2004 Disco 4.6, new thermostat, radiator, viscous fan clutch, hoses. Everything has been running cool and fine until the other day when the coolant system over pressurized. I have had no coolant loss or leaks. I run an ultra gauge and temps shot up to 235 with 10 minutes of driving. Got it cooled down with still no coolant loss. I am stumped on this one. I was think hg but there is absolutely no coolant loss. Thoughts?

 

What style thermostat? If factory style, have you bled the system well?

Could be a number of things. Sticky thermostat is a possibility whether it's factory or in-line. I'd move to an in-line thermostat if you haven't - tons of benefits. Cheaper parts, more readily available parts, considerably easier to bleed and work on, more stable and reliable.

If there's no coolant loss, then it is not a head gasket.

 

OEM 180 gray thermostat. No air in system. Just hoping that it is not exhaust gases pressurizing the system.

I am going to flush the heater core this morning and also pull the thermostat to check that it is opening.

I agree that it may be time to go inline.

 

@DiscoMerlin How do you know i t was over pressurized if there was no coolant loss ? Over pressure almost always causes some coolant loss assuming the rad cap is working. A sudden spike in temps can be a few things but start with water pump:

1. remove the rad cap
2. start the disco
3. Observe the lower rear left side of the expansion tank you should see coolant movement - that is the return from the lower manifold to tank it always moves if the water pump is working
4. If that is good, a possible as is a crack in the block can cause that kind of heat.

Generally even with a bad thermostat you should not spike that fast but it is possible

 

I was actually replacing the water pump as you were replying.

The cap is a month old but not functioning properly as during the event yesterday the tank was full to the top, hoses hard as a rock.

I had to do some running around so I will test the system some when I get home.

 

Typical symptoms of a thermostat sticking. Sounds like you are familiar with the inline mod so I won't post links to the sticky or the recent blog post but will mention this. Regardless of which thermostat you use I recommend running the truck with the reservoir cap one full turn loose from the full tight position. Running in that position will prevent the system from ever building pressure which prevents blowing hg, hoses, radiator end tanks, reservoirs and cracking the lower wye (all of which I cracked or blew before I started running it this way). If you decide to do that, then slip a length of hose over the overflow tube and run it up to the cowl area. Like most things in the cooling system BMW overcomplicated the tank and cap design and it can splash a little up in to the overflow landing and drip a little but not with the hose up to the cowl area. Not building pressure also has the benefit of lowering the boiling point of the coolant, so the much like a pot of boiling water on a stove cannot exceed 210 degrees until there is no water left an engine without a pressurized cooling system cannot exceed the boiling point of the antifreeze (typically around 230 degrees f)

 

Ordered an in-line from you this morning.

 

Will ship Mon morning

 

Running both D1s with loosened caps. With this hot weather, I upped the glycol concentration to 60/40, 232 deg F boiling point. 50/50 boiling point is 225 deg F, according to online information, a bit to close to operating peak temperature for me. Experimenting with Hy-Per Cool additive, label says it eliminates surface bubbles to improve heat transfer.

 

Thank you all for the comments. I think that I may be safe to conclude that I have a blown hg. New water pump flowing nicely, new thermostat, new cap, new hoses, new viscous clutch/ fan. No air in system. Fresh coolant 50/50 mix. 10 minutes of driving from cold, temps fine at first with heat on full blast. Then temps started rising, and rising. Hit 260 before I could get it off the road. 199-265 in less than 60 seconds. Coolant boiling and hoses and expansion tank look like they are going to explode. Coolant changed colors.