Although testing engine compression can be a process in itself, it's not that difficult. One thing that makes it easier is the fact that the spark plugs are quite accessible, as they need to be removed to test engine compression.
In this tutorial, I'll explain how to perform an engine compression test on the Crown Vic (Grand Marquis) and also how to interpret the results to determine if any of the eight cylinders have compression issues causing an engine performance problem.
Contents of this tutorial:
APPLIES TO: This tutorial applies to the following vehicles:
- 4.6L Ford Crown Victoria: 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011.
- 4.6L Mercury Grand Marquis: 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011.
Symptoms Of Low Or No Engine Cylinder Compression
Engine performance problems typically result in one of two major issues:
- The Engine Doesn't Start: The engine cranks but won't start, usually because several cylinders aren't producing any compression.
- The Engine Starts But Runs With A Misfire: In these cases, the engine starts and runs, but one or more cylinders (with low compression) misfire, causing rough idling or a lack of power under load.
When an engine compression problem is allowing the engine to start, you're going to see one or more of the following symptoms:
- Bad gas mileage.
- Blue smoke coming out of the tailpipe.
- 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.
- The check engine light is illuminated on the instrument panel.
- Misfire trouble codes illuminating the check engine light. You many see one or more of the following:
- P0300: Random Cylinder Misfire.
- P0301: Cylinder #1 Misfire.
- P0302: Cylinder #2 Misfire.
- P0303: Cylinder #3 Misfire.
- P0304: Cylinder #4 Misfire.
- P0305: Cylinder #5 Misfire.
- P0306: Cylinder #6 Misfire.
- P0307: Cylinder #7 Misfire.
- P0308: Cylinder #8 Misfire.
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.
TEST 1: Finding The Dead Cylinders
The test instructions below assume that you're gonna test all eight cylinders on your Crown Vic (Grand Marquis), but you don't have to.
Testing all eight cylinders will provide you with the information needed to interpret your test results accurately. However, if you only need to test a specific cylinder, you need to test at least two others. This way, you'll have at least three compression readings to help you determine if the misfiring cylinder has an internal compression issue or is OK.
If you don't have a compression tester, you can borrow one or buy one from your local auto part store. If you'd like to save a few bucks, 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, these are the test steps:
- 1
Disable the ignition system by disconnecting the ignition coil packs from their electrical connector.
This will prevent the ignition coils from sparking during the test. - 2
Remove the ignition coils.
NOTE: As you're removing the ignition coils, check them for any signs of damage, broken connectors, or if their boots are covered in oil (due to a leaking valve cover gasket). - 3
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.
NOTE: As you're removing the spark plugs, check them for any signs of excessive wear and tear, damage, or if they're covered in oil (due to a leaking valve cover gasket). - 4
Thread the engine compression gauge into the spark plug hole for the number 1 engine cylinder.
NOTE: Hand tighten the compression gauge only! Do not use any type of tool to get it tight. - 5
Have your helper crank the engine till the needle on the compression gauge stops climbing.
- 6
Record the value at which the needle stopped and the number of the engine cylinder on a piece of paper.
- 7
Release the pressure on the gauge and repeat step 4 and 5 one more time.
- 8
Repeat test steps 4 - 7 on the remaining cylinders.
Let's examine your test results:
CASE 1: One or more 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: All four cylinders had almost no compression or 0 PSI compression. This low or zero compression on all cylinders will cause the engine to not start.
Having a compression value close to 0 PSI or 0 PSI is usually due one of the following problems:
- A busted timing belt.
- Blown head gasket.
- The engine threw a rod.
CASE 3: The compression value of all four 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.
