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Leaking coolant reservoir cap

XJ4081
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Location: Germany (South-West)

Leaking coolant reservoir cap

Postby XJ4081 » Fri Jul 31, 2020 11:11 pm

Hi,

I repeatedly noticed a loss of coolant fluid through the reservoir cap, especially after driving mountainous roads or stop-and-go traffic. When examining the cap I noticed that the inner upper rubber sealing has almost gone. So I got me a new cap and ... the issue remains the same :shock: . In the past year I already had noticed a loss of coolant on hot days (by its sweetish smell when burned on hot surfaces), but I could never locate a leak. This year I don't have this issue, it seems all to come through the cap, maybe the weekest point in the system.

Several times I also noticed that the coolant temperature had raised beyond the N-mark in the gauge when the cap leaked. In think this could be no coincidence but be related somehow. Is it getting too hot in the system so the pressure is becoming too high and it leaks at the weakest spot? Or is there too much fluid in the system so the pressure rises too high when the coolant becomes hotter (how can I determine the proper fill level, the reservoir has no marks)? Or is the cap not tightened accurately (I simply tightened it with my bare hands, in order to untighten the old one I needed a gripper)?

Best regards
Oliver
1993 XJ12

XJ4081
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Joined: Fri Oct 23, 2015 12:12 am
Location: Germany (South-West)

Re: Leaking coolant reservoir cap

Postby XJ4081 » Fri Aug 07, 2020 10:59 pm

Just had a look under the hood after I had changed one of the front springs and found the cap has been leaking again, this time the engine did not run at all, I simply jacked up that side of the car in order to be able to change the spring.

Leaking cap

When tightening it the last time I noticed a kind of limit stop, so I assume the cap cannot (should not) be turned any further than I did. So is it normal that the cap leaks when the car is tilted (it has a kind of cavity inside the middle, maybe as a kind of pressure relief valve)? If not, since the cap is new, I guess it then could be only the coolant reservoir itself or its thread respectively (it actually looks ok) :? .
1993 XJ12

katar83
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Location: Cambridge

Re: Leaking coolant reservoir cap

Postby katar83 » Sat Aug 08, 2020 10:09 am

Cap should not leak unless you have too much coolant or too much pressure. Not sure how it looks on a 12 but on a 6 there should be a plastic tab inside the reservoir to which you should fill it up.
1996, 3.2 SWB X300 - Finally a rust free daily driver :)
1997, 4.0 LWB X300 - Unlucky car.
1988, 3.6S XJS - Never ending restoration project.
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1994, 3.2S XJ40 - Long gone.

XJ4081
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Posts: 272
Joined: Fri Oct 23, 2015 12:12 am
Location: Germany (South-West)

Re: Leaking coolant reservoir cap

Postby XJ4081 » Sat Aug 08, 2020 3:29 pm

Yes, the 12 also has a kind of tab in the reservoir, but no special marking line like I know it from other cars. So it should be filled to the bottom of that tab when cold? In that case there really might be too much fluid in it.
1993 XJ12

abercanadian
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Location: Vancouver Island

Re: Leaking coolant reservoir cap

Postby abercanadian » Sat Aug 08, 2020 5:35 pm

XJ4081 wrote:Yes, the 12 also has a kind of tab in the reservoir, but no special marking line like I know it from other cars. So it should be filled to the bottom of that tab when cold? In that case there really might be too much fluid in it.


Yes. Just filled so that the white tab is just wet. You have been overfilling and when the coolant expands it has to go somewhere!

Larry
94 XJ40 VDP vin 701729
Oyster Metallic/Doeskin on Aero Alloys
240,000 km/149,000 Miles
Daily Driver

94 XJ40 SOVEREIGN vin 703632
Diamond Blue Metallic/Barley
111,000 km/69,000 Miles
Parts Car

XJ4081
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Posts: 272
Joined: Fri Oct 23, 2015 12:12 am
Location: Germany (South-West)

Re: Leaking coolant reservoir cap

Postby XJ4081 » Sat Aug 08, 2020 9:38 pm

It actually wasn't me overfilling it, but the garage where the last maintenance was done, I actually relied on them filling in the right amounts of fluids :x . One more reason to do everything myself :roll: . Ok, so I will remove some of the coolant until it just reaches the white tab.

Would too much coolant and too high pressure also explain why it got hotter than N on stop-and-go or mountainous roads? Or is this just normal for the V12?
1993 XJ12

naki_k
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Re: Leaking coolant reservoir cap

Postby naki_k » Sun Aug 09, 2020 12:42 pm

XJ4081 wrote:It actually wasn't me overfilling it, but the garage where the last maintenance was done, I actually relied on them filling in the right amounts of fluids :x . One more reason to do everything myself :roll: . Ok, so I will remove some of the coolant until it just reaches the white tab.

Would too much coolant and too high pressure also explain why it got hotter than N on stop-and-go or mountainous roads? Or is this just normal for the V12?


If you want a job doing right..... as the saying goes.

Mountainous roads are extreme conditions for a V12 in an ideal world that needle should never move off the centre, but you will be testing the cooling system to the limits driving up mountains. Since the engine is working hard and there’s less air flow, relative to the heat being produced.
V12 cooling is marginal at best, so it doesn’t take much to top the balance.
Have you changed the thermostats? Cheap to buy genuine, and well worth it, to be sure the system is at its best.
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Breaking 40/ 300

naki_k
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Re: Leaking coolant reservoir cap

Postby naki_k » Sun Aug 09, 2020 12:46 pm

Overfilling is an easy mistake to make if you do not for know these cars. Ideally you should have an experienced mechanic to work on the car, as seemingly little mistakes could add up a to a bigger problem in some areas.
88 3.6 XJ6 PDH/CEM
90 Range Rover Vogue SE 3.9 V8 LPG
91 4.0 XJ6 Manual
92 4.0 XJR Manual
92 4.0 XJR
93 3.2s - 4.0 Manual conv
93 4.0 Sovereign
93 6.0 XJ81 DD6 x2
93 6.0 XJ81 XJ12
94 4.0 XJR
94 4.0 Daimler
96 4.0 X306 XJR
96 4.0 X300 D6
97 4.0 X300 Sport Manual
2000 LR Discovery ES TD5 A
Breaking 40/ 300

XJ4081
Snr Member
Posts: 272
Joined: Fri Oct 23, 2015 12:12 am
Location: Germany (South-West)

Re: Leaking coolant reservoir cap

Postby XJ4081 » Sun Aug 09, 2020 9:44 pm

It's actually not that easy to find an experienced mechanic for a XJ40 here :( . The one where I took it the last time did not notice the wrong coolant level, even though I told him to check all that kind of stuff. For unknown reason he went out of business in the meantime :? . So I decided to do as much as I can myself.

I'll keep an eye on the temperature gauge. When driving normally, even on hot summer days, the needle is sticky at the middle, it's only certain circumstances that cause it to go beyond. I actually thought the torque of the V12 makes each mountain an easy climb, but maybe I underestimated the stress it causes to the cooling system :D .

On hot days, usually the two fans cause more noise than the engine itself, so I guess everything is right with them. Anyway I have two new thermostats as spare here.
1993 XJ12

abercanadian
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Location: Vancouver Island

Re: Leaking coolant reservoir cap

Postby abercanadian » Sun Aug 09, 2020 9:58 pm

This may not apply to your car but I have owned cars where the thermostat had been removed, previous owners or mechanics under the impression car would run cooler. Not true!
The car I owned that had no stat was OK around town but on a warm day climbing a hill, the temperature would rise significantly and sometimes the car would boil over.
This state is caused by the coolant circulating too rapidly and not spending enough time in the rad to get cooled before entering the engine block again. The stat, apart from helping with warm-up, acts as a flow restrictor slowing down the circulation rate.

Larry
94 XJ40 VDP vin 701729
Oyster Metallic/Doeskin on Aero Alloys
240,000 km/149,000 Miles
Daily Driver

94 XJ40 SOVEREIGN vin 703632
Diamond Blue Metallic/Barley
111,000 km/69,000 Miles
Parts Car


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