I have a 2001 XJ8. Today while driving on the freeway I caught a red lightout of the corner of my eye. When I looked down I saw the temp. gauge in the red. I immediately started to change lanes to get off the freeway. By the timeI was ready to get in the off ramp lane the temp. gauge was back to normal. I decided to keep going. 3 minutes later the same thing occurred. temp. gauge went to red and warning light came on. 10-15 seconds later temp. gauge went back to normal. I turned the stereo off to see if I could hear anything unusual. Everything sounded fine. 3 minutes later it occurred again. This time with the radio off I could hear the fans kick on and 10-15 seconds later the temp. gauge went back to normal. Each occasion I was driving between 65 and 80MPH, the A/C was on. When the temp gauge went to red the A/C started blowing warm air. Driving at normal (45 MPH) street speeds never duplicated the issue. I stopped and went into my appointment. I came out 3 hours later and started driving back home. The drive back home never duplicated the condition even though the driving conditions were similar. What could the issue be?

I purchased the car recently from a friend that is an auto mechanic. He purchased the car from a lady who let it sit. When she went to drive it again a valve seat dropped and caused the need for repairs. Due to the cost for the repair she dumped it (for almost nothing). He bought it from her because of what she wanted for it. During the repair he took the heads off and completed a lot of maintenance to ensure the car was in good condition. Obviously due to the head removal he drained the coolant. Knowing the problems with thermostats he replaced it priort o completing the job and filling the car with coolant and completing the burping process. He is aware of the issue as I talked to him about it today and he is baffled but thinks it may be a flase over temp. issue due to a lose wire or faulty sensor.

I do have a Bluetooth OBD unit on order so I can monitor the engine coolant temp. and error codes from my android phone using the Torque 2 app. This is not due to arrive until Wednesday though.

Any help or thoughts that anyone might have would be helpful! Cheers!

 

The first thing to do is check the fluid level. You may have a faulty temperature sensor or a bad connection to the sensor, or the thermostat may be malfunctioning. There are a lot of possibilities.

Whatever the problem, you are taking a chance driving the car until it is resolved. These engines are very sensitive to overheating, and a dropped valve seat is one result of overheating. The fact that it happened previously would make me very nervous.

 

The coolant level is full and there is coolant in the overflow. There are no signs of a actual overheat occurring. Where is the temp sensor located at on this engine? Also do you have any data on the electrical layout and and attachments between the sensor, ECM, and gauge?

I am new here so I am still trying to locate some of this information.

 

The sensor is at the front of the engine in the right hand side of the plastic pipe that connects the radiator hose to the engine.

You can get electrical diagrams and everything else by downloading JTIS21 from the link at the top of the forum page. There are DIY articles also. You may need to send 10 messages and be a member for 3 days to download pdf docs.

 

It might be that your water pump has lost all its impeller vanes - ask me how I know?

My XJ8 suddenly showed red on the temperature gauge when I stopped at some lights, as soon as I started again the temperature dropped back to normal in about five seconds as soon as some ram air was coming through the radiator. Stopped again, up it went, moved off, down to normal. No overheat light came on in the central warning area, just the temperature gauge shooting up to maximum and the red light there coming on. No loss of coolant. Water pump changed, no problems since. Worth checking out.

 

That's interesting. If they are missing how do you verify that you have removed all of the broken pieces from the coolant system?

On another note... I received my new odb2 bluetooth tester. I paired it with my android phone and the app torque. This app allows you to monitor info from the odb port in real time. I went on a 50 mile drive trying to duplicate the issue. I was not able to duplicate the issue. I monitored the temp. the entire time. The temp stayed at 195.8°F while driving at 65-80 mph while on the freeway.while driving on the street (start and stop conditions) at 35-50 mph it stayed between 195.8°F and 203°F. The ambient temp was pretty close to 88°F. Are the temps for the coolant system normal or a little on the high side?

 

IMO that's the quite normat temp.
Mine have 210-215°F all the time. The ambient temp may vary from -20°F to 88°F, but the engine temp. stays the same.
The pump, thermostat and coolant are brand new, A/C is always on.

 

The hope is that the broken pieces are so small after they have been mashed up in the pump by the remaining blades that they have drained out with the coolant. At least, that's what my mechanic said, and I do trust his judgement. The pump is surprisingly small, the active area is probably only about 3" in diameter.