Most car owners know they should check the battery and tyre pressures before putting a vehicle into storage. Fewer think about what’s going on inside the engine, brake lines and cooling system while the car sits unused for months.
Your car’s fluids don’t just sit there waiting for you to come back. They degrade, absorb moisture and can cause real damage if they’re left unattended for months at a time.
Engine Oil Breaks Down Even When the Car Isn’t Running
You might assume that oil only degrades through use, but that’s not how it works. Modern engine oils contain additives that break down over time regardless of whether the engine is running. These additives protect against corrosion, reduce acid build-up and keep internal seals conditioned. Once they deteriorate, the oil loses its protective qualities and can actually become corrosive.
Old oil also absorbs moisture from condensation inside the engine. This is especially common in unheated garages or outdoor storage where temperatures fluctuate between day and night. That moisture mixes with combustion byproducts already present in used oil and creates acidic compounds that attack bearing surfaces, cylinder walls and camshaft lobes.
The fix is simple. Change the oil and filter before the car goes into storage, not after. Fresh oil gives your engine the best protection while it sits. If the car has been stored for more than six months without an oil change, it’s worth draining and replacing it before you start the engine again.
Fuel Goes Stale Faster Than You’d Think
Petrol has a shelf life of roughly six months in a sealed container at 20 degrees, and less if it’s exposed to higher temperatures. In a car’s fuel tank, conditions aren’t ideal. The tank breathes as temperatures change, letting in air and moisture. Over time, the fuel oxidises, loses its octane rating and leaves behind varnish-like deposits that can clog injectors, fuel pumps and lines.
Diesel has its own problems. Left too long, it develops a waxy consistency that blocks filters and makes cold starting difficult. Both fuel types also attract water through condensation, which can corrode the inside of the tank and fuel system components.
If you’re storing a car for more than a couple of months, you have two options. Either fill the tank completely and add a fuel stabiliser, which can protect the fuel for up to 24 months. Or drain the system entirely, though this is more involved and usually only worth doing for very long storage periods. A full tank also reduces the air space inside, which limits condensation.
Brake Fluid Absorbs Moisture Through the Lines
Brake fluid is hygroscopic, which means it actively draws in moisture from the surrounding air. Even in a sealed system, small amounts of water vapour can enter through microscopic pores in rubber hoses and seals over time. In a car that’s being driven regularly, this happens slowly and you’ll replace the fluid on schedule. In a stored car, the fluid sits stagnant while continuing to absorb moisture.
Water in brake fluid causes two problems. It lowers the boiling point, meaning you could experience brake fade or failure under hard braking after taking the car out of storage. Fresh DOT 4 fluid has a minimum dry boiling point of 230°C, but once it has absorbed just above 3% water by volume, that figure can drop to as low as 155°C.
It also causes internal corrosion. Brake caliper pistons, wheel cylinders and ABS components are all vulnerable. This damage often isn’t visible until you try to use the brakes and find they’re sticking or pulling to one side.
If a car has been sitting for more than 12 months, the brake fluid should be flushed and replaced before the vehicle goes back on the road. It’s a relatively cheap job that could prevent a very expensive problem.
🚗Related guide: 5 Common Mistakes To Avoid When Storing Your Car
Coolant Loses Its Protective Properties Over Time
Coolant does two jobs. It prevents the engine from overheating and it protects the cooling system from corrosion. The corrosion inhibitors in coolant have a finite lifespan, and they degrade whether the engine is running or not. Once those inhibitors are spent, the coolant itself can become mildly acidic and start attacking the very components it’s supposed to protect.
In a stored car, coolant will be present in:
- The radiator
- Heater matrix
- Water pump
- And engine block
If the inhibitors break down, you can end up with internal corrosion in the water channels, a failing water pump seal or a pinhole leak in the radiator. These are the kinds of problems that don’t show up until the car overheats on its first proper drive.
Most manufacturers recommend changing coolant every two to five years depending on the type. If your car has been sitting for over a year with old coolant, it’s worth draining and replacing it with fresh fluid mixed to the correct concentration. This is especially important if the car was stored in a space without climate control, where temperature swings can accelerate degradation.
Power Steering and Transmission Fluids
These often get forgotten entirely, but they’re not immune to storage problems. Power steering fluid can oxidise and thicken over time, especially in older hydraulic systems. You might notice stiff or noisy steering when you first drive the car, which usually points to degraded fluid or air that’s worked its way into the system.
Automatic transmission fluid is more resilient, but extended storage can still cause seals to dry out and contract. This can lead to small leaks when the car is driven again, particularly around the pan gasket and cooler lines. A visual check underneath the car after its first drive will usually reveal any issues.
What a Professional Storage Facility Will Do Differently
One of the biggest advantages of using a professional car storage facility is that fluid health doesn’t get overlooked. A good facility will carry out a full fluid check as part of the intake process and flag anything that needs attention before the car goes into long-term storage.
Climate-controlled, dehumidified storage also makes a significant difference. Stable temperatures and low humidity mean less condensation inside the engine, fuel tank and brake system. That alone slows down most of the degradation described above.
Regular maintenance checks during the storage period will catch problems early. A facility that inspects vehicles on a routine schedule can spot a coolant leak, a corroding brake line or discoloured oil before it becomes a costly repair.
Before You Put Your Car Away
If you’re planning to store a car yourself, change the engine oil, top up the coolant, fill the fuel tank with stabiliser and check the brake fluid’s condition before you lock the garage door. These are small jobs that’ll save you from much larger bills when you come to start it up again.On the other hand, if you’d prefer not to worry about any of it, a purpose-built storage facility will handle the lot for you.
