Winter weather is hard on both car and driver, but the summer heat presents a whole different set of challenges. No longer do you have to worry about having enough traction to make it up that hill, but you do have to worry that your car's cooling system can handle a mile long climb in 100+ degree heat. You may not have to worry about the cold lowering the cranking output of your battery, but you do have to worry about a hot engine compartment literally boiling the battery, for a similar effect.
Read on, and we'll give you the short list of things you should check out before the kids start summer vacation.
Engine Oil
You change your oil regularly, right? These days most cars have a little reminder computer that tells you when to get your oil changed, but if yours doesn't, you should be sure to change it at least twice a year. In the old days, cars would use a thinner oil in winter and a thicker viscosity in summer, but modern multi-weight oils have mostly done away with that. But if you take a lot of short trips in the dead of winter, your oil may not be able to evaporate out all the condensation, which can lead to internal rust and corrosion. That is why a fresh change of oil after a long cold winter is still required, even if you use the same weight oil year round.
Coolant/Antifreeze
Even if you do nothing, antifreeze becomes coolant during the summer months when there is no risk of freezing. Since keeping the engine cool is so much harder in the summer, start by making sure the system is full to the top with fluid; with the engine cold fill the radiator to the top via the cap, then fill the reservoir to the "cold" line. A 50/50 mix of antifreeze and water works best, but you are safe topping off with clean water no matter which formulation of coolant your car takes. Most modern cars have a recommended coolant change interval of 100k miles or five years, but cars made in the 20th century should get flushed and filled every two years no matter the mileage.
Tires
Many modern cars go without a spare tire, sending their driver out into the world with a can of sealant and a toll-free roadside assistance number, which is why regularly checking the condition of your tires is so important. If you don't have dedicated winter and summer tires, now is the time to check your all-seasons for wear and damage. The potholes and frost heaves of winter roads can cause sidewall damage, like the bulge seen above, which can cause a dangerous blowout if left to get worse. Summer heat causes accelerated wear on your tires too, so be sure they have plenty of life left in them (at least enough tread depth to hide part of Lincoln's hair on a penny). Often you can get a free or discounted car wheel alignment with purchase of new tires, and after a rough winter, your car would probably benefit from that too.
Belts and Hoses
The other major rubber parts of your vehicle, and the other major wear items in your cooling system, are the belts and hoses. The hoses, of course, carry coolant from the radiator to the engine and connect the heater core to the rest of the cooling system. When the ambient temperature goes up, stress on the cooling system goes up, and the hoses have a tougher time containing the pressure; look for signs of bulges or collapse, and check for cracks that may start out cosmetic but can become leaks. The belt (or belts) also have a harder time in the summer because of the air conditioning compressor, which on average needs the equivalent of 10 hp to run, not to mention the electrical requirements of the clutch in the system and the blower motor. Look for signs of cracking or shiny spots where the belt has been slipping.
Air Conditioning
Speaking of the AC compressor, now is the time to test your air conditioning and make sure it still blows ice cold. If you can't make cold air come out of the vents, and the AC compressor clutch is engaging, the most common issue is a leak causing low refrigerant. Your local auto parts store sells leak detector kits, allowing you to look for telltale red dye drips or a spray of gas which glows in UV light. There are also recharge/sealant kits which can replace the R-134a and seal up any tiny holes where it is leaking out.
Battery
One of the last things to check is the battery. These days, most people don't check the water level of their battery or test it with a hydrometer, and that is fine as most batteries are made to work with little maintenance for 3-5 years. However, since it can take a significant amount of juice to crank a hot engine in the summer, you do need to make sure everything is up to spec. Here are the details on How to Test a Battery with a Multimeter. Now is also a good time to go over the terminal ends and other connections and make sure they are clean and tight as well; since chemical reactions happen faster at higher temperatures, that corrosion will get much worse if left over the summer.