About three years ago I came to the conclusion that the pressurized cooling system in my Disco (Land Rover Discovery 2) fleet was causing problems, so I started running my coolant tank cap one full turn loose from full tight so it did not build pressure. Since that time I have generally been recommending that practice to the LR (Land Rover) community, and that has recently resulted in a few questions as to why I am making this recommendation. So I thought a blog post on the topic might be helpful as a reference going forward. We all know from experience that as engines get hotter, the probability of blown hg (Head Gaskets) and warping the heads go up. This is particularly true on aluminum heads as the aluminum becomes more ductile with temperature and the pressures inside the cylinders goes up and depending on what is going on so does the potential for the cooling system pressure. Also as temperatures go up the plastic and rubber in modern cooling systems gets more ductile, so the probability of those failing if the system is under pressure also increases. So let’s keep that in mind during this discussion.

I arrived at this personal conclusion in a moment that might be called the straw that broke the camel’s back, so to speak. Over the 10 years driving over 20 different D2’s I have had the unfortunate experience of multiple cooling system component failures including ruptured hoses, cracked coolant tanks, cracked hose tee’s and wye’s, leaks under hose clamps, etc. The final straw was on a truck equipped with an inline thermostat running normal temperatures for quite a while until one day it wasn’t. After pulling over I saw it was dripping coolant and the tank was low causing the temperatures to get higher than I liked. Inspecting closer, I realized the leak had come from a crack in the radiator end tank. What could cause this? The radiator was oem but had been performing fine. The only explanation was a combination of deteriorating plastic mechanical properties (due to age and heat cycles) combined with a pressurized cooling system.

So in the days/weeks afterward I thought a lot about the benefits versus the potential negatives of running a pressurized system versus an unpressurized system.

On the benefits side of a pressurized system:

1. It raises the boiling point of the coolant. This is very likely the reason automotive systems have been using pressurized cooling systems for so long. At sea level that change goes from from 225 degrees F to 265 degrees F ( 107 to 129 celsius) on a 15 psi system which may seem like more boiling resistance than needed, however the coolant boiling point at elevations drops considerably, from the unpressurized 225 F at sea level to around 208 F at 10kft elevation (107 to 97 degrees C), which is close to a normal engine operating temperature (ok, maybe a little hotter than we would like but within the safe zone). So pressurizing the system increases that boiling point back to 248F (120 c)which brings us a good bit of boilover headroom. So lets look at a little of this math (chart below for reference) , so the boiling point decrease is about 1.7 degrees F (0.94 degrees C) per 1000 ft of elevation, and the boiling point increase is about 2.7 degrees F (1.5 degrees C) per 1 lb per square inch. Just to close this discussion out for a D2, the two most common caps for D2’s are a 140 KPA (20.3 psi) cap and a 200 Kpa (29 psi) cap. So the boiling point goes from 225/107 to 280/138 on the 140 cap and to 303/150 on the 200 KPA cap, and at 10k ft elevation this translates to 258f/125c and 279f/137c.

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2. However, in my view this head room of coolant boiling points at very elevated temperatures is only a benefit if the engine is going to be operated long term at a temperature of greater than say 230 degrees. I have the unfortunate experience of operating a Disco engine in the 230 degree range for short periods of time without blowing the HG (due to not running the cap full tight). However I doubt very seriously you could operate a Disco engine much hotter than that for any significant period of time without blowing the hg. So if I have a pressurized system, I can operate at say 250 for an extended period of time. At temperatures above the boiling point, the coolant will boil and the pressure will increase in the cooling system, hopefully opening the pressure relief valve in the cap and allowing the pressure to escape before another component fails. If we continue operating with whatever is causing our system to run above the boiling point we will eventually boil the coolant away, however assuming we have a full cooling system that will mean boiling off around 2 gallons of anti-freeze.

3. Some people will point out that pressured cooling systems are necessary to maintain coolant contact with the internal engine cooling jacket surfaces, and that localized hot spots are possible in the combustion chamber area that would result in micro-boiling at those points without a pressured system.  This may have been true historically with cast iron cylinder heads but most modern engines use aluminum cylinder heads which have much higher thermal conductivity that virtually eliminates these micro hot spots.  This in part has been why we have seen manufacturers able to increase compression ratios on modern engines without serious concerns about detonation, the improved thermal conductivity of aluminum cylinder heads enables this despite fuel octane remaining significantly lower than in the '60s and 70's era.  Also, this is really only a concern with sustained high cylinder pressures i.e. really working the engine hard like pulling a load up a grade or WOT (Wide Open Throttle) as these types of hot spots typically do not occur on light load cruise or idle conditions.

On the negatives side for a pressurized cooling system:

1. System pressure is more likely to crack any non-metal components. I have cracked or blown hoses, radiator end tanks, coolant bottles, and lower wye fittings. I have even seen a factory thermostat split into two during an overheat (not my truck). I don’t think this issue was as big an issue when cooling system components were all metal or when the vehicle is new and there are zero thermal cycles and the associated degradation of any plastic components.

2. System pressure is more likely to push coolant out from under spring type hose clamps or push hoses off of the connected fitting even with a screw type clamp.

3. System pressure is more likely to blow the headgasket when operating at higher temperatures, either interior or exterior blows.

4. System pressure will force coolant past the seal keeping coolant out of the water pump bearing once the bearing begins to wear a little, we see that as water out of the weep hole but it hastens the demise of the water pump bearing.

5. If the system is running with the pressure cap full tight, and an overheat condition occurs to cause the coolant to boil, pressures will quickly rise to a level that will exceed the pressure caps ability to relieve. Causing 1-4 to definitely occur.

On the positives for a non-pressurized cooling system:

1. None of the negatives for pressured numbers 1-4. Quite simply it becomes almost impossible for any of the non-metal components to fail.

2. Lower boiling point of 225 degrees or whatever number corresponds to the operating elevation . What this means is if you do have a truck running hot, it will start boiling the coolant at 228 degrees. Boiling water absorbs 540 times as much heat as it takes to raise it one degree, so once we reach that temperature a tremendous amount of energy is being dissipated. Further, much like a pot of boiling water on a stove cannot exceed 210 degrees until the pot is empty of water, an engine full of coolant cannot exceed the boiling temperature of the coolant until it is empty. This means that the engine temperature cannot physically exceed 225 degrees. This is a much more survivable temperature for a Disco engine. If you operated it for a long period of time at temperature it would eventually boil the coolant away, but that would take a long time, likely hours.

3. Coolant can be easily added even with the engine running at temperature by removing the cap and adding coolant without fear of a coolant eruption.

On the negatives for a non pressurized cooling system:

1. See positives for a pressurized cooling system

2. Due to the design of the D2 cap and bottle neck, a small amount of splash overflow may occur and go out the overflow tube. I eliminate this by running a hose over the overflow tube up to the cowl area. Initially I ran it up under a pillar with the end poking out in the field of view so I could watch it but after watching it for more than a year I realized it never overflowed and I don't worry about it anymore.

3. Slight coolant loss due to evaporation. I consider this to be a cost of operation much like oil or fuel. Maybe a quart between oil changes? So about $1 for 5000 miles?

4. Coolant boiling temperatures potentially within the normal operating temperature ranges at higher elevations. However this can be compensated for by running a higher concentration of antifreeze in the system, potentially bringing the boiling point back to 230 degrees at 10k ft elevation.

I have never blown a HG nor a plastic component once I started running non-pressurized, and I have pushed several to 230+ towing at elevations of less than 2k feet. I now run all my vehicles in a non-pressured condition for the reasons listed above. Typically this is easily done by loosening a coolant tank cap from the full tight position until one can here the pressure escaping. Most modern cooling system caps are designed with this functionality in mind to enable venting the system safely before opening a hot system and adding coolant. Still, it is a good idea to do this as the engine is beginning to warm up and has not reached an excessively hot temperature, typically pressures begin to build in the 160-180 degrees f temperature range.

Hopefully this post provides some valuable information on helping to minimize detrimental cooling system events and understanding the various coolant cap options and coolant mixture ratio options. As always if you have any questions feel free to email me at Extinct@ExtinctMotorsports.com