Surprise overheating
 

Hi Everyone,

I was on my way to work today driving my 1998 XJ8 Vanden Plas and right before I pulled up I felt like something was a little off. I looked over and saw my temperature gauge was all the way in the red, however, no lights came on to warn me, so I don't know how long that was happening for during my 10-minute drive. I immediately pulled over and turned off my car while my engine fan roared on for an extra five minutes. My coolant levels appeared to be fine and couldn't find any leaks, no smoke was coming from my engine. After about 5 minutes I turned my car back on just to pull over to a nearby parking lot and my temperature gauge was back to normal. I didn't want to take any chances and had it towed to a repair shop but they won't be able to look at for a day or two. Does anyone have any theories on what could be wrong and the chances are I already have ruined a head gasket?

Thanks,
Nate

That's bad news as that temp. gauge is pretty much useless & for it to climb that high it had to be very hot for a while. I wouldn't stress the engine any more until after it's sat for hours to cool down & you've checked HG etc..

Wishful thinking, it could be you have a faulty gauge & nothing was out of normal range.

+1 to above.
The most likely causes of overheating without coolant loss are a stuck thermostat or water pump failure.

I couldn't have been driving for more than 10 minutes, and when I turned it on to load it up to the tow truck about an hour later it was below half way. Could that have been long enough to cause some serious damage or can I pray I just need a thermostat or water pump? I checked my driveway and didn't find any spills. From what I've been reading, since my fan was running so much its hopefully a stuck thermostat.

Hopefully it isn't anything more than a faulty engine coolant temperature sensor

My new to me 1998 XJR did something similar, i changed the thermostat but unfortunately it didnt help. Changed the waterpump and all is good. The original pump had the plastic impeller which literally fell apart upon removal.

Intermittant Cooling
 

Blown Income,

The 2 big questions are 1) What happened? What failed, can they fix it? and 2) What did that failure do? Did my heads, gasket or block suffer unseen damage?

Get the thermostat changed out no matter how it looks, or if it passes a shop-test. Earlier this year (fortunately while very cold out) I endured several weeks of a mystery problem that involved kinda-overheating, peculiar "pending code" heat sensor anomalies, and funny smells. I then (finally) caught the thermostat sticking (while driving with a scanner). I've had thermostats fail before, never the way this Jag thermostat did. Where it reluctantly opens, but not enough, causing the engine coolant temp to rise to just below the alarm level, cross above for a few seconds, drop into normal range, rise back up,... But not every trip.

The removed thermostat looked fine, a name brand, and the PO's maintenance file showed it to be only a few years old. And it passed the stove-top boiling pot test. (With thermometer).

But the problem disappeared as soon as it was changed out.

If you still have the original plastic housing, good time to change that, too.

If rest of engine & coolant passes Shop scrutiny, you still won't know if you're home free until you've got a few hundred miles of driving behind you with no more excitement.

But you've got another 'X308 Jag Event' notch on your gearshift!

Coventrywood,

Looks like the scanner tool you have really helped with catching an issue before major overheating took place.

For the OP questions, this is what my 1998 XJR symptoms were which is why I mentioned my issue and what was the problem.

My thermostat passed the boil test with a thermometer but I replaced it anyways but still had the overheating issue, in my case it was the water pump and replacing the original unit with a new one with the metal impeller cured the overheating issue. Being a supercharged engine I already have the metal thermostat housing so no issue there and I have done roughly 300 miles since having the water pump replaced and coolant system flushed and refilled with no overheating issues (at least per the gauge, stays right in the middle regardless of outside temps or traffic flow).

I knew going into these cars there would be some tinkering involved but I don't mind, it lets me get to know the vehicle better. This is my first Jaguar but the aesthetic lines and flow of the car are timeless.

I had this happen once... on the way home I looked down and the temp gauge was pegged. No idea how long it had been like that - one minute or ten. No warning lights accompanied.

The problem turned out to be a blown cooling fan fuse. I double checked the cooling fan relay and the resistance on the fan motors, but everything seemed fine. I drove around with a scan tool for weeks so I could know the exact temp. The problem never reoccurred (years have now gone by) and the engine didn't seem worse for it - no outward problems, great compression. Weird.