TEST 2: Wet Compression Test
If in TEST 1 your test results indicate a cylinder or cylinders with low engine compression, the next step is to do a ‘Wet’ engine compression test.
This simply involves adding a small quantity of motor oil to the affected cylinder and then checking its compression again.
If the compression value rises (from the value you got and wrote down in TEST 1), then you now know that the compression rings are behind the low compression problem.
If the compression value DOES NOT increase, then you now know that the problem is due to bad cylinder head valve seats or bad cylinder head valves (in that affected cylinder).
OK, this is what you'll need to do:
Add a tablespoon (or two) of engine oil in the cylinder you need to retest. I suggest using a small and long funnel so that the oil will reach the inside of the cylinder.
Once you've added the oil, install the compression gauge, and as before just hand tighten it.
Now, have your helper crank the engine till the needle stops climbing on the compression gauge.
As before, your job is to keep an eye on the gauge, and you'll see one of two results:
1.) The needle will climb higher than the previous compression number you recorded for this specific cylinder, or...
2.) The needle will not move at all or stay at the same number you recorded earlier.
What ever value your compression tester reads, write it down again.
If you have another cylinder that needs to be tested, repeat steps 1 thru' 4 on it now.
Let's take a look at what your compression test results mean:
CASE 1: The compression value shot up for the low compression cylinder This confirms that the low compression value registered in this cylinder in the dry test is due to worn piston rings.
The reason the compression value shot up is due to the fact that the motor oil you just added helped the piston rings to create a tighter seal. This type of test result only happens when the problem is due to worn piston rings.
CASE 2: Your compression value DID NOT shoot up (stayed the same), This result tells you that the low compression value registered in this cylinder (in the dry test) is due to worn/damaged cylinder head valves.
Which Compression Tester Should I Buy?
There are lot of engine compression testers to choose from and many places to buy them. I'm gonna' make two recommendations to you:
1) Which one to buy: The engine compression tester that I have always used is the Actron CP7827 Compression Tester Kit. My only complaint about this engine compression tester is that it does not come with a case to store it in.
Engine Compression Gauge Testers
2) Where to buy: You can buy an engine compression tester in any auto parts store in any neighborhood, in any city, but you'll be paying at least twice as much. Go to the above compression tester links, browse and compare, you'll see a big price difference!
More 4.2L Ford Diagnostic Tutorials
You can find a complete list of 4.2L Ford tutorials in this index:
Here's a small sample of the tutorials you'll find in the index:
- How To Test The Fuel Injectors (4.2L Ford).
- How To Test The Throttle Position Sensor (4.2L Ford F150, E150, E250).
- How To Test The Ford EGR Valve EGR Vacuum Solenoid, DPFE Sensor.
- How To Test The Coil Pack (Ford 3.0L, 3.8L, 4.0L, 4.2L).
If this info really saved the day, buy me a beer!