I took Extincts advice to leave the cooling system unpressurized by slightly loosening the reservoir cap. There is usually no problems with this. However, after driving up a mountain on a dirt road for almost an hour and 4k feet of elevation gain, at the top coolant was bubbling out of the reservoir cap. A look at this graph in the blog post shows the coolant would be boiling at 6k feet which is what happened. This was also with the engine off, and the gauge was in the middle the whole time. Could damage have happened even though it never showed that it overheated? Should I keep the cap closed all the way, or will pressure build up and crack things?

 

No issues. When at elevation you will need to close the cap. If I shut down my car unpressurized at denver altitude if it's warm out both of my discos boil out. Both my discos have sorted cooling systems and the inline mod and act the same way. It's far worse if you shut down shortly after climbing a hill for instance. This could be solved (maybe) by running an e-fan set up with a delayed off timer (similar to a turbo timer if familiar on boosted cars) to help keep the fluid from continuing to heat with no airflow/circulation upon shutdown.

 

If the majority of the boiling happens with the car off should I close the cap after shutting off the car at elevation or before the trip?

 

No damage was done, for reasons explained in the blog. Either before or after is fine, but of the two I would choose after, but be careful. If you live and drive at elevation you might consider running 70% coolant to raise the boiling point and continue to run the cap loose.

 

So long as there is still enough coolant to fill the block, then boiling does not cause damage. If so much boils out that the level drops low, below the heads or cylinders, then they could overheat locally and cause damage. So, besides running higher concentration glycol, make sure you bring a spare gallon. If I lived in Denver, I would definitely use higher concentration glycol. I live at 630 ft, so I went back to 50/50 after trying 60/40. Land Rover specs 60/40 max concentration for D1s, because more glycol reduces heat transfer properties of the coolant.

I like the cooling fan timer idea. My daughter's Volkswagen does that. Other fan consideration is the fan clutch. If the clutch is weak or failing, the system will run hotter, then boil easier. Maybe consider using the Hayden Severe Duty model, at least in summer at high elevations and low speed. If the cooling system is pressure capped while coolant is boiling, the pressure will increase and stop the boiling. But the higher pressure can cause undesired effects, which is why the cap was loosened in the first place. It's a real balancing act if you are way up high. Let us know where you land.

PS Be sure to get a device to read coolant temperature through the OBD port vs dash gauge. Knowledge is power!