After replacing the bank 1 RH side injectors, coils and plugs and tightening some screws on the valve cover, I went for a 40 mile drive and had the coolant spontaneously boil out of the expansion tank, nearly 2 gallons worth. Nothing in the cooling circuit was touched for the aforementioned service. I shockingly was able to successfully refill the coolant circuit on the side of the road without the need for bleeding anything, heater immediately went from cold air to hot air and I was able to drive over 100 miles without any further problems. Before I bought an airlift, I found manual filling and bleeding to be the most tedious and difficult procedure to perform but for whatever reason this went flawlessly even without bleeding. Later that night I decided to fit the airlift just to be certain and it added about a pint of coolant. I also re-placed the expansion tank cap with a brand new one (the current one was about 2 years old) an went for a short drive and it immediately began to boil over. I performed another vacuum fill and added a gallon of coolant to replace what boiled over. Went for another test drive and no further problems.

The following morning went for a short drive and it boiled over again, literally pouring out from underneath the cap. Yet again successfully refilled on the side of the road a gallon with no bleeding and went for a very log drive with no further issue though upon arriving at my destination low coolant appeared and indeed the tank was empty, refilled and went for another very long drive and still no further incident.

A few weeks ago after a 200+ mile drive I noted just a bit of coolant residue in the seam on top of the expansion tank and feared the 6-month old tank was failing (the original factory tank literally crumbled apart and it's replacement had a failed float after just about a year). I rinsed it off and drove over a 1000 miles with no further appearance of coolant.

Over the last 4 years I've replaced the radiator, water pumps, all hoses, tubes, pipes, thermostat, etc multiple times with success. In all this time I have never ever seen coolant boil over and out of the sealed expansion tank cap.

I don't understand the coincidence of this happening immediately after replacing fueling/ignition parts.

And finally one further observation perhaps experienced Discovery MK IV owners can help with, the upper radiator hose, until this time, has always been rock hard when up to temp and soft after cooling down for an hour or two. However now, it's NOT getting rock hard, just very firm with a little give when pressed. I have checked for leaks, but see none, anywhere.

 

Cold engine, coolant was a little bit below the top cold fill, decided to top up (as I've done previously) and went for a drive. Everything appeared ok and then after 20 miles, the temp started climbing rapidly, I stopped and found the tank overflowing again. When it was done I refilled with a gallon of coolant and continued driving. Made it 5 miles to my next destination without incident and had a two hour stop. When I resumed driving the temp started climbing and so I stopped again. The tank was not overflowing but it was extremely full. I went ahead and bled the tank at the screw and then continued driving for another 10 miles to the auto parts store to pick up more coolant. I was in the store less than five minutes before restarting the engine. WIth-in seconds I watched the coolant rise in the tank and started overflowing from the cap and I shut it off immediately. Then I refilled the tank again but this time not all the way up to the top line. By the way every time I checked and watched the coolant rising, the upper radiator hose was soft. Started up and drove another 10 miles and pulled over just to check and found the expansion tank nearly empty but the upper radiator hose was hard. I continued driving another 10 miles home without further incident, tank remained nearly empty and upper radiator hose very hard, like it is supposed to.

So the conclusion I've arrived at is the coolant's NOT boiling out, the expansion tank's filling up beyond it's capacity and is being forced out which then leads to lost pressure and coolant boiling. The mystery's what's causing the coolant to rise so high, even when the circuit's been vacuum filled at negative 25 PSI.

What IS common to every time the tank's overflowed in the last 3 days is I topped off the coolant on a cold engine before starting, so did I somehow eliminate enough room for coolant to expand? I don't think so because I've never had this problem ever, I've occasionally topped off coolant before (usually after a coolant repair), though it's been a few months since needing to and the tank level's remained consistent.

 

I don’t doubt the experience, but coolant does not expand that much.
I submit that you have air in the system and it is expanding.

 

@guy I don't disagree but how did air get into the circuit when changing plugs, coils and injectors? Nothing in the cooling system was touched...

Vacuum fill at -25 psi using an airlift apparatus, which I've been using for along time considering how many parts I've replaced, has always worked. All hoses collapsed, high confidence no air in.

And to add to the weirdness, I drove several hundred miles yesterday without ANY issues. Hoses became pressurised within about 15-20 minutes and remained so all day.

For the first hour+ drive, ECT A remained between 193-206 degrees. The second hour+ drive a couple of hours later the ECT A remained between 215-226 degrees F.

 

Going on a limb here:
I hear you. Weird. Intermittently the thermostat is sticking closed??

 

@guy Thermostat's just a few months old, genuine. The original worked but had started leaking at the seam. I have to assume the new one works, on cold start, it takes a very long time to climb above 100 degrees, when sitting idling from a drive, the T-stat temp will consistently climb to 200 degrees and while driving it fluctuates constantly from around 90 to 150 degrees when the ambient temp is high and lower when the ambient temp's lower.

I started this thread requesting ECT A & B data from other owners.

https://landroverforums.com/forum/lr...please-122195/

Drove another 100 miles yesterday and today and all working well, ECT A staying around 203.

One thing I noticed, when cold, coolant's at the cold mark on the expansion tank but when running, coolant tank's nearly empty (did get low coolant warning twice then it immediately went away). Could be mistaken but I'd swear coolant level only changed minimally at full operating temp.

 

@jahummer , this is peculiar.
With a proper thermostat, as I know, you know; this is supposed to aid the quick warm up of an engine.
And the tank near empty when running cold, equally odd.
I'd like to be able to explain this behaviour. Perhaps I'll be able to perform some observations in the morning.

 

To clarify, full when cold, empty when hot.

 

Empty when hot?!? ok, going for another spirited drive. I'll report back after a trip to the car wash.