OK, here's the situation

2004 D2 HSE currently with 190K miles
Engine rebuilt at 128K (prev. owner overheated it and slipped sleeve, gave it to my son for free to get out of driveway)
Upgraded to a 170 degree thermostat (the one for Middle East models) at time of rebuild (per recommendations on this forum went with an OEM translucent body, not aftermarket black motorad or similar)
Replaced the top coolant hoses as a set at 140K miles (Uro junk, more on that below)
New water pump at ~145K
New fan clutch at ~160K
New condenser fan motor at ~170K (it would overheat on hot days with AC on, but run normal if AC was off, so condenser was creating an extra thermal load in front of radiator when AC was running)
New lower hose set at 188K (the heater feed barb on the 4-way "Y" started to fail at the joint with the "Y")

Yesterday, while letting it idle for 10-15min in 98 degree, 90% humidity heat, it started leaking like a sieve. (Opened the overflow cap as I have done many times before on this engine when hot, and this time it exploded like a volcano, burned my hand and arm).

The leak appeared to be at the point where the top hose from the bleeder T goes into the top of the thermostat. That hose and the other two top hoses I had replaced as a preventative measure had become totally non-pliable -- they were literally as hard as PVC pipe. The hose did not split, so I suspect that because it had become so non-pliable that the clamp at the thermostat barb could no longer squeeze it sufficiently to create a seal under (excessive?) pressure.

Fortunately, I had kept the old top hose set "just in case", so I had a friend get them from my house, replaced the rock hard Uro hoses with what I presume are the original hoses, which were still soft and supple and a little thinner walled than the Uro versions. (lesson learned: if it ain't broke, don't fix it.)

After replacing hose(es), I refilled with 100% distilled water, set the ACC to economy and turned the heat to high to open the heater system loop and otherwise followed the fill-bleed procedure.

Drove it about 50 miles home. It overheated once (pulled over and turned off just as red light came on), and otherwise it registered at between 2/3 and 3/4 on the gauge, where for the past 60K miles since the rebuild it had consistently registered right at the halfway point (except when the fan clutch failed and when the condenser fan motor failed), (I know, I know, the gauges on these things are not very accurate.)

The only unusual thing during the whole "repair was that during the fill-bleed process was that the vent air never got hot (maybe the ACC, with a max "hot" of 88 degrees, would not let the vent temp exceed that or maybe It's a bad thermostat -- I have never been able to understand the operation of the external "artificial heart" thermostat?).

So IMO there are only 4-ish things that could be wrong?

1. Failed pump (unlikely. I think the pump I put in at 145K had a metal impeller and there is no leaking from the weep hole indicating a failing bearing)
2. Failed fan clutch (it's pretty new, I recall buying a quality one, and it shows resistance even when cool)
3. Clogged radiator
4. Bad thermostat

Thoughts on what is wrong and/or how I should proceed troubleshooting.

 

Well a couple of things :

• Replace the thermostat, either do the inline mode or get a new one
• Extended the overflow line add a pop bottle to catch the overflow so you can see if you are getting coolant out
• Replace the rad cap.
• Check your tailpipe at start for white smoke - more than 1 or 2 minutes you have a problem

You had a major over pressure event (rock hard hoses) there are only a few causes

• Rad cap
• Thermostat
• Water pump
• Head gasket / cylinder crack

 

after you do what Richard says, get an ultraguage. once the needle starts to move, it's already 230 degrees. once it's well up past half, it's 240 + and the red is like 260+ degrees. you'll do a lot of damage if you let it turn the light on. These engines do not like those temps. the temp gauge rests in the middle from about 150 degrees to about 230, so the stock temp gauge does not give you good, accurate temp information.

Always open coolant cap slowly to allow pressure to hiss out and use a rag covering it when you turn it so you don't burn yourself again just in case.

 

Start with an inline thermostat mod, see how it runs from there. You likely have damaged the HG enough that it will need HG in the next 6-12 months but maybe not right away. If everything the checks good after the inline mod and you want to keep going replace anything not metal that is original to the truck, this includes radiator, expansion tank, heater hoses, everything. Those old hoses can get too pliable and split. Under pressure.

 

@tnrangerover actually I discovered recently the overheat light comes on at 244. And the gauge tops out at 265 ish as I recall. But still way too freaking hot for an aluminum engine.

 

Thanks for the comments. No white smoke or milky oil, so fingers crossed, no blown head gasket (when we rebuilt the engine 6 years ago after the slipped sleeve by the previous owner, we used high quality head gaskets and ARP studs)

I stumbled onto the inline upgrade -- it showed up as a sticky -- and will try that (Extinct, living in TX, should I go with a 170 or a 180? and I assume that the silicone gasket option doesn't require any sealant).

I have an really good OBDII tool that shows temp real time, so I'll put that on and drive a bit to see what's going on with temp..

And how would the overflow cap affect anything? I guess that would let coolant escape, but that doesn't seem to be my problem?

Finally, am I correct that if the coolant system is operating correctly, I should be able to open the overflow cap without incident, even when engine is at operating temp?

 

I live in Austin and I went with a 170. In my in-line mod but a 180 would work fine too.

ps I would never ever open the coolant bottle on a hot engine. Every car I’ve owned builds pressure in the bottle to where hot coolant would come out like that

 

Thanks fellow Austinite. Didn't throw codes at the lower opening per Extinct's sticky? If not, then will go with 170 as well then.

Yeah, it was pretty stupid to open that cap in an overheat situation. Stupid is as stupid does. But why are some overflow tanks under pressure and others not (they just have press on caps like you would have on a washer fluid reservoir)?

Tracked temp real time with my ODBII on running engine, here are the results:

Ambient air ~80f (26.6c)
100% distilled water
AC on.

Before starting engine, the coolant temp reading matched ambient air (both read ~26c)

Gauge moved off bottom at about 45c
Reached 1/4 mark at about 60c
Got to halfway mark at about 80c (and then stayed there from 80-100c+, a swing of almost 40f, so gauge really is pretty useless)
Bounced between 94-95c when driving about at 55-60mph for about 10 min
Parked and let it idle a bit (<5min), got up to 100-101c before I turned it off

I assume that as the ambient air temp rises to a high of 97f today, the readings would also rise accordingly?

Under hood (and tailpipe) observations:

Fan spinning (and the fan clutch still looks bright and shiny, not that grey that aluminum gets after a while in the weather, so it can't be more than a couple years old)
Modest gurgling in the overflow tank (never recall seeing that before)
Oil not milky, shows no signs of water
No white smoke from tailpipe
Hoses were pretty stiff (pressurized?) but not hard

In spite of it being bright and shiny and spinning, could the fan clutch not be doing its job given the temp jump of 9-10f when idling?

And given that the overflow tank is "open", i.e., it has that overflow tube in the neck, how is the cap is holding any pressure?

BTW, per my recollection from the rebuild, the engine has two (2) temp sensors: one in the top front of the block and the other down low on the radiator. Which one am I reading (assuming the block sensor)?

 

Didn't check to see if the condenser fan was spinning as well with the AC on, need to do that.

 

Suspect of a clogged radiator. I wish to knew a method to diagnose that other than replacing or dismantling it apart to inspect it. Did it by un-bending tabs and removing the plastic 'tanks' once, to find a third of tubes stuffed with something whitish...
Never use the Disco as a personal airconditioner with wheels when idling at a parking lot waiting for someone as seen too often. That is Rover abuse to please comfort egos and will cost you, soon or later.
Distilled water poured in an impure engine block improves nothing.
If the thermostat works perfectly, opens the path to radiator fully when it should; replacing it even with a lower temperature rated one will not make the coolant run cooler. Applies to an inline thermostat too. If fully open, will not open more.

"I have never been able to understand the operation of the external "artificial heart" thermostat?" Unsure if you refer instead of a 'inline thermostat'

Hot coolant engine outlet------------>inline thermostat-------------->radiator inlet------------>radiator outlet----------->water pump into block

The 'inline thermostat' is made leaky by a perforation that allows a little coolant to pass to heat its guts and act.
When its rated temperature is reached; opens and becomes :

Hot coolant engine outlet-------------->radiator inlet------------>radiator outlet---------->water pump into block.