If you set a bucket of water down hard, you get a spritz of water back
up out of the bucket. What happened is the weight of water pressed
down into the bucket, pushing out on its sides expanding it slightly.
When the expansion snaps back, the concussion creates a wave in the
water that makes the small fountain. This is a low frequency response
of the water.
Reverse the fluids, so the liquid (now coolant) is on the outside of
the bucket (now cylinder), and the gas (was air, now air/diesel charge
in the cylinder) is inside. When the piston compresses the air charge
sufficiently to combust the fuel in it, the fuel explodes causing a
very high frequency concussion inside the cylinder. The concussion
expands the cylinder lining/jacket slightly; it rebounds- VERY
rapidly. This very high frequency vibration of the coolant jacket
wall can exceed the ability of adjacent fluid to remain in liquid
form, hence gas bubbles can form at these concussion sites, and
pitting type corrosion occurs where the gas bubbles form. The pits
can continue till the jacket has a hole or holes in it, and it can
ruin the block. If we used plain, unpressurized water instead of
coolant, we would ruin our engines in a short time; water has zero
tolerance for the vibration. Instead we pressurize the fluid, and we
use a chemically formulated fluid that resists the formation of the
gas bubbles. The Supplemental Coolant Additive (SCA) is the chemical
additive that fixes ordinary coolant used in gas engines, where the
concussion isn't critical, for use in diesels where it is critical.
The other thing that can contribute to cavitation is inadequate
pressurization of coolant system. As a regular precaution we should
have our radiator pressure cap tested & replaced when not up to spec.
I always have the new cap tested, since not all new caps (now
probably made in China) are created equal to the task. 14 pounds in
China may not be equal to 14 pounds North American.
ISL Owners: As mentioned previously in this thread, certain ISC
(8.3L) Cummins come with a coolant filter that dissolves SCA back into
the coolant at a rate approximating its decay. The ISL used in
motorhomes does not. There may be a filter can but it is a blank. So
testing for SCA level is a must biannually or so in the ISL. Unless
you change your coolant out to an OAT type coolant, in which case the
horses will be feeling their OATs longer than you will be needing the
coach in most cases.
On a related subject, I suggest placing a label at your coolant add
location specifying what coolant is used, and date of last SCA check
if applicable. This helps avoid mixing types of coolants or other
problems brought on by CRS.
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