This is more of a discussion topic than a request for specific help. Here's the deal.
It's not like I'm using/losing a lot; 50 ml in 2500 miles/5 months, but I don't see where it is going. I added a slug of UV dye to the coolant way back when, no external dye mark has ever showed up anywhere. No evidence of drips, leaks, or anything. The expansion tank, the usual suspect, holds pressure, no dye trace. No dye trace around the exhaust, no white smoke, head gasket problems unlikely. Naturally, the level fluctuates a good bit with external temperature, but on average, it drops very slightly.
Now then; some coolant loss is normal for a "normal" cooling system, with the expansion tank at atmospheric pressure. There's a bit of evaporation just from the hot coolant. I see about the same average rate of loss - 10 ml or so /month - from both my wife's Toyota and my Nissan truck. Absolutely normal, barely worth thinking about. Once a year I add a bit, every now and then I drain and refill.
But BMW is, naturally, different, and not in a good way, with the pressurized expansion tank. Nothing's at atmospheric pressure, as far as I can tell, so there's no real opportunity for evaporation. Or is there? Hoping someone with more intimate experience with the BMW cooling systems can weigh in here. Again, absolutely no trace of UV dye (except, of course, around the inside of the overflow tank fill. Nothing external), no coolant odors, nothing.
I realize this is perhaps a bit compulsive and nothing to get excited about, but I'm really curious about this. Does some water permeate the plastic (polyethylene is a bit permeable), or sneak past the cap? Possible, I suppose. Is it normal to add a bit now and then to a perfectly functioning cooling system? When I got the car I topped up with 750 ml of coolant; the service history showed that the last time the coolant had been checked and evidently flushed was almost 2 years and 20k miles prior, so that's not much to get worked up about, other than I'm due for a flush. Clearly, no history of significant coolant loss.
Ideas?
It's not like I'm using/losing a lot; 50 ml in 2500 miles/5 months, but I don't see where it is going. I added a slug of UV dye to the coolant way back when, no external dye mark has ever showed up anywhere. No evidence of drips, leaks, or anything. The expansion tank, the usual suspect, holds pressure, no dye trace. No dye trace around the exhaust, no white smoke, head gasket problems unlikely. Naturally, the level fluctuates a good bit with external temperature, but on average, it drops very slightly.
Now then; some coolant loss is normal for a "normal" cooling system, with the expansion tank at atmospheric pressure. There's a bit of evaporation just from the hot coolant. I see about the same average rate of loss - 10 ml or so /month - from both my wife's Toyota and my Nissan truck. Absolutely normal, barely worth thinking about. Once a year I add a bit, every now and then I drain and refill.
But BMW is, naturally, different, and not in a good way, with the pressurized expansion tank. Nothing's at atmospheric pressure, as far as I can tell, so there's no real opportunity for evaporation. Or is there? Hoping someone with more intimate experience with the BMW cooling systems can weigh in here. Again, absolutely no trace of UV dye (except, of course, around the inside of the overflow tank fill. Nothing external), no coolant odors, nothing.
I realize this is perhaps a bit compulsive and nothing to get excited about, but I'm really curious about this. Does some water permeate the plastic (polyethylene is a bit permeable), or sneak past the cap? Possible, I suppose. Is it normal to add a bit now and then to a perfectly functioning cooling system? When I got the car I topped up with 750 ml of coolant; the service history showed that the last time the coolant had been checked and evidently flushed was almost 2 years and 20k miles prior, so that's not much to get worked up about, other than I'm due for a flush. Clearly, no history of significant coolant loss.
Ideas?
other suggestions?