Guys, need help. I occasionally get a "Coolant is low" message on the computer in the Jag. Sometimes when I check it, it isnt low at all. Other times when I remove the cap, I can actually hear what sounds like a big "gulp" coming from somewhere in the cooling system. At those times, it is low so I refill usually with water. My exhaust guy tried to do a pressure test but didnt have the right adapter for my car so he couldnt. I dont see an obvious leak anywhere, but with this weather it may have one I just havent seen yet. The oil is luckily clean and free of any coolant. Ideas?

I should also mention the temperature NEVER goes higher than normal thankfully.

Could be a multitude of causes. Find someone with the correct adapter would be the first step were it me. With no obvious leakage seen.

First, the temperature gauge is an 'idiot gauge' that will sit right there in the middle over a 35*F range. Jaguar owners have no need to be bothered with such detail.

I have 'rented' pressure test kits from Advance, AutoZone and O'Rilley and none have the correct adapter for the Jaguar reservoir but every one had an 'universal' adapter that worked. It looks like a pointed ice cream cone and is inserted and expanded to allow pressure testing. Fifteen psi will show up the leak easily and you do have one.

Ok, heres a REALLY stupid question. Where the heck is the radiator cap? All I can seem to find is the reserve reservoir towards the left side near the firewall. What am I missing?

Hey Muti-

This is going to sound really odd. Are all of your gauge cluster lights working? I.E. When you turn on your lights none of the instrument cluster lights are out?

-Mitch

Yep, all gauge and dash lights are in good working order. Really curious as to what you were thinking

You have a leak that must be attended to before bad things happen.

In the interim, do not refill with plain water as this is diluting the antifreeze capability of the coolant. Use a 50/50 mix of the appropriate spec.

You should attend to this immediately.

If your small leak were to become a big leak the coolant level can drop below the temperature sensor and you may never even get an overheating indication. These aluminum engines can catastrophically overheat very, very quickly.

Broken tensioners and overheating are by far the two most common causes of failed early Jaguar V8 engines.

The equivalent of the radiator cap is the cap on the reserve reservoir you refer to.

Picked up a gallon of 50/50, the kind that can be intermixed with all others. I had been doing mostly water but also added antifreeze a time or two. Getting her pressure tested later this afternoon. Hopefully it isn't a big issue.

Another odd thing I've noticed is if I start the car with the cap off, once it's up to fully operational temp, the coolant level in the reservoir never goes down. In fact it ends up pushing some coolant out. I'd expect at idle and operating temp, the level should drop

This is a pressurized system, and if you take the cap off the boiling point is reduced, so the coolant will expand and begin to spill over. It is not intended to be operated with the cap off.

Thanks for clearing up my misconception

Again, thanks so much to all of you that have tried to lend a helping hand. I had it pressure tested and the only leak they could find, and a tiny one, seemed to be coming from the hose that goes from the top of the motor to the water pump. The timing chain was replaced just before I bought my car and its possible they did a sloppy job of putting it back together. When the tech went to check the clamps, they definitely seemed loose. So for now, he tightened up those clamps, cleaned and dried everything, than pressure tested it again and so far, knock on wood, nothing seemed to leak again. Im hoping it was just the loose clamps on the hose that were causing a tiny leak. Of course, my fear now is that it could either be the water pump or God forbid, the head gasket.

With all thats going on, I figured I may as well replace the coolant tank cap while Im at it. Does anyone know if the STANT 10238 will fit? Amazon says yes, Advance Auto says no. I bought one and screwed it in, and though it looks very different than the one my car had on it, it screwed down what seems like perfectly. Anyone ever use one of these?

Does our cooling system HAVE to be at 15 psi specifically, or is there a little room in there for variance? The cap I bought is 16psi but fits the car perfectly. Should I be worried I have the wrong cap?

I wouldn't worry about 1 psi difference. Could be more difference than that between the same type caps.

I am concerned about the STANT 10238 being the correct cap.

Advance AP calls for the STANT 10263 which has a 'neck' that extends below the cap with an 'O' ring to seal the reservoir from the overflow bottle. This is what the correct cap is supposed to look like:

Pictures of the 10238 do not show the 'neck' and you would therefore be running with a completely unpressurized system.

The 10238 does look different in person and the neck is not as long. I think it did have the O ring though and screwed right in nice and tight. As Im sure you saw, Amazon says it fits the car. Other vendors seem to be split 50/50 over if its the right cap or not
The one on the left is my old OEM cap, the one on the right is the replacement. Would you be worried that although the new one fits nice and tight, the neck on the OEM one is a bit longer? I doubt the new one would have me running unpressurized, as it is still afterall a radiator cap rated at 16psi and fits on to the reservoir tank perfectly. Do you guys think I should be worried about the new one even though Amazon says its a fit? Next to each other perfectly side by side, the new one is MAYBE 1/4 inch shorter.

I think I answered my own question. Started her up and let her run for about 10 minutes and coolant started to seep out, so I wouldnt recommend buying the 10238. Hope this helps someone as Amazon has this listed incorrectly

An OE cap is only about $20, partno MJA4440BA.