Here is a lengthy explanation I copied from a member (Gooch) on another thread:
Some errors and corrections....
I've been getting lots of PM's about Bob's coolant/SCA article (the sticky that was closed), and I would like to address a few things to help folks better understand. I think Bob realizes that much of his information is out-dated, pertains to certain engines other than our Powerstrokes, and is just plain wrong.
To give you a little background, first let me say that I've had extensive training and education in the field of coolants, some by the coolant manufacturers themselves. I worked with International during their cavitation crisis of the 6.9L and 7.3L IDI, helping to write bulletins, provide technical field data, and testing several different types of coolants, SCA's and mixtures thereof. I've operated and maintained a large fleet (100+ vehicles) of International/Ford 7.3L Powerstrokes. I am also a Mechanical Engineer with a background in Materials Science who has studied cavitation in cylinder walls and water pumps. Coolants are my specialty.
Where diesel cylinder cavitation is concerned, the bubbles are formed by the rapid flexing of the cylinder wall liners as the high compression, high energy diesel combustion process takes place. Much like if you filled a plastic liter pop bottle up with water and rapidly flexed the sides of the bottle back and forth with your hand. Bubbles form without any heat present. The bubbles in diesel cylinder wall cavitation don't explode, they implode due to pressure. It is this implosion against the metal surface that causes the pitting to form in the outside of the cylinder wall. Eventually the prolonged pitting become a hole. Water/coolant enters the cylinder, and thus we have engine failure (usually via hydrolock). Unlike gasoline engines, all diesel engines experience some level of inherent cavitation, some worse than others. FWIW, the Ford 6.9L and 7.9L IDI's origianlly had serious cavitation issues because Ford did not initially require (or add) SCA to the coolant. Ford later issued a bulletin to address that, and problems decreased dramatically.
As Bob stated, SCA's are one method of inhibiting cavitation by providing a barrier on the coolant side of the cylinder wall. However, I would like to say that it is not necessarily the SCA's displaced layer that can cause scale and coolant system plugging, but the components in the SCA itself, particularly when mixed with H2O or glycol. The use of conventional SCA is effective, but it requires testing, careful dosing, and frequent flushing. It will reduce heat transfer, and exacerbate water pump and other engine component failure.
One thing that importantly needs to be corrected, and a mistake than many misinformed people make, is that not all OAT coolants are like what GM uses. GM typically uses a "Dexcool" forumulation, which is specifically not recommended for your diesel engine. Dexcool type coolants were never intended for diesel applications. There are many modern Heavy Duty Extended Life Coolants specifically designed for diesels. Delo ELC, Rotella ELC, International's Fleetrite ELC, CAT ELC, Mobil 1 ELC, and a host of others. They will address diesel cavitatation, provide superior metal corrosion protection, provide better heat transfer, require no testing and maintenance, be free of harmful abrasive silicates, borates, and phosphates, and have an operating life up to 1 million miles (Delo ELC). In fact International, who made your engine, recommends and factory-fills with these Heavy Duty Extended Life Coolants (Fleetrite ELC/Shell Rotella ELC). And in fact almost all heavy duty diesel manufacturers use Heavy Duty ELC's meeting the most strict requirements in the industry...Caterpillar's EC-1. Very little to do with GM's OAT coolant.
No where has Ford said you can not use a heavy duty ELC in your Powerstroke due to seal incompatibility. That is simply more misinformation. As we know International makes the engine and uses the HD ELC coolant, generally in more harsh, commercial applications. Seals are fine. The only coolants in the Owner's Manual Ford recommends you don't use are Dexcool and Ford's Specialty Orange. Owners and fleets have been using HD ELC successfully, and in fact with better success, for many, many years.
Another correction...Ford's Gold (G-05) coolant is nothing like a Heavy Duty ELC coolant. Thus is will not meet the more stricts specs of other Heavy Duty ELC coolants, or provide the higher level of protection. The Gold is simply a universal hybrid, one-size-fits-all coolant that Ford uses in its entire line up that, with the exception of the Powerstroke, are all gasoline engines. Ford simply uses the Gold in your diesel out of convenience, simplicity, cost, and uniformity throughout it's product line and dealerships. The Gold coolant contains conventional (green coolant) components like silicate, which eventually form microscopic abrasives that eat water pump seals. It provides a lower level of cavitation protection using traditional SCA's like Nitrite, it has a shorter life, it reduces heat transfer compared to HD ELC's, and it in fact does require testing and SCA maintenance if you are an enthusiast about protecting your engine. In fact Ford recommends you add SCA to the Gold in certain F-Series applications. Caterpillar and John Deere also recommend adding SCA if it is going to be used in their diesels. Ironically, Ford's Gold will not meet International's heavy duty diesel B-1 spec. HD ELC's will. The Gold will work, but it's not the best.
For the record, adding SCA to a HD ELC will not produce anything close to "mud" or "goop". That is more misinformation. HD ELC's and SCA's are completely compatible, although not recommended only because you will lose the long-life properties of the HD ELC. There is no need to add SCA to HD ELC's. "Extenders" are available to add to HD ELC's if you want to further extend their life from the 300K, 500K, or 750K mile mark (depending on brand).
I have no intention of discrediting Bob's comments about the Evans and RMI-25, only to inform you. Evans is a good product, although expensive and hard to find. Less expensive, easier to find modern HD ELC's provide similar operating lives and similar (or better) protection. And the Evans does require modification of your cooling system to run low pressure, when your water pump seal is designed to run (and seal) under higher pressure, as documented by Ford. I have not used RMI-25 and don't plan to for reasons I won't discuss here. I would not recommend putting anything into your cooling system other than coolant and SCA.
Additionally, these days it is not appropriate to identify coolants by color. Color means nothing. A "green" coolant could be anything from a conventional, to a pre-charged, to a G-05, to an ELC. Same with red, purple, pink, gold, etc. You have to know what type of coolant it is.
There are 4 choices for your Powerstroke....
Conventional coolant (usually green) with the addition of SCA at initial fill. Frequent SCA testing and maintenance there after.
Pre-charged coolant (usually purple or pink). Comes with an initial dose of SCA. Frequent SCA testing and maintenance required thereafter.
G-05 coolants. Comes pre-charged with SCA package. Fush required at 50K miles. Semi-annual SCA testing and maintenance recommended.
Heavy Duty Extended Life Coolants. Come pre-charged with carboxylate inhibitors. No testing or maintenance. Super protection. Super long life.
Specialty coolants. Evans. Waterless, pressureless. Long life. No maintenance or testing. Expensive.
Not to be used - Dexcool, Universal coolants, All-makes-all-models, etc.
I use Chevron's Delo HD ELC. Good for 750K miles/8 years or 1M miles with addition of an extender.
2003 F-250 XLT Crew Cab 7.3L, Chrome BigTex Grille Guard, Quad pillar - 3 ISSPRO gauges (trans, pyro, boost) and DP-Tuner F6; Roush fuel pressure / temperature / oil pressure gauges, Ford Severe Duty AIS, 31 row 6.0 transmission cooler, ScanGauge II, Marinco mod, Walker BTM