Ok, thats a sympthon of air in the system. When hot, the pressure rises and air bubble shrink by pressure, so you are low in expansion tank and when cold there are less pressure, so bubble grow and push coolant to the expansion tank. Have you noted that there are bleeding screw on heater hose?

 

I've done the bleed screws multiple times, and I've raised the expansion tank. I've done vacuum fills, with all hoses collapsing under vacuum and I've tried leaving it under vacuum for hours with no loss of vacuum.

I agree air trapped somewhere seems to be the most likely scenario, which's why I think I need to take it to a specialist for another drain and vacuum fill or perhaps only a vacuum.

 

I did run the engine with heat set to max, but not under vacuum. I cycled between this and vacuuming as it took quite some time before the air started blowing hot, each time more air came out and eventually the heater started working.

I think there's only 3 possible scenarios, somewhere a blockage, trapped air or a gasket leak (though all evidence suggests this isn't possible).

 

Did you also open the heater while draining & filling?
I had them open - I had a battery charger on the battery while doing so since the XJ complains if it's not running and ignition is on for more than a few minutes.
Although you changed the radiator, remember that there are also the oil cooler & transmission cooler in the cooling circuit. They could also be restricted.
But that wouldn't have changed with the new radiator. SO not likely important.

 

Do you mean the bleeders? No they were all closed when filling. Actually the vacuum would not be able to work if the bleeders were open.

Yes, there's the intercoolers and trans cooler, etc.They all get hot so coolant's flowing and the intercooler pumps only a few months old and verified working.

 

No, the valves that control coolant flow though the cabin heater core/elements when the heat is on.
When you drain and fill coolant, you need to
1 - turn on the ignition (but don't start the engine)
2 - turn the temperature settings inside the cabin to the highest setting - front & back & both sides.
3 - drain or fill the coolant
This is to include draining/filling the coolant from/into the heater core and lines to it.

 

I’ve never done a vacuum fill with the engine running. I do run the heat after the fill and bleed/vacuum until the heat starts working. This usually takes 4 or more cycles before the heat starts working.

 

x351 heater circuit do not have valves. (nor any modern car) The coolant are circulating in the heater matrix all the time and temp in cabin are controlled by airflow flaps. (faster and more accurate)
You might have heater matrix partially blocked and air might not find the way out from it. Heater matrix have thinniest passages of all cooling system, so it will easyly collect everyting going around the system. I have been using carden hose for cleaning blockedes from various vehicles by removing both heater matrix hoses and sending water to the opposite direction thru matrix.

 

I think we're a little confused here.
I never said to vacuum fill with the engine running.
Did you know that you can turn the ignition on, and have the electrical systems of the care turned on without the engine running?
What I tried to describe clearly but succinctly is to push the start button without putting your foot on the brake.
If you do that, it will turn the electrical system on without starting the engine.
But as Vasara said, you don't need to do that anyway on the Jaguar - as opposed to it's Land Rover brother vehicle that does control coolant flow through the heater coil.

BUT step 13 of the JLR Topix procedure for vacuum fill is to set the heater control to max
Step 14 is to start the engine & idle until the air coming from the registers is hot.
Step 15 is to shut the heater off.
Step 16 is to let the engine run at 2000 RPM for 8 minutes.
Step 17 is to shut the engine off & let it cool.
Step 18 is to look at the coolant level & make sure it's at the full level, or fill it if needed.
And that's the end of the JLR Topix vacuum fill procedure.

 

Apologies, you did say without starting the engine. But my answer's still the same.

 

Basicslly what I did, I used the workshop manual steps for both vacuum and non-vacuum traditional method.

 

I know this is an old thread but I was reading with some amusement, the one thing that was missed initially; the cap should have been tested or replaced as they can fail to open and relieve the pressure which does occur everytime you drive the car, it will open slightly to relieve pressure and the open the other way to avoid vacuum when cooling down.
If you suspect over pressure from head gasket other combustion leaks into cooling system then with a cold engine install a pressure gauge (we use the one on the pump to pressurize for leak testing) run the engine at fast idle and see if the pressure rises too quickly before the coolant expands, leaking headgaskets usually cause over heating due to air (gas) locks preventing proper flow.

 

Post #29 Jahummer said: “The problem is the circuit’s remaining fully pressurized, even if it sits cold for days. That’s not how it’s supposed to be, once cooled pressure should be gone.”

Post # 33 12Jagmark said: “I often still let pressure out after long drives, it still doesn't lose coolant often unless I neglect to relieve pressure for a while. It still has the problem of slight pressure leak into the coolant system while at running temps, then if not at least slightly relieved before cooling down, will reflow a small amount of coolant back into a few cylinders as or when it cools.”

Post #35 12Jagmark said: “…I'd just replace a cracked head or keep on with a cracked block as it is if it couldn't be sleeved or welded.
These aren't likely though, as nothing has happened that would cause either.
The head gasket is the best remaining cause other than those, and I haven't heard from anyone knowledgeable with any other plausible possibilities.

Post #38 Jaghummer said: “My biggest concern’s having the cooling circuit under constant high pressure which may lead to further or premature failures of any parts of the circuit.

Mines only had this problem for a few months since the radiator replacement. Since then other circuit parts have been replaced. Multiple bleeds and vacuum fills, even removed and lifted the expansion tank several inches higher and no difference.
As I said, 2 oil changes done and no coolant, coolant contains no oil, coolant tested for exhaust and none present, compression test passes, no coolant in the exhaust and no coolant loss. Circuit holds pressure or vacuum so it’s sealed. Thermostat works as it should and coolant temps always where they’re supposed to be.
trapped air’s been a suggestion but where?
A new development this week last, not certain what it means but when the engine’s been running, the expansion tank level goes below the low line but when it’s cold, it’s nearly overflowing despite the fact the last coolant refill, as always, was at the cold mark. So, when it’s hot and over pressurized, where’s the coolant going and when it’s cold where did the extra coolant come from?
Despite having done numerous drain and fills and parts replacements over
the last 2+ years with no issues before, I’m tempted to take it to a specialist and pay them to drain and fill.

Bill400 response: You both seem to be saying it: Everything points to combustion gas pressurizing the coolant system. The new development is likely rubber hoses stretching further than normal due to unrelenting exposure (24/7?) to elevated pressure, and higher than normal (30PSI max/200F?) when running. I am sorry to say: Consider using the professional to check for combustion gas rather than another vac/fill.

 

If I've neglected to mention it, yes it's got a new genuine cap fitted about 2-3 months ago.

 

Except. in 2 previous posts including the one you cited above I said it was already tested for combustion leaks...

 

Finally got round to testing the tank cap and it opens at 25 psi, so should I assume despite the upper radiator hose getting rock hard and staying that way when cold, the cooling circuit's safe from any bursts or ruptures due to excessive pressure (the cap would keep it from going higher than 25 PSI)?

Also trying to figure out how the thermostat's supposed to operate and how this may affect the pressure issues. Manifold temp typically stays between 195 and 215 and thermostat temp stays between 95 degrees and 125 degrees F. If it sits idling for extended time, the manifold temp drops and the thermostat temp increases until both temps are in the 180-190 degree range, then once moving, thermostat temps drops and manifold temp increases.

I spoke with a general mechanic yesterday and the issues I've described lead him to an immediate conclusion there's a coolant flow issue somewhere, either a blockage, thermostat not operating correctly or a valve somewhere not opening.

I've replaced perfectly good parts in the past so I hesitate replacing the thermostat, plus it's been on global back order for nearly 6 months, but I'm tempted to do it any way and at least eliminate that variable.

 

Did you try to flush heater matrix backwards?

 

No, but the heater's working perfectly fine and I never have overheating issues. When drained, the heat doesn't work, when refilled it works perfectly.

 

Well, you live in Florida. Just 1/4 of the heater matrix capasity will be enough for your climate for hot air. The matrix full heating capasity by my own experience are somewhere -38C (-36F) when it was just able to push warm out on one weekend in Lapland. (front radiator of car fully covered, since diesels donīt give so much heat than petrol engine. Still took some 15min drive to warm up the engine)
If your cooling system have some foreing objects running around or somebody have add an leakstop in the system, the air pocket propably are in matrix by blocked passages. You can try to bypass the matrix by connect matrix out and in hoses together with hose joints and some extra piece of hose and test if you still have same issue. If not, then you know where your problem is.