I have a 2000 S Type. It occasionally overheats. However, it does not overheat when either the heat is on or if the A/C is on. When it does overheat, the temperature will almost immediately return to normal when the heat is turned on. I understand why it would not overheat when the heat is on, but why does the A/C prevent it from overheating? I am trying to track down the problem, and learning why the A/C prevents the overheating may give me some insight.

Thanks for you help.

 

It may be that it's to do with the (engine cooling) fan. It should be on when the a/c is, I think.

I'd want to use OBD (easy and very cheap to do) to see what the temperature sensors are reading.

It may be that either a sensor is wrong or the thermostat is not operating properly (which would be seen in a dubious sensor reading I think).

As you have the 4.0 you have a bigger issue - it will need its timing chain tensioners & related parts changing.

Oh, and the 4.0 will NOT tolerate overheating - warped heads are a serious risk - so you need to fix it ASAP or not drive it.

 

. . . or, and it might read as unlikely because it would entail failure of your coolant level sensor . . . your coolant level is way low and/or you have significant air in the system. Activating circulation of the added coolant in the normally closed HAV circuit when either the heater or blended AC system opens the circuit, would aid engine cooling across the area where the temp sender is located.

As JagV8 advises for the 4.0L V8, be ultra careful of both major issues of TC gear and overheating. Many a sad soul has emptied their wallet and exhausted their Jaguar experience on each. Above all, be informed . . . eg read furiously rather than drive until you are on top of both. For overheating prevention, read how and when to check correct topping up and air bleeding procedure. All the resources you need are in Stickies here or made available by another of our stalwart gurus, Gus, at JaguarRepair.com - and do drop him a "thank you".

Cheers and best wishes,

Ken

 

That's how my '02 V6 is. I'm not positive about the V8, but think the logic for the fan is similar.

What road speed are you at when the temp rises? Generally, above 30 MPH there is plenty of ram air flow through the radiator and the cooling fan is not needed. When my fan motor failed a couple of years ago, I was still able to limp home by taking the scenic route and avoiding traffic. As long as I kept moving, the temp stayed in the same range. Stopped at a light, I'd only have a couple of minutes before the temp would start to climb.. As JagV8 has said, the 4.0 is much more susceptible to overheat damage, so do be careful, but some more details about your road speed may help narrow down the root cause.

Also, be aware the temp gauge is deliberately dumb. There's a wide temperature range at which the needle is misleadingly centered. Without knowing this, you might miss a temperature increase until it's too late:

https://www.jaguarforums.com/forum/s...tation-152099/

 

DO NOT neglect the advice you got about the timing tensioners!!!!!
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+1 . . . tick, tick, tick, tick!
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Unfortunately, the overheating can occur at any speed, including 70+ mph on the highway.

I appreciate all the advice, and I have not been driving the car. I am afraid of causing damage. I am wondering if it could be a faulty sensor. When it shows to overheat, the temperature rises from normal to overheating in 1 - 2 seconds. When the temperature returns to normal, it also does it in 1 -2 seconds. I have never had any other symptoms of it overheating, and have considered using an IR thermometer to check the actual temp. Of course, I am leery in doing this, because if it is really overheating, the time it takes to check the actual temp. could cause significant damage. I am going to try to find a post on how to bleed the air from the cooling system and see if that helps.

Thanks

 

I think you are going about it the right way. May not be over heating as I have a hard time seeing anything over heat at 70+ mph. That's a LOT of ram air for cooling.

Unless the water pump is not pumping but that would not go away like your over heating does.

Stay with it and whittle down the problems one by one!
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Maybe a broken up impeller whose pieces move.

 

Sorry, but respectfully, I disagree that you are going about this the right way. Until you fix the timing chain issues, any further running of the engine to chase down causes of (perhaps) transient overheating or (perhaps) aberrant temp sensor or gauge . . . is to ignore the very real risk of turning your V8 into a very under-powered V6 or V4.

I know I'm bashing this warning, so I'll retire my comments after this. Even after all these years, it's sad to see examples still not addressed. I recently saw the internal damage to ignored warnings that resulted in a high RPM loss of 4 cylinders . . . and that dead engine led immediately to the car's write-off.

Added benefit in addressing this, is that you will access and can check the water pump but, more importantly IMHO, you will have the priorities right.

Ken