My car just started to overheat yesterday. It will operate at normal temperature (per the dashboard gauge) for several miles after warming up then the temperature starts to rise and will go to maximum on the gauge if I don't shut the engine off. The weird thing is that when the engine is shut off the temperature starts dropping immediately and rather quickly (20-30 seconds). I was able to drive home by shutting the engine off at lights and/or coasting with it off when the temp reached 3/4 on the gauge. I used an IR gauge on the hoses and sensor. The passenger side hose read 85 C and the sensor reached as high as 95 C. The electric fans were operating almost continuously even before it started overheating.

I bought a new thermostat at an auto store for $14 that is supposedly for my Jag and will install it tomorrow. What other things should I be prepared to look at?

 

Is the coolant circulating?
I would also take a look at the water pump as well.

Do you know the service history of the car? Was the water pump changed to the metal one?

 

The gauges in these cars are basically useless. They just dress up the dash.

XK8 / XKR / XJ RealGauge - TheJagWrangler

For an accurate reading you need either the "Real Gauge" or an OBDII reader.

I wouldn't start replacing anything until I had an accurate reading. There's absolutely no way an overheated engine is going to cool down in 20-30 seconds!




Stu

 

Don't attempt a thermostat swap unless you have a new Aluminum Thermostat tower on hand.

I had to destroy my old tower with an 8mm socket, a cut off wheel, bench vice AND 4lb sledge hammer to get the old thermostat out of it. The 3 nuts spin in the plastic and it goes downhill from there.

New AL Towers are $55 shipped of Ebay.

 

95 C is only 203 F. So assuming the pressure cap is holding pressure, it's not over heated. The thermostat itself is a 195 F. I believe. Defective dash gauge? restricted radiator?

 

If the gauge is showing rapid cooling then i would suspect the temperature gauge or sensor. Plug in a code reader and read off the temperature, if that matches the gauge, change the sensor, if not then you are looking at a gauge or wiring issue? I think these gauges have a feedback pot on them rather than being a dumb gauge, so if the feedback pot isn't reading what the system wants, probably through a dirty pickup on the pot, it will increase the voltage to the needle until its happy with the feedback. Hence the occasional high reading. Working the needle back and forth a few time can be enough to clean it. Just a thought?

 

Lots of good information here.
I will answer questions in the order they were asked:
I don't know the water pump history. I bought the car in Jan. 2014 and other than a low coolant warning a couple of years ago and a cracked plastic water outlet pipe which I replaced last Nov. 2016 the cooling has been fine.

I have no idea whether the water is circulating or if the water pump has been changed. What is the easiest way to check for water circulation? If not circulating I imagine the likely cause is the water pump, correct?

I have an inexpensive OBDII gauge, but I don't remember ever seeing the coolant temperature on it.

I'll postpone the thermostat swap until I know more or have a new aluminum thermostat tower, thanks for the warning.

The cap is holding pressure and nothing is leaking from the engine when it's hot (or says it's hot), though it does smell hot. I have looked for the "normal operating temperature" in the manuals I downloaded from this forum and haven't been able to find it. That seems like the most basic of information for an engine and its either not there or well hidden. I have seen forum threads refer to an 85 C (185 F) thermostat.

I will check the OBDII to see if I can get a temperature history and let you know.
Thank you for all the great ideas.

 

Just for a reference, My coolant temperature constantly sits at 96 C (OBDII reading) during the hottest months in summer, which is about the same as summer temperatures in Houston.

 

My OBDII shows two readings related to temperature: ETC (F) 205; TAT (F) 109
I'm not sure what either one represent and I haven't had the opportunity to research them. Does anyone know?

While I was warming up the engine I did see the temperature gauge go from zero to about mid range in a few seconds and then back down. This was after I had wiggled the sensor plugs. Later I removed two sensor plugs and sprayed them with WD-40 to provide a better connection. I am going to test drive it later with my wife so that I can have her watch the gauge while I play with the sensor wires if it decides to report an overheat again. I'll let you know what happens.

Thanks again for the great info.

 

ETC is the coolant temperature. TAT is is the inlet air temperature at the Mass Airflow meter in the intake pipe.

It does sound like a possible intermittent at the coolant sensor. If you get the temperature gauge to read incorrectly due to a loose coolant sensor connector, you should get a comparable bad reading of the IAT over OBDII at that moment since they both reference the same sensor. The difference is that the gauge data is computer filtered so the needle does not move from 185 to about 240 F while the OBDII data is not filtered .

 

OK, I went out with my wife and it again reported an overheat. I wiggled the sensor plug and wires while my wife watched the gauge and it had no effect.

The thermostat I bought is labeled 84 C (183 F). Since that should be the opening temperature I don't think 205 F would warrant the gauge spiking to maximum. Does anyone know what the normal operating temperature range is suppose to be for a 1999, 4.2 Liter, XK8?

 

There is always some variation due to cooling system age, but 195-210 probably covers 90% of the population.

 

I just went out to recheck the coolant temperature with my OBDII scanner. The car has been idle for several hours and the temperature gauge was reading near its lowest point. On the other hand the OBDII ECT read 216 F. I started the engine and the reading remained at 216 F even though the engine was clearly cool.

The OBDII was also showing a P1642 fault code which I think means there is CAN system fault. Maybe the fault is in the temperature sensor.

Any thoughts?

 

A quick update to let others know what the solution was. I finally installed the new thermostat I purchased in May and it solved the problem. The old thermostat looked great, but I tested it in a pan of water on the stove using an infrared temperature gauge to track the temperature. The thermostat opened a crack at 190F, but that was it. When I pulled it out of the water it closed and didn't reopen even with the water boiling. I should note that I did have the plastic thermostat housing, but unlike another member who reported problems with removing the old thermostat, mine came out easily.

 

glad to hear thermostat fixed issue - marking thread as resolved

 

Urge you to add RealGauge. Works great and takes a lot of stress off my aging mind.