readings should be uniform in all eight cylinders with less than 20 to 25 pounds variation, from best to worst. If the head gasket is leaking (cylinder-to-cylinder), it can cause a loss of 50 to 70 psi in pressure. The engine’s mechanical compression ratio, camshaft duration, displacement, and other specs also affect the numbers but should affect all cylinders equally. The absolute gauge reading is not as important as the indication that any cylinder is way below the average.
| Engine | Gauge Pressure | Acceptable Pressure Variation |
| 347–392 | 140 psi | 25 psi |
| 393–426+ | 150 psi | 25 psi |
Documentation Required
Keep a notebook for any engine that you rebuild. Give the engine a name or a number so you can keep rebuilds straight. As you take pieces off the engine, put all of your observations and measurements into the notebook. ■
Pro Mechanic Tip
Similar to the compression test, you only need to remove one of the Gen III Hemi’s dual plugs for the leak test. Remember that the Gen III Hemi engines use a longer 1-inch reach plug. The tester’s fitting should work fine with the extra clearance as long as the O-ring on the fitting seals to the head. ■
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.
The engine control module (ECM) or computer is basically flat and about the size of a book. It mounts to the engine compartment’s firewall by three bolts. The two plugs have lots of wires coming into the wiring harness plug (about one wire for each pin). Half of the pins are shielded from view by the lower wall of the plug. It doesn’t need to be removed for a rebuild project. One unique feature of ECMs is that they are based on on-board diagnostics (OBD II), and if you have a OBD II reader, you can ask it if anything is wrong and it will answer. Generally, a Chrysler dealer has the readers required.
Tools Required
You’ve run your tests and determined it is time to rebuild your Hemi. With any project as complicated as an engine rebuild, you will need a lot of tools (some common and some special).
Basic Tools
These tools are commonly used in many different projects, not just engine building. The basic tools