The parts manual seems to indicate a 15lb cap for my 8L40403 engine, but I seem to find some rhetoric indicating I should use a 13lb cap. Any ideas? I do know it's important that the cap have two rubber seals - one below the spring and one to seal top of cap.

 

the higher the cap pressure calibration, the more pressure buildup before it opens. That is what I have been told.

For example, my '65 3.8 engine came from the factory with a 4lb. radiator cap, then it was "factory updated" through a Service Bulletin to 7lbs., to avoid blowing up the radiators of the 1960s I suppose.

12lb. seems to me like a better radiator cap calibration, not 15lbs. not 13lbs.

 

Higher pressure = higher boiling point.

From memory, at 15 psi a 50/50 water/coolant mix will boil at 258ºF at sea level. A 13 psi cap would lower that to about 250ºF. It's all a bit academic since our engines typically run in the 180º-210ºF range and we pull over to the side of the road (well, most of us) long before reaching boil-over point.

15-16 pounds is almost an industry standard; very common since the 70s on most cars.

Cheers
DD

 

12 or 15? Either should do just fine.


Carl

 

Yep, I use 13 on ALL the Jags, never had an issue, yet.

I am a firm believer that as things AGE, the less stress the better, us humans included. I have only done one heater core on a RHD Series car, and never again, so lower the pressure, lower the stress, and motor on.

 

Now I'm confused. Just received my new 13lb overflow cap and thought the cap was supposed to have TWO rubber seals - a smaller one at the bottom under pressure by the spring (which the one pictured has) and a second one just under the top of the cap to seal the top (which this one does NOT have). What is correct?


Thanks, Bill

 

What you've got looks correct. Are you getting confused with the blanking cap at the front end of the water rail on the cylinder head. This has a seal under the securing lip, (obviously), but of course is just a cap without any pressure spring and seal.

 

Not, not confused (a little dizzy maybe). It just seems I read somewhere in some forum that the overflow cap also has the upper rubber seal (just like the thermostat housing flat cap.

 

Well your memory may be correct ! It's just that I don't remember having anything like you describe on my Series 3, but it was a long time ago that I had it, from 1988-2002

 

That cap is correct for the Series 2 cars as made by Jaguar.

There was NO atmospheric overflow tank, so no need for the upper cap seal. The excess coolant simply spurted on the ground, and the system found its own level.When an owner fitted an aftermarket "catch container" AKA an atmospheric tank, then YES, the upper seal is then needed, to allow the return of coolant to the radiator from the atmospheric tank as the system cools.

 

Many thanks - onto other worries.

 

I have a Series 2 overflow metal tank and it has a cap with the following markings:

CAC 4568 Unipart 15 psi.

the cap does have the top and bottom rubber seals.

 

Well, now I'm confused! Does your Series 2 have an overflow to the overflow metal container?

 

I don't have a Series 2, it's just the metal tank with cap I got in a parts purchase I made and I got a bunch of Series 2 parts.

what I posted is what the cap has on it, and please note I said 15 PSI, not LBS.

 

apparently it should have both seals, at least based on the metal expansion/overflow tank I have, which has a cap with two seals. you can have the cap if you want it. Just send a pm.

So does my 1984 XJ-6 by the way..

 

i don't know if this is how i send a pm or nt:

 

According to the parts catalog......

https://www.jaguarclassicparts.com/u...r-vehicle-info

The correct cap for a Series II is a JLM9652...which does not have the upper seal...for the reason Grant mentioned

https://www.ebay.com/itm/JAGUAR-DAIM...-/201477981249

Cheers
DD

 

no, you just published your email to the russian spies. Erase it by editing your post. Not a good idea to post your e-mail address.

if you want to send someone a private message (PM), you click on the forum member's name, a menu pops up, and you select "SEND PRIVATE MESSAGE",
this takes you to your private mailbox in this forum, where a window will open with a text box where you type your message, the recipient's name is already in the box.

 

maybe he should try both types and keep the one that works best. Personally I do not like the metal-to-metal contact and wear of the non-seal cap, the rubber seal eliminates that.

 

Oh boy.

You can use a double seal cap on a car without an atmospheric catch tank, the car has no idea. The air being allowed back in when cooled will still do the same.

The other way round, WITH an atmospheric catch tank, you NEED a double seal cap NO exceptions. The coolant flows TO the catch tank, as the system heats up, BUT, as it cools down, the lack of top seal will allow air to be sucked into the system, in lieu of coolant, and the catch can will stay at its HOT level.

Single seal caps down here were very hard to find a few years ago, as the manufacturers went with double seal cap only, as it will work on either spec car, as I stated. Single seal caps are now reappearing for some bizzare reason. We have both is stock at work, which I find DUMB, as stock is duplicated, and most classic cars requiring a single seal cap, do have an owner fitted catch can, so a double seal cap is mandatory.

All my old stuff have catch cans and a double seal cap.