‘Wet’ Engine Compression Test
After you've found the ‘dead’ cylinder (the cylinder with very low or no compression), the next step is to find out if the compression result is due to bad cylinder head valves or bad piston rings. To find out... you need to do a ‘Wet’ compression test.
You might be asking yourself, “What's a Wet compression test?”... this is a variation of the same compression test you did in the previous page... the difference being that you're gonna' add about two tablespoons of oil to the engine cylinder that showed a low compression reading in the ‘Dry’ compression test.
If the low compression test result you got (in a specific cylinder) is the result of bad piston rings, the oil will help in sealing the rings and the compression value will shoot up/catch up to the rest of the other values.
If the problem is in the cylinder head valves... then no amount of oil will help them seal and the compression value will stay the same.
OK, this is what you need to do:
Add a small amount of engine oil to the cylinder that reported low compression or no compression in the ‘Dry’ compression test
- The amount should be about 1 to 2 tablespoons of oil.
Install the compression tester onto the cylinder.
- Do not use any type of tool to tightened the compression tester... hand tight is fine.
When all is set up, have your helper crank the engine.
You'll get one of two results, either the compression value will go up (from the one you recorded before) or it 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 (block) of the engine in your Dodge van, pickup or SUV.
Why did the compression pressure shoot up? This is due to the fact that the oil you added sealed the space between the worn out piston rings and the cylinder wall. This then results in the cylinder compressing the air that was escaping between the ring and cylinder wall, which in turn increases the pressure that the compression test gauge is recording.
CASE 2: The compression value stayed the same. This confirms that the problem is in the cylinder head valves.
To explain why the blame lies in the cylinder head valves... it's because no matter how much engine oil is added to the cylinder, the oil could never help seal worn out cylinder head valves. Since the oil you added made no difference in the compression value, this then points to worn cylinder head valves.
Why an Engine Compression Test?
One of the things that can cause a misfire condition (also known as a miss or dead cylinder), is an engine cylinder or engine cylinders that produce less than normal compression.
When this happens... not matter what gets replaced (like spark plugs, spark plug wires, fuel injectors), nothing solves the miss. This is when the compression test comes in to save the day.
Over the years, I have solved many unsolvable misfire codes, rough idle, lack of power issues by doing a simple engine compression test and if you're faced with something similar... I highly recommend doing an engine compression test.
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!
Related Test Articles
To see all of the Dodge 3.9L, 5.2L, and 5.9L specific articles here at troubleshootmyvehicle.com, go to: Chrysler 3.9L, 5.2L, 5.9L Index of Articles
Here's a sample of the tutorials you'll find there:
- How Can I Tell If My Automatic Transmission Needs An Overhaul?.
- How To Test The Starter Motor (Dodge 3.9L, 5.2L, 5.9L).
- How To Diagnose Misfire Codes (Dodge 3.9L, 5.2L, 5.9L).
- How To Test The Fuel Injectors (3.9L, 5.2L, 5.9L).
- How To Test The MAP Sensor (P0107, P0108) (Dodge 3.9L, 5.2L, 5.9L).