Engine Cooling

Cooling System Flushing

Cooling System Flushing

Personal opinion on cooling system service - you don't do it often, and it isn't too much more trouble to do it really well.

1) After you have completed all replacement activities, fill the system with clear water. (At this stage, if you would drink it, it's OK for the car.) Turn the heater on, and get the engine warm enough to open the thermostat. Remove the lower radiator hose from the radiator (faster draining gets more crud out of the system.) Repeat the filling and draining until the drained water is clear and clean, with very little crud entrained.

2) If you have access to distilled or clean condensate water, fill and drain the system a couple of times using it. (One source of good water is the drain on your home air conditioner. Put the condensate drain hose in a jug for a few days. I use condensate from a dehumidifier.)

3) The last time that you drain the system, fill it to the brim with pure Dex-Cool coolant. This is a type of GM-approved non-phosphate, non-silicate coolant. Several brands are available. Target usually carries Texaco Dex-Cool. You fill with pure coolant because the system doesn't drain completely, and this gives you close to the desired 40/60 mix. The shop manual doesn't mention any bleed valves or operations.

4) Run the car (heater on!) for several minutes, checking the coolant level. The coolant should be at the seam in the coolant tank when you finish, but it won't hurt anything to have it high at this point. Check it often for the first few days. If you have access to a hydrometer, it will show you whether you need to add pure water or pure Dex-Cool to get the exact 40/60 mix as you add coolant over time.

Note: If you want to completely drain the system, you can remove the two block drains and the radiator drain, but the block drains are sometimes VERY stubborn, and there have been cases where the plugs break off, the threads in the block strip, the threads in the radiator strip, etc., etc.. If you do choose to go to the extra hassle and risk, you can then refill the system with a 50/50 mix. The water left in the system will then get the final mix close to the desired 40/60 mix.

Wally Plumley
928 Specialists

I've typically used distilled water in the past. While working in the garage, I also typically listen to 'Car Talk' from NPR on Saturday mornings. So, I was surprised when this subject was addressed two Saturdays ago. The guys went through the process discussing what a typical 'system flush' should entail.

Apparently, they contacted an engineer with Zerex, I believe, to get technical information about the process. More specifically, the engineer recommended AGAINST distilled water. Their research showed that such mineral-less water actually had the effect of leeching (for lack of a better term) minerals from the cooling system, which is detrimental. The water itself performs the same, but there is a negative effect on the system using 'pure' water.

I guess I'll go back to good ol' tap water.

JP Rodkey
79 Euro

Quite correct. Pure water is natures perfect solvent. The most aggresive of waters is RO (reverse osmosis)whereby nearly all minerals have been extracted from the water(remember those old biology classes about osmosis). Anyway, RO water tends to have a very low TDS rating (total dissolved solids). If you know of anyone with an RO system for drinking water, notice that the faucet is non-metallic. RO water loves to eat metal. FWIW

Joe Rausa '89S4

I'll bet they were refering to deionized (DI) water, not distilled water. DI water is created by running tap water through a cation and anion collection bed. This produces purified water that has a much greater negative charge due to the minerals (Ca, Mg, etc) with positive charge no longering binding with and neutralizing the negative H2O charge. The only materials impervious to DI water in plumbing applications are stainless steel and variants of PVC - it is highly corrosive. Distilled water on the other hand still retains the electro-chemical bond with these minerals, though suspended and disolved solids are removed - these being the stuff that causes build-up in a cooling system.

Randy Venier
San Diego, CA

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