Anti-Freeze.
 

Since the anti-freeze used in our X is different from regular ant-freeze, can you go by the hydrometer readings for regular anti-freeze to determine the cold rating of the BMW anti-freeze. Temps are expected to go below -20F for the next couple of days.

Good question. I often wondered the same. But, I swapped all mine out for the yellow/green Prestone stuff that says you can mix with any color antifreeze (although mine isn't mixed with anything but water). Hope I didn't mess up there. It's just so much more convenient to buy at Walmart.

Below -20?? Really? Ouch! And here I am thinking the +15 we are supposed to get here in Mobile is going to be the end of the world.:chilled:

FWIW, I read on wikipedia that hydrometer type testers work by relating freeze protection to the density or concentration of the antifreeze, but that different types of antifreeze will have different relations between density and freezing point. So, they may not be that accurate with the special types.

An idea: If you have some of the BMW antifreeze sitting around, you could mix up a small sample with the correct proportion of water to give you the concentration that you ideally need, and see what your hydrometer tester reads. Then check your BMW and see what its antifreeze reads on the tester. If it's showing less protection, then drain some out and add enough of the good stuff to give you the "ideal" reading.

As long as you are using an ethylene glycol (not a propylene glycol) based antifreeze, a cheap hydrometer will work. It won't be very accurate (reading depends on fluid temperature) but it gives a good starting point.

If you are using a good refractometer for testing, you will get much more accurate results.

The real issue is that if you are worried about the effectiveness, it is usually because water has been added. In that case, even if the freezing point is sufficient, the additive concentration (the important things that protect cooling systems) will likely be low. The safest thing to do is to put new coolant in, at the correct ratio. As soon as it isn't -30 C out anymore.

I have always use BMW anti-freeze in the X. I have four gallons since I brought a case of it. One gallon was already mixed according to the instruction. When I used the hydrometer to sample the coolant in the radiator/expansion tank it said that I was good for -40F.

Sounds like you will be fine.

It would still be interesting to see what the gallon you already have "mixed according to instuction" tests with the hydrometer.

I did my own little experiment while checking all my vehicles over this afternoon. I have 3 GM vehicles (yes, I know this is a BMW forum, but I have a point to make here). The GM's use the "Dexcool", which is their own special brand/formula antifreeze. I don't know how it differs from the BMW stuff or the yellow/green Prestone stuff, except GM says you gotta use it, and it is orange.

I mixed a little of the Dexcool I had on hand in my garage 50/50 with water, which the label says will handle down to -34F. Stuck in my Prestone brand hydrometer type tester and it read -20F. So, maybe that says something about variation in the readings obtained with the tester for one type antifreeze when testing another type?

At least in my case with the Dexcool, it appears the reading was conservative. However, I will echo JCL's remarks about having the right concentration for other reasons, besides freeze/boilover protection. One needs to be sure to have enough of the additives in the solution to protect all the metal parts, water pump seals, etc. I think the labels of most of the brands will tell you a 50/50 mix is good for that purpose.

You should use BMW coolant because it protects the water pump and radiator...........I did the same on my 4.6is and ruined the whole system over time.

It says more about the accuracy of the hydrometer.

There are two bases for antifreeze, propylene glycol and ethylene glycol. As long as it is an ethylene base the hydrometer works, with the usual hydrometer limitations on accuracy.

The differences in specs are related to additives/chemicals related to seals, aluminum components, etc. These differences have no impact on hydrometers.