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Chromenut 11 Apr 2012 10:21 pm

Why HOAT? The PT radiator coolant thread...

Many people are asking, “why must we use HOAT in our PT Cruisers?”, “Why can’t we go with some cheaper mix?”, “Can we mix in the “green” antifreeze with our existing HOAT?” To help answer these questions, I’ve taken a lab course on Ethylene Glycol and have posted below all of the information I’ve collected related to these questions. Back when I was in the Military Service, I worked for a year on a mountaintop in Sardinia – Monte Limbara – where we maintained a heavy tropospheric-scatter ultra-high powered radio station. Six 60 foot parabolic dishes, plus untold numbers of microwave, AM and FM antenna systems. The tropo radios utilized a 10KW Klystron for the amplification of main power, and this was cooled by a system fed with poly-propylene glycol. We had to learn everything about it to become certified on those radios. Since then, I’ve worked diligently to keep the correct coolant fluid in my vehicles. The information below comes directly from the lab course and can also be found online.


The main ingredient in the vast majority of automotive antifreeze on the market today is ethylene glycol. Virgin ethylene glycol is produced from ethylene which is produced in the petrochemical industry by steam cracking. Steam cracking is a process that reduces complex hydrocarbons into simpler hydrocarbons, and is used to produce the many products that come from crude oil and other complex hydrocarbons. (Jet fuel, gasoline, diesel fuel, motor oil, ethylene and propylene glycol, etc.). The main uses for ethylene glycol are listed below.


The major use of ethylene glycol is as an automotive antifreeze. Due to its low freezing point, it is also used as a deicing fluid for windshields and aircraft. Ethylene glycol is also commonly used in chilled water air conditioning systems that place either the chiller or air handlers outside, or systems that must cool below the freezing temperature of water.


Ethylene glycol has become increasingly important in the plastics industry for the manufacture of polyester fibers and resins, including polyethylene terephthalate, which is used to make plastic bottles for soft drinks. The rapidly expanding economies of China and India have led a worldwide increase in demand for ethylene glycol, and have helped to fuel recent price increases.

The antifreeze capabilities of ethylene glycol allow it to be used for airplane deicing and have made it an important component of vitrification mixtures for low-temperature preservation of biological tissues and organs. Obviously, the purity requirements for soft drink bottles, deicing and preservation of biological tissues is much greater that it needs to be for automotive antifreeze or chiller fluid, and virgin EG is required for those applications.

Antifreeze Types

Automotive antifreeze/coolant is made of ethylene glycol, water, and a chemical additive package. For many years, there was only one type of antifreeze on the market, conventional green. Antifreeze technology, however, has become significantly more complex in the last several years with the introduction of Dexcool and other extended life antifreeze formulas. There are three main types of antifreeze on the market. They all contain an ethylene glycol base, water, dye and an additive package. Colored dye is added for leak detection and is not a reliable way of identifying coolant type. The additive package is what makes them different.

Conventional green antifreeze formulations usually contain a number of inorganic corrosion inhibitors that provide immediate corrosion protection because they maintain the pH of the solution (buffer it), but are consumed or transformed chemically as they perform their functions. As the coolant is heated and cooled, and exposed to air, the components of the conventional antifreeze additive package are depleted over time. This causes the pH to drop, and is why the coolant should be changed out every two years or 24,000 miles.

Dexcool-type extended life coolants use organic acid technology to inhibit corrosion, and are referred to as “OAT” based coolants. OAT antifreezes are touted as having longer potential service life than conventional antifreezes because of the fact that the components in the additive package are not chemically consumed as they perform their function of inhibiting corrosion. The chemicals used in the OAT type formulas protect metals from corrosion by forming a thin, molecular coating on them, and because of this, are not as fast acting as conventional inorganic formulas. However, as long as the cooling system is kept sufficiently full and coolant is not lost due to leakage nor diluted by top-off with water or conventional antifreeze, it will continue to function properly. Unfortunately, if the cooling system is not properly maintained, a “red muck” is likely to form and could cause serious cooling system problems.

An example of the “red muck” buildup, this is from a Jeep radiator top filler neck:

Another example of the “red muck” buildup, this is from a Jeep radiator with the top removed:

The third type of antifreeze on the market today is the hybrid OAT, known as “HOAT” (or G-05). One of the primary problems with OAT formulations is that they are not compatible with conventional antifreezes. The chemicals used in OAT antifreeze react to some extent with some of the inorganic salts and other components in conventional antifreeze. The result of this interaction is the generation of cloudiness and precipitates. HOAT formulations are called hybrid because the additive package contains ingredients from both OAT and conventional formulas and is compatible with both. Hybrid OAT antifreezes provide both fast acting and extended life corrosion protection, eliminate the problem of antifreeze compatibility, and therefore are compatible with all types and colors of antifreeze.

An example of Valvoline’s Zerex G-05 HOAT antifreeze:

Unfortunately, many automotive professionals consider all extended life antifreeze to be “Dexcool” and associate all the problems related to “Dexcool” with both OAT and HOAT formulas. The result of this misconception is often replacing the extended life coolant with conventional green. Industry experts say that this can be done safely if all of the green antifreeze is removed from the system. However, auto manufacturers recommend specific formulations and replacing the factory fill coolant with a different type that doesn’t meet the manufacturer’s specifications could cause liability issues down the road. A much safer solution is to replace OAT systems with HOAT coolant which meets OAT specifications.

In the Cooling System

Regardless of the type of antifreeze used, the additive package will eventually break down and cease to provide adequate corrosion protection. The antifreeze will also pick up contaminants from the engine cooling system. In other words, antifreeze wears out and gets dirty. The service life for conventional green antifreeze is 2 years or 24,000 miles, and for OAT and HOAT is 5 years or 150,000 miles, and the manufacturers recommend fluid replacement at these intervals. However, although the antifreeze loses its corrosion protection and picks up contaminants, the ethylene glycol base does not break down and remains intact. In other words, the ethylene glycol retains its ability to lower the freeze point and raise the boiling point of the solution. This simple fact is the basis for the antifreeze recycling industry.

Now for those of you who would like to see another example of what mixing your antifreeze can do, here’s is a photo taken recently of a 2002 Honda Prelude water pump assembly. The owner bought the car, mixed in standard ‘green’ antifreeze and never really took care of his car properly. The engine started making loud banging noises from around the timing belt, so the new owner pulled the water pump housing off. The impeller had shattered, broken free from the shaft and was laying in the bottom of the water pump housing. Pretty bad deal, and the mixing of antifreeze types contributed greatly to this failure.

Now before this happened, the original owner had a radiator failure. He replaced it with a used radiator and continued to pour in the ‘green’ antifreeze, with no thought at all about the outcome of his mixing up the antifreeze types. It is quite likely that if he had used HOAT only, he would not have had these issues.

Chromenut 11 Apr 2012 10:21 pm

Re: Why HOAT? The PT radiator coolant thread...
Oddly, I came across a coolant chart that really contradicts itself. As shown in my post above, the back of the G-05 bottle states that you should NOT use any other type of coolant if the OEM states that you must use HOAT. It also states that it meets and/or exceeds the industry and engine standards for Chrysler.

That all being said, if you look at these charts, especially the second one, you'll see that they state that several of these coolants work for Chrysler too. So all that being said, since Chrysler states that the PT should use HOAT, and only HOAT, why on earth would you put something else in it? I personally find that foolish behavior and you're asking for trouble, but hey, it's your PT, so do with it as you wish!

cruserdad1976 11 Apr 2012 11:27 pm

Re: Why HOAT? The PT radiator coolant thread...
well if no one else says it wow that is a lot of info thanks

we use the stright glycol in some of our inks im not sure why but i am sure it has to be there

Busted_PT 12 Apr 2012 08:38 am

Re: Why HOAT? The PT radiator coolant thread...
The examples here have more going on here than antifreeze. There is obviously stop leak products in play in these examples. See this in a lot of car brands regardless of antifreeze type. The old brown bars-leak is famous for this very thing. I'm only making references to the pics only. I'll leave the rest alone!:rolleyes:

Chromenut 12 Apr 2012 12:05 pm

Re: Why HOAT? The PT radiator coolant thread...
Personally, I've gone through 12 vehicles now of various types, mostly Chrysler and Dodge, which had overheating problems. Every single one of them had issues either in the coolant system or the radiator. The technicians I was working with showed me a shortcut to find out if the rad has issues. Very simply, remove the lower radiator hose and let it drain, then look into the opening. Every one of the 12 we worked on had piles of goo in there. Some white, some rusty red, some a sort of greenish-blue. But all had stuff in them that you would not want running through your engine block.

In all 12 cases, except for the Honda, the radiator fluid was tested first before anything else was done. 9 of the 12 vehicles had manufacturer's recommendations to use HOAT only. All 9 had mixtures, three had nothing but the original green ethylene glycol. Of all 12, not one had just HOAT in it. One even had about an 80% mixture of water, and the pH indicated that it was some incredibly hard water. That builds up scale all through the coolant system.

So in every case, all 12, any warranties that were involved were voided. All 12 were owner responsible. 5 of the Owner's complained vehemently that they didn't change the fluids, they were flushed by some other company, all five had it done at quick-change oil joints.

All this shows me that Chrysler is right - go with HOAT and nothing else. If you can imagine it - you go cheapo and buy the cheap stuff and you start having overheating issues within 12 months. The cost to clean up the cooling systems outweighs the cost of using HOAT by about 20 to 1. So why even deviate from what Chrysler suggests?

Chromenut 12 Apr 2012 01:14 pm

Re: Why HOAT? The PT radiator coolant thread...

Originally Posted by Busted_PT (Post 421035)
The examples here have more going on here than antifreeze. There is obviously stop leak products in play in these examples. See this in a lot of car brands regardless of antifreeze type. The old brown bars-leak is famous for this very thing. I'm only making references to the pics only. I'll leave the rest alone!:rolleyes:

Nope, these were confirmed cases of use of multiple types of antifreeze, no stop leak or any other chemicals were added, other than straight water.

rcktpwrd 12 Apr 2012 03:01 pm

Re: Why HOAT? The PT radiator coolant thread...
great info! :coolblue:

Samson 22 Apr 2012 12:02 am

Re: Why HOAT? The PT radiator coolant thread...
Ok I don't mean to sound dumb, but which ones are HOAT, the 50/50 antifreezes?

Chromenut 22 Apr 2012 04:40 am

Re: Why HOAT? The PT radiator coolant thread...

Originally Posted by Samson (Post 422732)
Ok I don't mean to sound dumb, but which ones are HOAT, the 50/50 antifreezes?

Nope, the Zerex G-05

Samson 22 Apr 2012 08:40 pm

Re: Why HOAT? The PT radiator coolant thread...
So that is the only HOAT made, the zerex G-05?
I see the chart says "not recommended" for Asian vehicles. So I shouldnt use it at all in my 2005 Miata?
Thank you very much for the information.

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