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For styling purposes, engines have been covered with a removable plastic cover. The engine shown has a large cover; some engines have two smaller covers. You can’t do much troubleshooting until these covers are removed and you have access to the actual engine components. (Photo Courtesy FCA US LLC)
The Gen III Hemi spark plug is unique because it has a 1-inch thread depth rather than the 3/4-inch depth used on the Gen II Hemi and all Mopar small-blocks. The troubleshooting tests focus on the end or tip of the plug. Some engine builders use a magnifying glass to look at the electrodes more closely.
Next, remove the eight spark plugs (one per cylinder since all Gen III Hemi engines feature dual plugs). The Gen III Hemi plugs are 1-inch reach plugs rather than the Gen II Hemi and Mopar small-block 3/4-inch reach plugs. When you remove them, keep them in order (1-3-5-7-2-4-6-8) then look at each one closely. There are about eight aspects to the plug’s potential failure listed in the plug manufacturer’s guide. What you are looking for is one or two plugs that don’t look like the others. Check for one that is black and/or sooty, looks oily, or smells of fuel.
Pro Mechanic Tip
Vacuum leaks are the number-1 drivability problem. It is a good idea to check all the vacuum hoses and connections, but it is not an official troubleshooting test. ■
Head Gasket Test
If the head gasket fails, the engine should be rebuilt. That is because in order to replace the head gasket, you have to remove the cylinder heads, and if you have the heads off the engine, you might as well look at the whole engine and not just the head gasket.
The head gasket can fail in one of two ways: from adjacent cylinder-to-cylinder or from cylinder-to-water jacket. The adjacent cylinders in a V-8 engine are 1-to-3, 3-to-5, 5-to-7 on the left bank and 2-to-4, 4-to-6, and 6-to-8 on the right bank. They are most prone to fail because they have the thinnest support area and the highest loads. The area from the combustion chamber to the oil feed or the oil feed to water could also fail, but this is less common.
To test for adjacent cylinder failure, run the compression test outlined in the next section. If a gasket failed in this manner, you will see a 50- to 70-percent loss in compression pressure.
An observation test to detect failure to the water jacket is easy to execute, but it is not as accurate as the compression and leak tests that require special gauges. To perform the observation test:
1 Remove the pressure cap from the radiator.
2 Start the engine and allow it to warm up until the thermostat opens.
3 Look for bubbles in the coolant in the radiator, which indicates a large combustion pressure leak exists.
Compression Test
The third engine test is the compression test, which requires an inexpensive compression gauge. Nearly every engine builder has one. The compression test is best run after the engine is warmed up, but it also can be done cold because you are looking for differences not absolute values.
Special Tool
It is helpful to keep a battery charger hooked up to the battery during this compression test to ensure that the cranking speed (RPM) for each cylinder is maintained for each cylinder. The RPM that the starter turns the engine over will factor into the compression gauge reading. ■
With the simple head gasket test, you don’t usually get to look at the actual gasket. This can only be done at the disassembly step. Note the extra row of smaller head bolts across the top of the gasket.
There are several styles of compression gauges. This one has a short piece of hose that threads into the spark plug holes. The extra-long reach of the plug shouldn’t cause any problems as long as the O-ring seals to the plug seat. The actual gauge connects to the hose with a quick-connect fitting (on the end of the short hose) a few inches above the valve cover. Only remove one spark plug.
All Gen III Hemi engines have dual 1-inch reach plugs rather than the shorter 3/4-inch reach plugs. For this test you only need to remove one plug per cylinder. The typical compression gauge fitting is designed to work in the shorter plug heads, but it should work fine with the extra clearance as long as the O-ring on the fitting seals to the head.
Compression Test
1 Remove the engine cover(s) to access the coils and plugs.
2 Run the engine up to normal operating temperature (180ºF) and then shut off the engine.
3 Disconnect the eight coil wires and remove the eight coils (two bolts).
4 Remove eight spark plugs (one per cylinder).
5 Remove the air cleaner.
6 Hold open the throttle blade on the single throttle body (1/2 throttle to wide open) to allow air in.
7 Thread the compression tester into the cylinder number-1 spark plug hole.
8 Crank the engine over on the starter for at least four cycles.
9 Write down the maximum gauge reading for cylinder number-1.
10 Repeat this procedure for all seven remaining cylinders.
Compression 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
