1991 Mercedes-Benz 300SL Coolant Leak Diagnosis Cost

Common symptoms of a coolant leak include:

The engine runs hotter than normal, or overheats

You see a puddle of coolant under the car, or on parts under the hood

You notice the somewhat sweet smell of coolant in or around the car

The check engine light or temperature warning light comes on

If you notice a coolant leak, or if your car is overheating, don't drive it — doing so can cause serious (and expensive) damage to the engine.

Engine coolant leaks are typical for all vehicles, old and new, without regard to mileage. Driving conditions, driving style, age of the car, and lack of maintenance can all make coolant leaks more likely. Maintaining the cooling system will dramatically reduce the number of leaks the vehicle will have over time.

The first thing mechanics check when dealing with coolant leaks is the radiator cap. This part keeps the cooling system properly pressurized, so when it fails, problems ensue. After the radiator cap, the vehicle will be inspected for obvious leaks, damage to cooling system components, and contamination in the coolant reservoir. If contamination from combustion exists in the cooling system, the engine cylinder block, cylinder head and head gasket are suspected. If no leak is noticed, and there's no obvious sign of damage, the technician will apply a pressure tester and UV dye to the cooling system, causing the leak to become visible with a UV flashlight.

Repairing a coolant leak typically requires draining the engine coolant from the entire system, removing the faulty part, and either cleaning or replacing that component before reinstallation. Internal leaks tend to mean expensive repairs — a new head gasket or a new or refurbished engine being the most likely. Most often, though, repairing a leak involves a simple hose replacement and coolant flush.

On older vehicles, it's often recommended to flush the cooling system every two years or 30,000 miles. Many newer vehicles have extended-life coolant that can last up to 100,000 miles. It's always recommended to follow the service intervals in the owner's manual. Following the required services listed, whether it's a coolant change or coolant flush, will help ensure the longevity of the cooling system, as well as the rest of the vehicle. Outside of routine maintenance, coolant may need to be drained and replaced when repairing a cooling system leak. In that case, a full coolant flush should be performed if excessive corrosion is present, or if the factory scheduled service interval has already been passed.

The average DIYer can maintain the engine cooling system without much trouble, but diagnosing the point or cause of a leak can be tricky.  For instance, replacing a leaky hose is extremely low in difficulty. In contrast, replacing a water pump requires the removal of timing components and is not recommended for the DIYer. Don't use stop-leak products, as they can eventually cause clogs or contamination in the cooling system.