator height is different or is there something that I could have done to screw this up?
Got it. One of the latches had clicked itself shut.

For anyone that's wondering I did end up getting an oem water pump and returning the aftermarket. There was a huge difference in the build. The casting was totally different shapewise. There was also a much bigger gap between the pump body and impeller on the aftermarket.

I ended up testing the thermostat and it was in fact bad. It did not open in boiling water. The Mopar thermostat looks much more substantive than the AutoZone model but I didn't have a replacement for OEM so I just put the aftermarket in after verifying it worked. I did not use additional sealant on the gaskets and they seem to be holding.

The specter radiator looks almost identical to OEM and went in without any problems. For 80 bucks on Amazon it's hard to justify anything else.

Crawling uphill with the air conditioning on I could not get it over 200.