Drain Petcock for the Radiator?


Can someone please settle the issue here … Does our face-lift 4.0s have or not have a drain cock on the bottom of the radiator? I remember my XJ40s had one, so I can’t believe Jag didn’t think to have them also on the XJS models (but then again, I have been surprised many time by Jaguar :roll_eyes: ). I have looked under my radiator on Superblue as best I can, but can’t spot one. Also, I notice the factory service literature always seems to instruct, as a first step, “raise the car and remove the lower radiator hose to drain the coolant” every time something in the cooling system of the XJS has to be worked on. Reason I’m asking at this time is b/c I need to drain off a bit of the fluid and replace it with 100% coolant, as I tested the coolant in the system recently and it’s only good to about 25 deg. f. (i.e. only one ball floating on top in the testing hydrometer). We have had a cold snap here of late and I’m worried the temps may drop close to the 20s. :grimacing: I’m worried if I pull off a main hose it may not seal back up 100%.

(JLo) #2

I cannot speak for your facelift however my 1989 xjs did NOT come with a petcock. I had an 88 which DID have one. I can only assume that around that time they forewent adding them. You may have to do the messy bit of pulling the hose.
I had added this drain plug and hose for future endeavors.

(Kirbert - author of the Book, former owner of an '83 XJ-S H.E.) #3

Cute. Have you installed it yet? I’m wondering if you can even get to that lever.

What I did, having a car that originally had the petcock, was to replace the petcock with a simple hose barb – which was anything but simple because the port had BSPP threads. I connected a hose permanently, as apparently you did, and routed the hose out the bottom of the car. Then I plugged the end of that hose with a plug and a clamp. That way when I want to drain it, I only have to loosen the clamp and pull the plug out of the end of the hose. Much, much easier than fiddling around at that bottom corner of the rad.

(Veekay) #4

No drain petcock on the facelift radiators.

(Bernard Embden) #5

Instead of a plug at the end of the hose you could install a petcock.


(JLo) #6

Will take pictures of it in the car sometime soon. It’s a very tiny ball valve and it’s very accessible when compared to the choice of pulling the lower hose off.
Forgot to say; I added a second valve at end of hose. Reason for this is overkill. But I didn’t care for the hose to always be pressurized. But if for some reason my tiny little ball valve does open there is a second line of defense.

(JimD in Alabama) #7

A method I use to drain some fluid out is to stick a siphon hose all the way into the Expansion Tank, and with the Cross-over cap also removed, let gravity and atmosphere transfer fluid to an empty jug on the garage floor. Ought to help you out for what you intend to do there in Dallas.

(Greg) #8

all previous car’s I’ve dealt with have flimsy petcocks (usually plastic that breaks off), i know it’s messy, but I always just unhook the lower radiator hose.

Will this work on the V12?

(JimD in Alabama) #9

yeah, for full drain that is the way unless you have an older car that has a drain petcock or has been modified.

Since attydallas3 just wanted to change his solution, i suggested siphon off a gallon or so.

(Kirbert - author of the Book, former owner of an '83 XJ-S H.E.) #10

That was my concern. Draining the coolant is supposed to get all the crud out of the end tank, but if the passages in this petcock are too small – as, IMHO, they are in the OEM petcock – the crud won’t pass through, it just drains the liquid and leaves the solid chunks in the rad. If I were designing a rad, the drain plug would be at least a 1/2" opening.

(Kirbert - author of the Book, former owner of an '83 XJ-S H.E.) #11

Disconnecting the lower hose will work, but it makes a helluva mess on the garage floor.

As the lower radiator hose on the XJ-S radiator is far above the bottom, it doesn’t work well to clear the crud out of the tank either. Plus it’s on the downstream side of the core, so the only crud there is crud that already made it through the core.

And I agree, the drain petcock on many cars – including my Hondas – is too restrictive to drain solids, only liquids will get drained. It’s almost as though carmakers want your car to have cooling problems when it gets older.

(JLo) #12

I used 3/8”. My theory is that if I flush it at least once a year coupled w coolant filters, it should stay fairly clean. Time will tell.

(Greg) #13

I assume we’d have to worry less for crud if the radiator was made of aluminum instead of steel?


Copper, with some brass bits I believe. The crud mostly comes to the rad not from. Basically a big filter for anything that can’t make it through the core tubes.


Removing the lower radiator hose to drain the radiator is almost impossible and can be life threatening

LOL! :laughing:


I thought about that possible method, too, Jim … Guess I’ll give it a try (btw, mine is 4.0, so no “crossover cap” - right?). :slightly_smiling_face:

(Kirbert - author of the Book, former owner of an '83 XJ-S H.E.) #17

Presuming you meant aluminum instead of brass? If your rad is made of steel, your car is probably overheating!

Correct, though, because the tubes in an aluminum radiator are much bigger and therefore harder to plug up. Once plugged up, though, you’ve got to throw the rad away and buy a new one. “Rodding out” a rad doesn’t work with an aluminum rad.

(Greg) #18

yeah, sorry, I meant brass.

My Volvos have aluminum radiators, and are super clean. Only issue? These aluminum radiators have the brilliant idea of mixing plastic with aluminum at some of the joints. Of course they contract/expand at different rates, and sure enough, after about 10-15 years they start leaking at the seams. Why don’t manufacturers make 100% aluminum radiators?

(JLo) #19

Right. Always wondered that too aside from the cost reduction from manufacturers. Often many models share same aluminum radiator core, and based upon the model they place the corresponding plastic top onto each.