In this tutorial, I'm going to explain in a step-by-step manner how to do an engine compression test. I'm also going to explain how to interpret its results.
You'll easily find out if an engine compression problem is behind a cylinder misfire problem or an engine no-start problem.
NOTE: You can find the 4.0L Ford Explorer (Aerostar and Mercury Mountaineer) engine compression test here:
- How To Test Engine Compression (1991-2010 4.0L Ford Explorer, Mercury Mountaineer) (at: troubleshootmyvehicle.com).
Contents of this tutorial at a glance:
APPLIES TO: This tutorial applies to the following vehicles:
- 4.0L V6 Ford Ranger: 1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011.
- 4.0L V6 Mazda B4000: 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009.
Symptoms Of Low Or No Engine Cylinder Compression
Engine compression problems will cause one of the following two conditions:
- An engine no start problem.
- The engine starts and runs but runs with a misfire or a rough idle condition.
If the engine compression issue allows your 4.0L V6 Ford Ranger to start and run, you're going to see one or more of the following symptoms:
- Bad gas mileage.
- A heavier exhaust smell coming out of its tailpipe.
- Engine is not as peppy as it was once.
- Rough idle that goes away as soon as you accelerate the engine.
- If your vehicle is OBD II equipped, you'll see one or more of the following misfire diagnostic trouble codes:
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
TEST 1: Finding The Dead Cylinders
Before you disconnect the spark plug wires from the spark plugs, label all of them with the cylinder that they belong to. This will save you from losing the spark plug wires' firing order when it's time to put everything back together.
You'll need a compression tester to test the engine's compression. If you don't have one, you can borrow one from your local auto parts store for a small deposit that they'll return to you when you return the tool.
If you'd like to buy your own and save a few bucks in the process, check out my recommendations here: Which Compression Tester Should I Buy?
IMPORTANT: If the engine has been running for any length of time, let it cool down completely before removing the spark plugs.
OK, let's get this pot boiling:
Disable the ignition system by disconnecting the ignition coil pack/ignition control module assembly from its electrical connector.
This will prevent the ignition coil from sparking during the test.
Remove the spark plugs. Remember, the engine can not be hot!
When removing the spark plugs, be careful not to drop any on the floor, or you run the risk of having the spark plug's porcelain insulator crack and then you'll have a misfire on your hands.
Thread the engine compression gauge into the spark plug hole for the number 1 engine cylinder. Hand tighten the compression gauge only! Do not use any type of tool to get it tight.
Have your helper crank the engine till the needle on the compression gauge stops climbing.
Record the value at which the needle stopped and the number of the engine cylinder on a piece of paper.
Release the pressure on the gauge and repeat this step one more time.
Repeat test steps 3 - 5 on the remaining cylinders.
Let's examine your test results:
CASE 1: One or two cylinders had a much lower compression value than the others. Up to a certain range, this could be normal.
To further interpret these test results go to: Interpreting The Compression Test Results.
CASE 2: Three or more cylinders had almost no compression or 0 PSI compression. Having three or more cylinders with almost no compression or 0 PSI compression will cause your Pontiac Grand Am's 3.3L engine to not start.
Having a compression value close to 0 PSI or 0 PSI is usually due to a problem in the cylinder head valves or piston rings of the affected cylinders.
We can find out which of the two it is by doing a wet compression test. For this test go to: TEST 2: Wet Engine Compression Test.
CASE 3: The compression value of all six cylinders was similar and above 120 PSI. This test result lets you know that an engine compression problem is not behind the misfire condition and/or engine no-start problem you're troubleshooting.
I'll explain why: If the engine had a compression problem causing a misfire or a no-start condition, then your test results would have indicated one or more cylinders with low or very close to 0 PSI compression.
Since your test results indicate that all 6 cylinders have very similar compression values, you can rule out engine compression as a source of the misfire or engine no-start problem you're trying to diagnose.