Hey all,
I have a 2004 XJR that has been having some issues lately.
I had the thermostat changed last Friday since the coolant temperature gauge would go down to about a quarter of the scale and then suddenly shoot up (without the car actually overheating) whenever the car was driven at interstate speeds for a long amount of time.
I drove at about 80mph for roughly 300 miles the day after and the needle stayed at the mid marker of the gauge the entire time, so I thought the issue was fixed. I did have the P0171 and P0174 codes popping up, but found out that this was due to to the engine breather hose not being reattached correctly to the air cleaner outlet pipe. Test drove the car for about 20 miles after I reattached it, everything seemed to be fine.
The next day the check engine light came on along with the P0128 code (Coolant Temperature Below Thermostat Regulating Temperature). The fan started running (which it has prior only done on hot days when I got stuck in traffic) even though we have 10 degree Celsius (50 Fahrenheit) weather this time of year. Also, the temperature gauge was fluctuating between the second and third quarter. When I pulled over, the coolant in the expansion tank was boiling. I checked the coolant level after the car had cooled down and noticed that I was about 500ml low, thinking that the low coolant level may have been the cause of the issues that came up that day.
I didn't use the car for about five days after that. Yesterday, I refilled coolant and tried to drive to class. About two miles into the journey I noticed the coolant temperature gauge wasn't moving at all. When I pulled over, coolant in the expansion tank was already boiling, so I just left the car there.
Do you guys think that the new thermostat I put in last week may have been bad?
My expertise and experience concerning the subject is about as limited as it gets, so I will be very thankful for your replies.
Thanks in advance!
-Vincent

 

Sounds like air in the system. That would cause the gauge to read low while the coolant boils. The bleed knob on the expansion tank should be opened up while the vehicle is running to bleed it out. It's possible to get air bubbles stuck in the housing that holds the temp sensor and it's right near the thermostat. It's probably warming up just fine and if the sensor isn't reading the coolant temperature properly, the fan won't kick on.

I've been fighting to get all the air out of mine. I bleed it a bit more every few days now. I've had the temp take a long time to register half some days, but the heat is plenty strong within minutes of starting. I've yet to get an engine code like you've had happen.

 

Thanks for the reply, Torrid! I will try to bleed it first thing tomorrow. For how long have you been having the issue?

 

I just noticed it last week with the cool down. I must have caused it to happen when I replaced the expansion tank and flushed the fluid. No problem the last couple of days now, I must have got it.

 

You say you had the thermostat changed. I presume it was at a shop? I'd take it back and have them bleed the system.

If you have an infrared thermometer you can check temps on either side of the thermostat so @ 180 degrees, assuming that's the one you have vs 195. Watch to see that it opens. One side of the thermostat will be hotter until it trips the thermostat and you have equal temp on the other side.

Because, believe it or not, people sometimes install them backwards...

I'll admit I haven't changed mine so I don't know if the space allows for backward install but if you've bled the air out, an infrared thermometer is a great investment.

 

I hope that works out for you! I bled it yesterday and let it idle for about five minutes and afterwards the gauge reacted normally, so I'm thinking that may have been it. Haven't had a chance to drive it yet but I will try that today.
I changed the thermostat with the help of a friend in my driveway so I don't really have the chance to have it drained by anyone .
I know there is no chance that we installed it backwards, but it is possible that the new thermostat is faulty (it's a reproduction thermostat and I read in other threads that it's not unusual for these to fail).
The tip with the thermometer is great though, so I'm probably going to buy one. What kind of thermometer do you have?

 

Sounds more like air in the system as Torrid suggested. I don't recall the brand of the infrared thermometer but I think it was about $30. I initially bought mine to check the surface temp of a catalytic converter.

You can get them at your local auto parts store, Harbor Freight etc. No need to go high end on $$$. the cheap $30 ones work just fine. Point the laser and read. It's instantaneous readout. They work great on other things as well, like the trans pan change, Checking the air temp on the drivers side vs passenger side (point it into the vent), checking either side of the auxiliary pump to see if it's performing. Window and door leaks in your home, a quick little point and shoot at Grandpa napping in the recliner after Christmas dinner etc....

 

Yep, mine is officially good to go now and I was starting to worry either the thermostat was part stuck open or the sensor was bad because my heat still worked great even though the temp read low. After my last bleeding it's been good. People aren't joking when they say this car is a pain to bleed.

 

I have been driving mine the past few days now and everything seems to work perfectly.
I'm glad your's is working, too, Torrid! And thanks a million for the advice!!! If it wasn't for it, I would've probably had the car towed to my go-to mechanic who is 300 miles away.
Also thanks again for the tip, Sean. My dad just ordered an infrared thermometer and that will sure come in handy in the future.