The Coolant Flush

The Coolant Flush

A lot of maintenance items get sold by some repair facilities. You have likely experienced some of the sales pitches. As an ASE certified mechanic, I am only impressed with the value of a few of them.

As the weather is quickly getting colder, let’s start with the coolant flush. There are possibly three different levels of cleaning involved with this important service. Some coolant flushes are more aptly named "exchanges". This provides little to no actual cleaning, it involves draining the coolant in the radiator, and hopefully at least most of the coolant in the engine, then refilling with fresh. More aggressive flushes add a chemical cleaner to the coolant first, then the engine is run for a half hour or so, until the engine gets hot. Then a machine is hooked up to one of the coolant hoses. This machine drains all the old coolant from this hose into one tank, while simultaneously pumping fresh coolant back in where the hose was going to originally. This service does a good bit of cleaning under normal circumstances; however, factory coolants often leave heavy mud looking deposits inside the radiator. A normal cleaner is unable to remove these deposits. To get rid of these deposits, the most aggressive flushing is necessary. This usually involves an acid type cleaner, more run time (likely actual driving will be best), and some very important clean water flushing when done. A coolant flush machine can be used if it runs a of couple clean water cycles before the final coolant, but often a garden hose is used. The hose provides lots of clean water, can back flush all the individual hoses (after removal), and the pressure will dislodge blockages.

Despite the thoroughness of the most aggressive service, I will usually recommend only the second method described, and never the first. The problem with the third service is not a matter of extra cost, but has more to do with the acid cleaner. One major manufacturer recognized the tough sludge left by its coolant and recommended a powerful acid cleaner as a one time procedure, with strict instructions as to how long the cleaner was left in, and how to clean it out. Interestingly, the cleaner burned the nose with a similar smell to battery acid. The obvious danger here is that the cleaner will eat completely through some portion of the cooling system. Dangerous? Yes. Would I be willing to use this on a car that already has a decade of use? Not mine. No car has come to my attention yet, that had overheating problems because of this muddy sludge, so, I think it best to leave it alone.

This may leave you wondering about what type of coolant to use. The old fashioned green coolant still seems to work best on everything. The newer "yellow" coolants that cover both applications also seem to work fine. I often recommend against using the "orange" type coolants, but will never try to persuade a customer with a preference. Much has been made of the problem of mixing two different color coolants. My experience has been that it makes for an ugly soup of coolant, but still functions normally. A good coolant flush should leave you with one color or the other.

Should you buy the sales pitch? Most new cars state that the coolant is good for 5 years, some even say lifetime. Five years is a long time for the coolant to break down, but, honestly, lots of cars come in for there first flush several years after the first five! So why do it at all? Two reasons.

One: Freezing point. Fresh coolant usually freezes around -40 degrees F. As it gets older, the freezing protection gets weaker. Often cars have old coolant in them that would freeze at 10 degrees above zero! Temps are very likely to get lower than that on any given winter, and if your coolant freezes, it will easily split open the cast iron your engine is made of! It will also boil easier, leading to overheating.

Two: Acidity. As the coolant ages, it picks up more contaminates, and grows weakly acidic. On newer cars (get used to hearing this caveat) this is hard on the aluminum parts, specifically aluminum cylinder heads. A good number of readers are likely familiar with a popular v6 engine that seems to often develop head gasket failure around 150,000 miles. Most of these have obvious craters in parts of the cylinder head that are supposed to hold some very high pressure gaskets. Changing your coolant 10,000 miles sooner than a failure of this type will not help anything!!!! Coolant needs to be changed before the damage is done. I would urge, well before 100,000 miles.

So, are you going to give in to this often high pressure sales item? Perhaps if the data supports it. By that I mean, do you know the coolant is really old? Or have you been provided with actual test? Yes it is not that hard to actually test the coolant to determine both freezing point, and acidity! Otherwise, it is just opinions fighting against each other, and I am too busy getting dirty to want to get involved with that!

by Jarret Swank