Cavitation is the phenomenon in which vacuum bubbles are created in a liquid due to an external force. A common example of cavitation is the bubbles that emerge from the prop of a boat; they are in fact small pockets of vacuum, not air. These pockets are created when forces are rapidly applied to a liquid. In a diesel engine, cavitation is created when the cylinder walls rapidly expand and contract under the immense pressures created in the cylinder. At 2,000 rpm, each piston in an engine is moving nearly 34 cycles per second (the piston is moving down, then up 34 times per second), which translates into nearly 9 power strokes per second. In effect, this cyclic motion creates a vibration that causes a small vacuum pocket to form between the engine coolant and cylinder wall. The problem with cavitation is that the vacuum area must dissipate, and when it does so the engine coolant slaps against the outer cylinder wall. While the force against the wall is low, it occurs continuously and slowly erodes away the cylinder wall. Over time, the cylinder walls can wear to the point that a hole or crack will propagate. Coolant in the engine oil and/or engine oil in the coolant is often a sign of a hole in the cylinder wall for engines that are prone to fatigue failure due to cavitation.
International's 7.3L IDI, offered in Ford pickups from 1988 to 1994, is extremely susceptible to cavitation due its high compression ratio and thin cylinder walls (relatively speaking - the cylinder walls are not abnormally thin). Neglecting to maintain the cooling system on a 7.3L IDI may result in cavitation damage in as little as 150,000 miles. In essence, all diesels are prone to cavitation. However, since the cause and effects of cavitation are now well understood, most modern engines come factory equipped with pre-treated engine coolants and the concern is minimal over the usable life of the vehicle. Regardless of what engine make you have, cooling system maintenance is extremely important, as is understanding and meeting the OEM coolant requirements.
Preventing cavitation is as easy as using the proper SCA/DCA (supplemental coolant additive, diesel coolant additive). An SCA/DCA works by chemically bonding to the outer cylinder walls. As cavitation occurs, the adhering additive provides a layer of protection between the cylinder walls and crashing coolant column. Ford's VC-8 SCA (previously FW-16), International's DCA-4, and Fleetguard's (Cummins) DCA4 seem to be the most popular additive choices. Furthermore, they are all compatible with one another. NAPA sells its own SCA, but it is not compatible with any of the aforementioned additives. You will also need to purchase test strips so that the SCA/DCA level can be monitored and maintained. It is recommended that the additive level be checked at least every other oil change. SCA/DCA test strips are available from both NAPA and Fleetguard (part # CC2602A). You may not use NAPA's test strips to test the additive concentration unless you are using NAPA brand SCA and visa versa. Also keep in mind that the coolant test strips have a shelf life, so there is no need to buy 100 of them at a time. The proper ratio of SCA/DCA to engine coolant is 1 pint (16 oz) per 4 gallons. The 7.3L IDI has roughly an 8 gallon cooling system capacity, so 2 pints are required after a cooling system flush. SCA/DCA should be used with a low silicate, ethylene glycol engine coolant concentrate. This should be used in a 50/50 mix; 50 percent coolant and 50 percent distilled water.
An alternative route is to perform a complete coolant drain/flush and upgrade to a pre-treated coolant, such as Fleetguard's FG ES Compleat Lifetime coolant (part # CC2820). This has a 150,000 mile service life, and the SCA/DCA level only needs to be checked about once per year. Fleetguard and NAPA also sell external coolant filters that are pre-charged with SCA/DCA. The system should still be checked and monitored, but the filter will be replaced as necessary to maintain the proper levels, as opposed to adding the additive directly.
If the SCA/DCA level drops below the recommended level, cavitation may erode the cylinder walls. If the concentration becomes too high, the coolant may "sludge" and/or water pump damage may occur. Properly maintaining the coolant system on these engines is therefore a careful balancing act.