Finding the Leak

Nov 30, 2023

You’re pretty sure your customer's vehicle has a leak, but what's the best way to find it? When the source of a leak isn't obvious, leak detection can be one of the most frustrating automotive service challenges. However, with the proper tools and equipment, a fast, safe and reliable diagnosis and location identification of the leak can bring about a profitable repair and a satisfied customer.

A/C System Refrigerant Leaks

One of the easiest ways to find a leak in an A/C system is to add a small amount of ultraviolet (UV) dye to the system, run the system for some time so that the dye circulates through the system, and then look for the leak stains with a UV light and/or with the aid of UV glasses.

There are now a variety of sizes of UV lamps and flashlights to suit your needs. Partially lowering the hood or turning off the lights in the service bay may also help make it easier to see the dye. The UV glasses will also help you find the leak by increasing the contrast between the dye and the engine compartment.

But some A/C leaks like to hide. Your best bet for finding leaks in hidden areas (such as inside the HVAC unit or in an evaporator) is an electronic leak detector. Look for a model that is capable of detecting HFC, CFC, HCFC and refrigerant blends.

Sensitivity depends on the model, but most can detect leaks as small as 0.1 to 0.4 oz. per year. Valuable features include audible and visual leak indicators, volume control and a gooseneck probe for one-handed operation.

Some electronic detectors also include a UV lamp on the tool, giving you two ways to locate leaks with one tool.

Coolant Leaks


If your customer's car is overheating or running too hot, you know there's probably a coolant leak.

There are several methods that can be used to find coolant leaks. Just as with refrigerant leak detection, UV dye is available to add to the cooling system. Then use your UV light to look for leaks to help pinpoint the source. This can be helpful for finding things like leaky freeze plugs, pinhole leaks in radiators or very small leaks in the heater core that may be hard to detect.

If an engine is using coolant and/or overheating but no coolant leak can be seen, the engine probably has an internal leak, which can mean a bad head gasket or a cracked head or block. Besides being hard to find, these unfortunate leaks are usually very expensive to repair.

A cooling system pressure tester is a vital piece of equipment to help you find this kind of leak. The tester hooks up to the radiator cap or coolant reservoir and uses a hand pump or shop air to pressurize the system to its normal operating pressure. If the cooling system cannot hold pressure for 10 minutes or so, it indicates an internal leak.

An infrared exhaust analyzer can also be used to sniff out internal coolant leaks by sampling the vapors at the coolant reservoir or radiator cap opening when the engine is running. If the analyzer picks up any HC or CO readings within the cooling system, it signals the leakage of combustion gases into the cooling system past a bad head gasket or cracks in the head.

Engine Vacuum Leaks

Have you ever been confronted with an engine that just doesn't seem to run right no matter what you’ve done? You may be dealing with a vacuum leak.

Diagnostic leak detection tools are essential to help technicians find these types of leaks. These systems can inject a gas or smoke into a system and the leaks can either be visible or detected with an electronic sniffer tool.

These machines feed artificial carbon dioxide or smoke into the intake manifold. The smoke may also be mixed with an ultraviolet dye to make leaks easier to see. You then look for smoke seeping out of hoses, gaskets or cracks in the manifold and/or use a UV light to find the leak. In the case of carbon dioxide, leaks can be detected with an electronic detector that can detect higher levels of carbon monoxide.

A general-purpose smoke machine typically operates at very low pressure, only about 1 to 1.5 PSI. The machine heats mineral oil or a glycol-based solution to create a nontoxic vapor. A small pump then blows this smoke through a hose, which can be connected to the PCV hose, brake booster hose or any hose or vacuum fitting to check for leaks. The smoke is white and can be seen with a light.

A general-purpose smoke machine can also be used to find a variety of other leaks including exhaust, compression, oil and even coolant leaks. Almost any part that normally holds pressure or vacuum can usually be tested for leaks with a smoke machine, including turbocharger intercoolers, brake vacuum boosters, vacuum plumbing and motors in HVAC systems, air inlet ducts between the throttle and air filter – even tires. A smoke machine can be a versatile shop tool, saving you considerable time finding leaks of many types.

EVAP Leaks

Leaks in evaporative emission control (EVAP) systems can be difficult to detect with smoke due to the size of the leak. Some older EVAP systems use a bladder or diaphragm calibrated to detect a 0.0014" to 0.0020" leak. Seeing smoke coming out of a hole this small may be a real challenge. Filling the system with an inert gas like carbon dioxide and using an electronic leak detection device may help you find the leak faster.

The EVAP system usually requires no maintenance, but faults can turn on the Check Engine light. Finding leaks in the EVAP system isn't always easy.

EVAP leak detection requires a smoke machine designed to operate at a very low pressure (only about 1/2 psi) so there is no risk of over-pressurizing the system. The key thing to remember when testing for a leak with a leak detection device is finding the right pressure. Forcing too much pressure into the EVAP system might blow off a hose or create a leak that wasn't there in the first place. This can result in replacing components unnecessarily and not uncovering the real leak. Often, high pressure will cause leaks from the charcoal canister's outside air intake valve and filter. The canister will be replaced and the real leak will go undiagnosed. Once the EVAP system starts completing key-off tests, the vehicle will be back with the same problem.

Modern systems have up to three fuel vapor circuits in the EVAP system that are used to purge, isolate, vent and test the system for leaks. These solenoids are controlled by a module and can generate codes if an open or short is detected or if an action did not generate the expected result for the EVAP sensor in the tank.

In order to test the entire system, it is necessary to set the positions of the purge, vent and other valves to fully seal the system. Doing this requires a scan tool that can bi-directionally use Mode $08 data to control the EVAP solenoids. On some non-factory scan tools it may take some time to find the correct menu to control the EVAP system.

Another advantage of a scan tool is using Mode $06 data to monitor pressure or vacuum sensor data from the EVAP pressure sensor. This information can be used to double-check the information coming from the leak detection flow meter.

Sources for this article include: Andrew Markel, editor of Underhood Service, and Larry Carley, Babcox Media technical editor.