There's a bleed screw next to the cap, have you tried that?

 

Is your coolant low. Have you checked it when its cold? Also does your coolant have the proper level of coolant? A 50-50 mix of the correct coolant and distilled water is what you should be looking for.

 

Yes it is at the full mark and is 50/50 mix. But there it is building air in the reservoir? I tried the bleed screw but water mix just comes out no air the only way to get coolant light off is loosen the cap and bleed the air from there. Crazy

 

If it keeps happening you could have a head gasket leak so do a test urgently.

 

Good point. If that is the problem, you don't want to run it before getting fixed. Could cause a major $$$ issue if not repaired soon.

 

If it is the head gasket would it not run hot or something? Thanks for your help. James

 

The pressure release valve in your cap may be bad. Try a replacement cap.

If that does not fix it have your cooling system professionally examined. It is a false economy to delay repairing any cooling system problem...the negative consequences of possible overheating in these engines are just too high.

 

Same thing happened to me, had to replace everything. I was told if I didn't the pressure would be miss-calculated and could harm the engine

 

I will replace the cap first that is the reasonable thing to do and if still acting up I will take it to the jag dealer. Thanks, James

 

There was a seriously long thread about this - it turned out the float had gone porous. When the system pressurised the float sank.
http://en.wikipedia.org/wiki/Cartesian_diver
The OP fixed it by adding a bit more buoyancy to the float.

 

Interesting... but he says it's "building air". That needs thorough checking.

 

Agreed 100% - unless he's just noticing the initial pressurisation.

 

Realizing this is an old Post, but seems it is a common problem. I just wanted to share my own experience w/this issue. I bought my 2006 Jag on 06/03/13, and within less than a week, I was getting the "Engine Coolant Low" Error. It THANKFULLY was still under the 90 day Warranty, so the Selling Auto Lot where I purchased it, was still responsible for the repair. I stopped by Autozone 2x's after the warning came on intermittently before it finally came on for good (knowing nothing about auto repair or autos in general), I had Autozone check the fluid levels, which they both times told me were max'd out (they also told me, if I put any more fluid in it, then I could cause an overflow, as the tanks & reservoir were full). After the selling Auto Lot looked at my Jag, they determined the code I was getting was a "False Read". I returned for the repair, but seems they tried to save $ the 1st time around and so the bought a used tank (unfortunately, the tank did not come w/a sensor..Duh! Might have wanted to check that before buying). The selling Auto Lot then purchased one new from the local Jaguar Dealership in Baton Rouge, LA (Perretti's). As I lived closer to the Jag Dealer, I volunteered to drop off their check and pick up their purchased part. The Jag Dealership sold the brand new tank to the Selling Dealership for $110.04. I went this past Thursday to have the repair done. They swapped out the tank, and it 'of course & thankfully' resolved the Error Issue!!
The cost of a new tank may be different where you are located geographically, but for anyone local to Baton Rouge, LA, the $110 is basically what you are looking at for a new tank currently, just so you can factor that into your self repair vs replace & your shop for used vs new part decision.
Good Luck & Best Regards!!

 

There's no float in the 03+ stypes. I'm not sure if you're talking about an older model but are you even talking about an stype?

 

 

I have also found that on the occasions coolant has been drained or lost and refilled after repairs. Even though the car has been left running for a while with the expansion tank cap off and the coolant level topped up until it remains fixed, for the next 2 or 3 days of use the coolant will need to be topped up every day.