Yesterday, I opened the hood (bonnet) to show the engine to a new friend who has an XJS convertible.
I noticed that the upper radiator hose, that comes from the top of the radiator and connects to the engine, was collapsed, like it was under vacuum.
I removed the cap from the expansion tank and the hose returned to normal.
What is the cause of this?

 

Your COOLANT tank cap is faulty or the atmospheric catchment tank/hose is faulty/clogged.

 

Which one is the atmospheric catchment tank/hose?
And, is it best to just replace the entire tank and cap, or just the cap?

 

The catchment tank is below the left headlamp and the black plastic hose to it is the uppermost one on the reservoir.

When the coolant drops in temp, it is supposed to be drawn back into the reservoir but your cap is NOT allowing that or the overflow system is faulty/clogged?

 

Wrong the catchment tank is behind the front wheel plastic wheel arch liner to the rear on the right hand side... You dont need to see it just disconnect at the expansion tank and blow through to hear it bubble. Your problem is a faulty cap. To test wash it and put it in your mouth and suck

 

I cleaned the cap and sucked on it and it wasn’t blocked. I could hear a sound. I haven’t blown through the hose yet. Is it the hose connected to the back of the reservoir?

 

SORRY, I thought I was in the XJ8 sedan part of the forum.(never mind)

 

At the risk of playing Capt Obvious, rubber hoses do get soft as they age, and this one likely needs to be replaced. The hose in question has to have some stiffness to be able to sustain the lower pressure of a cold engine and bring back the coolant from the expansion tank. FWIW, the same happened to my car a number of years ago, and I replaced the hose itself. It was a much thicker-walled Uro part that has not collapsed since. And yes, I have replaced the cap as well since then.

PS: Double check the condition of the 2 heater hoses running under the intake manifold. They are harder to replace and often neglected. Fortunately, the end of one of the hoses is easily reachable as it connects to the thermostat housing in the front. Squeeze it with a cold engine to assess its condition.

Best of luck, keep us posted.

 

At risk of being accused of heresy - I found that after my car had cooled down some, if I loosened the coolant tank cap just a little, I would hear the coolant being sucked back into the tank from the atmospheric tank. So, rather than doing this every time I got home, I now tighten the cap, and then loosen it a quarter turn or so. My coolant level seems to stay fine that way, and I've sat in traffic plenty of times without any problem of losing pressure.

 

What you describe hearing is normal. If you did nothing the coolant would continue to be pulled
back from the overflow tank on the wheel well for hours and hours.

when the engine and the coolant has retuned 100% to the ambient air temperature, then the coolant will quit moving from the overflow to the expansion tank. And that takes a long long time.

The coolant is being sucked back long after the engine has stopped running, when the engine feels “warm” the coolant is still being returned to the expansion tank. It doesn’t matter if the cap is tightened hard, or firm or whatever. Just tightened enough for tue o-rings to seal is all that matters. Loosing it the cap a bit after a full tighten doesn’t change the o-ring sealing properties.


Keeping the cap just snug might help with the cap’s propensity for warping, but has nothing to do with the coolant being returned to the expansion tank, or not .

Z