"I don't mean to hijack the thread, but what is an "open system" cooling system?"
I'll let others give you the expert reply, such as any advantages, or not.., but simply an 'open system' is recognizable where there is a radiator cap on the radiator, unlike some of the earlier XJ'x with the 'closed system' which does not. RENIX XJ's for example. have a screw on cap on the overflow bottle in which to deliver the fluid needed. The traditional former cap on radiator method is easier to deal with, i.e., to 'burp' system after a complete drain, and resupply of anti-freeze fluid(s). Whereas the latter closed system requires a bit more finesse, i.e., removal of the temperature sending unit on the back of the block, (with the rear of the vehicle jacked slightly uphill in order for the back of the block to be higher than the front of the block), running the engine, pouring fluid in the so-called overflow bottle, and enduring a whole lot of over-spray, lol. There are some helpful shortcuts that can be used in conjunction with the pour into the bottle, i.e., a PRESTONE "T" spliced into the top hose to the heater, in which one can also wiggle that portion of hose, or otherwise help to remove air, pour fluid into said connected fitting, to help towards a more complete fluid fill.
One disadvantage of the closed system is the fact that the bottle, and cap, have a half life that is yesterday, today, or tomorrow. Meaning they are problematic in that the bottle becomes opaque, and brittle, and the cap is best replaced with a Volvo cap, (forgot the year). New non-OEM replacement bottles seem to have a massive quality control issue since they are made by slave labor under the stuper-vision of their red army masters.
