I noticed that my low coolant light came on and saw the thermostat housing had a hairline crack and was leaking coolant. I replaced the complete housing (minus the neck portion to the top of the engine). All was easy to complete, but when I started the car up after filling the reservoir tank the temperature gauge would spike when driven just a few feet. It seemed to do fine while it was idling, but when driven it spiked into the red zone very quickly.

My first thought was a stuck thermostat as that is the only moving part in this assembly. I took it back apart, the thermostat moved freely. During the process of reassembly I torqued one of the screws attaching the thermostat housing front to the middle section too tight and stripped out the brass threaded fastener. So, another tstat housing on order.

While I am waiting, I was wondering if there is there a special procedure to fill the XJ8 coolant that I am not aware of? One would think that after the tstat opens up it would suck coolant out of the reservoir. The reservoir was full, and the level did not change during the heat up.

I thought this would be a quick repair. I put a transmission in a Miata today faster than this little side adventure has taken.

One handy tidbit I learned today, use a telescoping magnet to hold the four screws that attach the tstat housing to the heads. You can insert the screws directly into the holes and turn a few times and then pull the magnet back off of the screw. Voila! It saves burnt/cut knuckles and is so much faster.

 

Off the top of my head it sounds like a air lock or possibly a pump failure. Did you replace with the upgraded aluminium bodied thermostat housing? May be worth putting old thermostat in to see if you have a faulty new one. How fast after starting does temp spike? Also with a thermometer check temp in resovoir as you start it and check temp until fully warmed up or use a laser thermometer on other parts of cooling circuit as there might be a temp sensor failure. I hope this helps.

 

I was thinking a possible vapor/air lock also, although it seems remote to me at the current time. That is why I was curious if there was a particular trick to filling the coolant system after cracking it open. The temperature spikes within 20 seconds after putting the car in gear and idling away slowly. It seems to be fine at idle. I have a digital pyromter which I can check temperatures with accurately.

 

I put on the second thermostat housing yesterday. I undid the bleeding cap on the coolant reservoir, filled the reservoir, replaced the bleeding cap and the reservoir cap. Started the engine and turned the heater on high. The fluid started flowing and no overheating now.

The other thermostat housing was probably just fine, the problem was I didn't undo the bleeding cap before adding coolant. Thanks to Brutal in Houston for helping me out with the coolant addition steps.

 

Glad it is fixed!
I had that same problem years ago on an old 289 v8.
Second time fixed it too. Followed directions and presto. Fixed.