The leak tester connects to a compressor or shop air at 90 to 100 psi (not visible). The short piece of hose with quick-connect fitting threads into the spark plug hole and the gauge connects to the hose using a quick-connect fitting. The gauges show the incoming air pressure (top left), which would be shop air, and the amount of leakage in the cylinder (top right).

      The chart below shows some approximate compression pressure readings with the engine warm, all plugs removed, throttle open, and the battery fully charged.

       Leak Test

      The fourth engine test is the leak test, which is similar to the compression test. The leak test will give you more information than the compression test but the leak test gauge is less common, more expensive, and also requires 100-psi shop air supplied by a compressor. The leak test is best to run after the compression test or by itself.

      Run the leak test after the engine is warmed up. Always write down the test results in your engine build book. To perform the leak test:

      1 Remove the engine cover(s).

      2 Warm up the engine to normal operating temperature (180ºF to 200ºF) and then shut off.

      3 Disconnect the eight coil wires and remove the eight coils (two bolts per coil).

      4 Remove eight spark plugs (one per cylinder).

      5 Rotate the engine to top dead center (TDC) on the number-1 cylinder, which is the end of the compression stroke where both valves are closed. The Hemi has no distributor and no timing marks, so this may require removing the valve cover. Then mark or tape the damper and front cover’s TDC for future reference.

      6 Connect the leak test gauge to shop air (90–100 psi).

      7 Screw the test fitting into the number-1 spark plug hole.

      8 Connect the hose fitting to the leak test gauge (typically a quick-disconnect fitting).

      9 Remove the radiator cap.

      10 Remove the air cleaner.

      11 Remove the crankcase filler cap or breather.

      12 Turn shop air on, which applies full air pressure to the selected cylinder. Be sure that the 90 to 100 psi shop air maintains its pressure throughout the test.

      13 Listen for air escaping from the carburetor, crankcase, headers, throttle body, and/or exhaust manifold.

      14 Look for bubbles in the radiator water.

      15 Repeat this procedure for the other seven cylinders. Because there is no distributor or timing marks, use the engine’s firing order sequence (1-8-4-3-6-5-7-2) and 90-degree rotation to completing this step. To do that, rotate the engine 90-degrees from the TDC mark and install the test gauge into the spark plug fitting of cylinder number-8, which is the next to be tested. Once completed, rotate the engine another 90-degrees and test cylinder number-4, then 90-degrees and test cylinder number-3. Then follow the same procedure for number-6, then number-5, then number-7, and finally number-2.

      When troubleshooting, you will find that an engine will always lead a small amount. A lot of air escaping through the throttle body indicates a poorly seated or a bent intake valve. Air escaping through the tailpipe or header indicates a bad or a burned exhaust valve. Air bubbling into the radiator indicates a blown head gasket or air escaping through the water pump outlets. Air in the crankcase (listen at the valve cover breather) indicates a bad ring or ring seal.

      One of the nice features of a leak test gauge is that it gives you a numerical answer for how bad a leak is: 3 percent, less than 10 percent, 15 to 20 percent, more than 30 percent, etc. Similar to the compression test, all cylinders should be close to the average in leakage number. Leak test gauges vary, so always try to use the same gauge when you make leak test comparisons. Write down your results.

       Ask the Engine

      Production MPI engines can troubleshoot problems by scanning the engine control module (ECM) to find out what may have failed. This is done using the on-board diagnostics (OBD II) system. The federal government required all vehicles to have OBD II beginning in the late 1990s, and all Gen III Hemi engines have one. These computers have special features based on modern electronics and depend on multiple sensors to run properly.

      The OBD II system can tell you which component has failed as well as other important information on a used vehicle. The ECM can’t tell you if you need a rebuild. The OBD II reader is an expensive tool, and one that is not commonly found at repair shops. This test may have to be done at a Chrysler dealership service department, and it can be costly.

      When rebuilding an engine, it is very important to handle the OBD II sensors with care. You must locate them, disconnect them, and label all wires. They must all be reconnected once the rebuild process is complete.