TEST 2: ‘Wet’ Engine Compression Test
If you've gotten a very low compression reading from any one cylinder, the next step is to add a little bit of engine oil (to that cylinder) and re-check its compression to see if the compression pressure will go up or not.
Adding engine oil and checking the cylinder's compression is known as a ‘Wet’ compression test. This ‘Wet’ compression test will help you determine if the piston rings or cylinder head valves are the ones causing the low or no compression reading you obtained in TEST 1.
The amount of oil you're gonna' add is about 2 tablespoons. If the piston rings are worn (and the cause of the low compression reading), this oil will cause your compression tester to record a higher compression value.
Now, since oil could never help seal the cylinder head valves, if the compression value does not go up (from the previous one), then you'll know that the problem lies in the cylinder head valves.
OK, this is what you need to do:
Add oil to the cylinder.
- You should add about 2 tablespoons of oil.
- Remember, you're only adding oil to the cylinder or cylinders that reported low or no compression in the ‘Dry’ compression test.
Install the compression tester.
- Once again, hand-tighten the compression tester.
Crank the engine when all is ready.
Once the needle on your compression tester stops climbing, have your helper stop cranking the engine.
You'll get one of two results:
1.) The compression value will go up (from the one you recorded before).
2.) The compression value will stay the same.
Let's take a look at what your test results mean:
CASE 1: The compression value shot up. This tells you that the piston compression rings are worn out and thus the problem is in the bottom end.
CASE 2: The compression value stayed the same. This confirms that the problem is in the cylinder head valves.
More 4.7L Dodge Tutorials
You can find a complete list of tutorials in this index: Dodge 4.7L Index Of Articles.
Here's a small sample of the 4.7L Dodge tutorials you'll find in the index:
- How To Test The Fuel Injectors (Dodge, Jeep 4.7L V8).
- How To Check Engine Compression (Dodge, Jeep 4.7L V8).
- How To Test The COP Ignition Coils (Chrysler, Dodge, Jeep 4.7L).
- How To Test The Fuel Pump (4.7L Dodge Dakota, Durango) (at: easyautodiagnostics.com).
- How To Test The MAP Sensor (2002-2009 4.7L Dodge) (at: easyautodiagnostics.com).
If this info saved the day, buy me a beer!