Professional car washes add up over time, so saving a few bucks by washing at home is tempting. Even a commercial do-it-yourself wash can approach $10 when you’re thorough. It may have been at one of those spray-your-owns that you drew the parallel between car washes and pressure washing your deck. That pressure washer is just sitting there in your garage.
Water under pressure might be the only thing that a commercial car wash has in common with your deck cleaner though. You can, of course, use a consumer pressure washer on your vehicle. The real question is, should you?
Can a pressure washer damage car paint?
Pounds per square inch (PSI) is only part of the car cleaning equation, but it’s an important one. Too much pressure and you can do damage to your car’s paint. Too little pressure and you may not be cleaning or rinsing effectively.
PSI also needs some situational awareness. Pressure at the nozzle is higher than it is 6, 8, or 10 inches from the nozzle. Nozzles have different spray angles, each of which affects the output pressure.
The amount of water pumped in gallons per minute (GPM) is also a factor. It combines with PSI to create a value called Cleaning Units (CU). Simply, PSI X GPM = CU.
A garden hose has a low PSI of about 40 with a high GPM of 7, resulting in a CU of 280. You may know that you can change the flow of water from a garden hose by placing your thumb over the end. You’re simultaneously increasing the PSI and reducing the GPM, resulting in a higher CU number.
This relationship continues to change with a pressure washer. The washer’s pump increases pressure while the nozzle restricts volume. A pressure washer that can produce 1500 PSI may have a GPM of 1.5, resulting in a CU value of 2250, almost 10 times that of the garden hose you connect to the washer.
How much psi should a pressure washer have for washing a car?
As it turns out, a PSI of 1500 is the upper limit of safe pressure for washing a car. However, consumer pressure washers can produce higher pressures than this. Electric pressure washers for the consumer market produce about 1900 psi while gas-powered washers are generally stronger.
There’s no “more is better” relationship when it comes to cleaning power. Above 1500 psi, there’s no appreciable gain in cleaning units. Do the math. You’re only increasing the chances of damaging paint.
Then, there’s the angle of spray. Obviously, a wider angle lowers the overall pressure while giving greater spray coverage. Car cleaning products manufacturer Turtle Wax recommends 25-degree nozzles for wheels and tires and 40-degree nozzles for glass and painted surfaces. Always use fan tips instead of bullet tips to reduce the chances of paint damage.
The tip of the nozzle of your pressure washer also has a “no-fly” zone. Keep the tip at least 6 inches away from glass and paint, and 12 inches away from plastic bumpers and trim. If your car already has chips in the clear coat or paint (the leading edge of the hood is commonplace for this), boost your minimum distances to avoid expanding the damage.
Yet, none of these values, PSI, GPM, CU, or spray angles tell the complete story when it comes to cleaning your car.
The importance of water quality
Touchless commercial car washes depend on high pressure to clean your car, so we know that there’s an answer to the water pressure question, otherwise there’d be class action lawsuits against those touchless wash facilities.
So let’s assume for a moment that you have a pressure washer that precisely matches the output of a touchless car wash, ignoring variances like nozzle distance. If your pressure washer delivers the same water pressure and volume as a commercial wash, you’ll get the same clean, right?
Nope, sorry, not even close.
Your grandmother knew that hot water cleans better, and with apologies to the makers of cold-water detergents, the type of grease and grime that gets on your vehicle demands water that’s warmer than what you can get from a hose. The film of dust, pavement byproducts, exposure to exhausts, and the general greasiness that always seems to collect around cars won’t budge.
Along with hot water, another advantage commercial car washes bring to the table is water softening. You might be lucky enough to be in a municipality with soft water, or if you have a softener at home, great, but your hose probably still isn’t connected to a hot water line.
Is it okay to pressure wash a car engine?
If your goal is to open the hood and give your engine a blast to clean it up, then the answer is no. If you’re prepared to understand ingress protection (IP) codes and wrap sensitive electronics with plastic to avoid damage as well as holding the nozzle a minimum of three feet away from the engine using a 40-degree tip, then maybe it’s okay to pressure wash your car’s engine.
Your car’s alternator, distributor, electrical connections, and alarm system all need protection from pressure washer spray. These components should first be encased in plastic bags which are then taped securely to prevent the bags from being blown off by pressurized water. If this seems like a lot of work to you, then it’s probably best to leave it to a professional detailer.
Can I wash my car in the driveway?
The question about pressure washing your car may be moot in many communities. For instance, you face a $3,000 fine in Calgary if you wash your car with soap of any kind, even biodegradable, and the run-off enters the city’s storm sewers.
Even using just water could net you a fine if there’s any petroleum by-product washed from your car and entering the sewer. Even though you now know that cold hose water probably won’t do a thorough job of cleaning, there’s still likely some grease and grime entering the Bow and Elbow Rivers without the benefit of treatment.
The story is similar in many towns and cities across the country, so be sure to check your local laws before firing up your pressure washer.
Or perhaps it’s easier to visit the car wash around the corner. The choice is yours.