Coolant in bottle stays ice cold
 

Hi Guys 86 XJ6 I've changed thermostat and I get heat inside but coolant in bottle stays ice cold both hoses upper and lower are warm.please help me understand. Thanks in advance!

Tom Huntsville Ontario Canada

My thoughts from some V12 oddities over the years.

YES, I know yours is the half engine version, but the workings are the same for any car.

1) The hose that travels FROM the header tank TO that bottle is blocked.
2) The cooling system is NOT full, so there is air in the top of the header tank, so when the engine heats up the air is all that is expelled. Wait until it is COLD, lift that filler cap, and FILL the system, making sure the heater tap is ON, and then run it up to temp again, and note the temp changes and fluid level changes in that atmospheric bottle. Most engines only purge about 600ml from COLD to HOT.

TY Grant for the response where would I find the heater tap? Tks in advance

Heater tap, DEAD centre rear of the engine, mounted on the firewall. It has 2 heater (coolant) hose attached, and a small vac hose.

If you follow the fender braces to where they bolt at teh firewall lip, and just below that is teh tap.

Not the easieast to get at, but is doable.

If you remove the small vac hose the tap SHOULD default to full ON. They do jam very frequently. Note the lever (clearly visible) and it should be in the down position without vac.

The fact you say you now got heat indicates the tap is at least flowing some coolant.

I have never seen a 6 cyl XJS but I think this may be normal, at least for most cooling systems. The great majority of the coolant will remain in the engine and radiator. The only amount that I would expect to go to the overflow bottle is the portion that must go to allow for thermal expansion. This would not amount to much, and I could easily see the reservoir staying cold. If you do not have any other issues, you may not have a problem, don't try to fix what is not broke.

To clarify, are you talking about the brown plastic header tank? Or are you talking about the white plastic overflow bottle ('atmosphereic tank') inside the fenderwell?

Cheers
DD