The first stage of fixing car problems is often figuring out exactly what the problem is. To your clients, a certain complication might only be an unfamiliar sound or a small difference in the way the car drives. In order to determine what the source of the problem really is and how it can be fixed, you will have to perform a series of tests.
One test that you will almost certainly have to do at some point is a leak down test. The leak down test can reveal critical information about a vehicle. Read on to find out more about what a leak down test is and how to perform one!
Understanding the Difference Between Leak Down Test and Compression Test
Even though they are often performed together, a leak down test and a compression test reveal different information. A compression test can tell you if the engine is not producing enough pressure. Insufficient pressure can cause a loss in power and may prevent the engine from running. A healthy engine should have a compression of over 100 psi per cylinder with no more than 10 percent variation between the highest and lowest readings. If the results of a compression test show that pressure is lost, the leak down test can be used to determine where and how much of it is lost.
The leak down test allows you to calculate the percentage of pressure that is being lost because of leakage. Loss of pressure in the engine is not uncommon, especially in older cars. Usage can cause the piston ring, valve, or cylinder wall to wear down, which affects pressure containment. As with the compression test, there should be minimal variance in leakage between cylinders.
In your career after automotive courses, make sure you have the following equipment to perform a leak down test:
- Air source (i.e. compressor)
- Leak down gauge kit
- Socket wrench
- Notepad (to record and monitor your results)
How Auto Technicians Perform a Leak Down Test
Step 1. Remove the spark plugs and rotate the cylinders to Top Dead Center (TDC) position. Top dead center is the furthest point a piston can travel away from the crankshaft. If a cylinder is not at TDC, air might escape and provide a false reading.
Here’s a trick auto technicians use to find TDC: Place a long screwdriver into the spark plug opening and turn the engine manually with a socket wrench. When the screwdriver stops rising and before it starts falling again, you’re at TDC. If you miss the TDC, do not turn the engine backward. Take it through the cycle again.
Step 2. Connect the leak down tester to an adequate air supply and set the pressure gauge to 0.
Step 3. Install a gauge adapter into the spark plug opening and connect it to the leak down tester.
Step 4. Pressurize the cylinder. Turn the knob clockwise.
Step 5. Listen attentively for hissing or disruptions that might indicate a leak. After a couple of minutes, verify your leak down test gauge to determine how much pressure is lost. Repeat steps 1-5 for each of the vehicle’s cylinders.
If the leak percentage is above 20%, it is excessive and more verification will be required to determine which parts need to be replaced. If the leakage is around or below 10%, it means the pressure being lost is normal and that the vehicle’s cylinders are in good condition.
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