Cooling System Pressure Tester: Diagnose Leaks Efficiently

How to Test a Cooling System for Leaks Like a Pro

If your vehicle is losing coolant and you don’t see any obvious leaks, you need to take a methodical approach to diagnosing the problem. As someone who has worked on countless cooling systems, I can tell you that the most effective way to pinpoint leaks is by using a cooling system pressure tester. Knowing how to test a car cooling system for leaks isn’t just for professionals—it’s a skill every DIYer should have. In this article, I’ll walk you through how to properly use a cooling system pressure tester to diagnose and fix cooling system leaks.

Why You Need to Pressure Test a Cooling System

A properly functioning cooling system is critical for engine performance and longevity. If you notice the coolant level dropping over time but there’s no visible puddle under the car, the leak could be small, internal, or only occurring under pressure. That’s where a cooling system pressure tester comes in.

By pressurizing the system with a cooling system pressure tester, you’re essentially simulating the conditions the coolant experiences when the engine is running. This allows you to spot slow leaks that aren’t obvious during normal operation.

Step 1: Eliminate the Obvious Leak Areas With a Visual Inspection

Start by checking the upper and lower radiator hose connections. You’re looking for signs of dried coolant, loose clamps, or hose deterioration like cracks or splits. Next, check the radiator for signs of wet spots.

Follow the heater hoses from the top of the engine to the firewall, where they connect to the heater core. Again, check for signs of dried coolant, loose hose clamps, or deteriorated rubber hose. Lastly, check for signs of leakage around the thermostat housing.

If you find signs of dried coolant in any of those areas, fix them first before moving on to the cooling system pressure test.

Buy or Rent a Cooling System Pressure Tester

Two inexpensive cooling system pressure testers from Orion Motor Tech

The image on the left shows the Orion Motor Tech Coolant Pressure Tester Kit, 19pc Radiator Pressure Tester Kit with 15 Radiator Adapters & Wrench, Cooling System Pressure Tester Kit with Vacuum Pump & Probe Thermometer. It’s $44 at Amazon.com

The image on the right shows the Orion Motor Tech Coolant Pressure Tester Kit, 31pc Master Coolant Vacuum Refill Kit with 23 Radiator Adapters, Radiator Cooling System Pressure Tester Kit with Vacuum Pump Vacuum Purge & Refill Tool. It’s $78 at Amazon.com

Step 2: Testing the Radiator Cap With the Cooling System Tester

First, I remove the radiator cap or coolant reservoir cap (if the system uses a pressurized reservoir). Check the cap and seals for signs of torn or worn rubber or mineral deposits that might cause the cap to leak. If you find any of those issues, replace the cap. Next, using the cap testing adapter, test the cap.

Most caps are labeled with their pressure rating. For example, if the cap says 18 psi, that’s the maximum pressure the system is designed to handle.

Step 3: Use the Cooling System Pressure Tester To Pressurize the Cooling System

1) Using the proper adapter, connect it to either the radiator or the pressurized reservoir.
2) Begin pumping the system up, but don’t exceed the manufacturer’s pressure rating. I typically pump it up to 14-15 psi, even if the cap is rated higher, to stay on the safe side.
3) Listen closely for any hissing sounds and visually inspect for coolant seeping from hoses, connections, or the radiator.

Once you have pressure in the system, the goal is simple—does the pressure hold steady, or does it start dropping? If pressure bleeds off, you’ve got a leak somewhere. Double-check all hose connections and the radiator for signs of a leak. Check around the water pump, radiator, heater core, and any hose connections. If there’s no external leak, the problem may be internal.

If you can’t find any external leaks, then a drop in pressure means you have an internal leak. An internal leak can mean coolant is getting into the engine oil or being burned in the combustion chamber. At this stage, I check the oil dipstick—if it’s milky, that’s a red flag. A block test can confirm if combustion gases are entering the cooling system, which could indicate a blown head gasket or a cracked head.

Step 4: Using Leak Detection Dye

Sometimes, a leak is too small to spot with the naked eye. That’s when I take it a step further and use a fluorescent dye designed for cooling systems. After adding the dye and running the engine for a bit, I go over the system with a black light. The dye makes even the tiniest leaks visible, making it easier to pinpoint the issue.

The Take Aways From This Article

Knowing how to test a cooling system for leaks is a critical skill for diagnosing coolant loss before it turns into a major engine problem. Learning how to use a cooling system pressure tester properly can save you from costly repairs down the road. If the system holds pressure, you’re good. But if it doesn’t, keep digging until you find the source of the leak.

I hope this guide helps you master how to use a cooling system pressure tester and perform a leak test with confidence.

Whether you rent a cooling system pressure tester or buy a kit from the links above, you’ll save at a lot by conducting the leak test yourself versus paying a shop to do it for you.

NOTE: Ricksfreeautorepairadvice.com receives a commission for any purchases made by using the links in this article.

©, 2025 Rick Muscoplat