Lesson Learned – Do Not Overfill the Cooling System. I described my experience changing the thermostat housing on my 1997 528i, but I did not describe the problem (a result of my mistake) that followed. As background, all the cars I previously worked on were older, and used a more antiquated cooling system design. “Back in the day”, the cooling system was filled directly into the radiator, and the radiator cap has a pressure relief valve built into it. Excess pressure and (important for this discussion) excess coolant vented from the system through a tube to either the ground or an atmospheric reservoir.
I have discovered that “modern” automobile engine cooling systems (like the one on my 528i) have an entirely closed cooling system; there is no vent for excess pressure or excess coolant. The “radiator cap” (which is not on the radiator, but rather on the pressurize reservoir) has a relief valve, but the vent tube reconnects back into the cooling system.
My mistake was filling the cooling system like I used to on older cars. I filled the system up to the top, and made sure that I got all the air pockets out by revving the engine and heating up the engine until the thermostat opened.
Do not fill up your cooling system that high.
Because I did, when the engine heated up fully, it blew the upper radiator hose off of the nozzle on the radiator. For me, this happened during morning rush-hour, in the middle of a six-mile construction zone on a local freeway, where there were concrete barriers on each side and only 18 inches of berm beside the road. It was also dark (it was 6:15am in late November) and 12 degrees F outside, with a stiff wind blowing. A bit inconvenient.
I have spoken to a few folks about this experience, and have been told that the reservoir tank is the usual failure spot, with the vertical seam on the side splitting open. So, I guess, I was lucky that my radiator hose popped off, instead. I did not have to buy any replacement parts.
The key point is that an air space must be maintained in the reservoir, to permit the liquid coolant a little bit of space to expand when heated. If no air space is provided, the heated coolant can develop an enormous amount of pressure, which will vent through the weakest point in the system.
After reconnecting the radiator hose, I re-filled the system to top again, revved the engine a few times, and then removed coolant (using a 50cc syringe with a length of tubing on the end) until the fluid level was below the baffle in the reservoir. That left about six or seven inches of air space in the top of the reservoir. I removed 300 cc of coolant.
The car has been performing well for the last two weeks, so I am declaring the repair a success. I just thought that it might be helpful to others working on their E39s.
Thank you Brian from Ohio for this insight!