CARS.COM — What is underbody flushing? It's simply a means of washing a vehicle's underside to remove anything that could cause rust or corrosion over the long term. GM has, for example, recommended for years that owners flush the underbody of their vehicle at least once a year to remove salt deposits, mud, dirt and other accumulated crud. Other manufacturers may recommend that kind of cleaning as well, and even where it isn't recommended, it is a good idea for those who engage in off-road driving or frequently travel unpaved roads.
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GM doesn't make a big deal about underbody flushing, advising that owners should use plain water to clean the undercarriage of cars so road salt and other debris doesn't take up permanent residence. This can be done at a car wash (which sometimes offer underbody washes as an extra-cost service), with a garden hose in your driveway or — if you're the type of owner for whom maintenance expense knows no bounds — you could take it to a dealer. We can't help but wonder, though, how much cleaner the underside of your vehicle would be as a result of having a dealer do it (and how much it would cost).
Perhaps going to a car wash on a regular basis and making sure some water is squirted where the sun doesn't shine is all you need to keep the underbody clean, especially if you do so not long after snow storms once road salt residue has been washed away. Just don't get carried away, because you could do too much of a good thing: Though GM advises having an underbody flush annually, it also cautions, "Do not directly power wash the transfer case output seals" on all- and four-wheel-drive models in an attempt to hit all of the nooks and crannies. High-pressure water could damage the seals and contaminate the transfer case fluid, which would then need to be replaced.
Likewise, getting too enthusiastic with your cleaning and spraying up from below with a nozzle can result in water pooling on top of the engine where it may short out spark-plug wires and cause rough running or stalling. If you find this has happened, leave the hood open and dry the water thoroughly before driving your vehicle again.
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