A compression test will let you know if the hard-to-diagnose misfire (or rough idle) condition or no-start condition is due to something wrong inside the engine (like worn piston rings or worn cylinder head valves).
This tutorial will help you do a compression test and, more importantly, how to interpret your compression test results in a clear step-by-step way on your 4.0L Ford Explorer (Aerostar, Mercury Mountaineer).
You can find the 4.0L Ford Ranger (Mazda B4000) engine compression test tutorial here:
- How To Test Engine Compression (1991-2011 4.0L Ford Ranger And Mazda B4000) (at: easyautodiagnostics.com).
Contents of this tutorial:
You can find this tutorial in Spanish here: Cómo Probar La Compresión Del Motor (Ford 4.0L) (at: autotecnico-online.com).
APPLIES TO: This tutorial applies to the following vehicles:
- 4.0L V6 Ford Aerostar: 1991, 1992, 1993, 1994, 1995, 1996, 1997.
- 4.0L V6 Ford Explorer: 1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010.
- 4.0L V6 Mercury Mountaineer: 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010.
ENGINE NO-START DIAGNOSTICS: If you're dealing with an engine no-start problem and you find that compression isn't the issue causing it, the guide below will help you move forward.
Tools You'll Need:
- Compression Gauge Tester.
- A Helper.
- Pen and Paper.
Symptoms Of An Engine Compression Problem
An engine compression problem will usually cause one of two issues:
- An engine no-start problem (engine cranks but does not start).
- The engine starts and runs, but overheats within minutes and for no apparent reason.
If your Ford Explorer, Aerostar or Mercury Mountaineer's engine starts and runs but has has an engine compression problem, you'll 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.
- Misfire trouble codes (on OBD II equipped vehicles):
- 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.
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 three recommendations to you:
Disclosure: As an Amazon Associate, I earn from qualifying purchases. If my tutorials help you, using these links is an easy way to support the site at no extra cost to you. Thank you!
TEST 1: 'Dry' Engine Compression Test
As you might already know, every engine cylinder in your 4.0L Ford Explorer (Aerostar, Mountaineer) needs spark, fuel, and air to start and maintain the combustion process.
If any one of these is missing, that cylinder will not produce power, and you are gonna' feel it as you drive down the road or when you come to an idle.
Unfortunately, the compression test is one of the most overlooked tests when troubleshooting a misfire (rough idle) or an engine no-start.
I've found many misfiring cylinders with fuel and spark, but their mechanical condition was in such a bad state that the cylinder could not produce enough power.
My recommendation to you, if you're diagnosing a misfire issue, is to perform the engine compression test after you have verified that all cylinders are getting spark and fuel.
OK, to get this show on the road, this is what you'll need to do:
- 1
Disable the fuel system. You can do this by simply removing the fuel pump relay. This will prevent fuel from being injected into the cylinders as you perform the test.
- 2
Disable the ignition system. You can do this by simply disconnecting the electrical connector from the ignition coil pack. This will prevent the ignition coil pack from creating and delivering spark to the spark plug wires.
- 3
Now, disconnect the spark plug wires (label them before you disconnect them) and the remove all 6 spark plugs.
As you're taking them out, be careful and don't drop any of them on the floor, or you could cause the spark plug's ceramic insulator to break, and this will cause a misfire! - 4
Thread the engine compression gauge into the spark plug hole for the number 1 engine cylinder (this is the spark plug hole closest to the drive belt).
Hand tighten the compression gauge only! Do not use any type of tool to get it tight. - 5
Have your helper crank the engine.
- 6
When the tester's needle stops climbing, have your helper stop cranking the engine.
- 7
Now, at whatever number the needle, on the gauge, stops, write this number down along with the number of the cylinder.
This number will be the maximum compression value for that specific cylinder. - 8
Repeat steps 4-7 on the other 5 cylinders.
Let's take a look at how to interpret your test results:
CASE 1: No compression in 2 or more cylinders. This test result tells you that the engine has serious internal problems.
The most common issues behind this test result would be:
- Broken timing chain.
- A blown head gasket.
- The engine threw a rod.
CASE 2: Low compression in one or more cylinders. To a certain point, it's normal for the compression to vary a little between cylinders (as the engine accumulates thousands of miles). But if these values vary too much, then you're gonna' have a bonafide misfire on your hands.
So, your next step is to do the math and find out if these compression values are within normal parameter or not. Go to: Interpreting Your Compression Test Results.
Interpreting Your Compression Test Results
There's a good chance that the compression values you obtained in TEST 1 vary between each other.
Up to a certain point, this variation will not affect engine performance or cause a problem. But if they vary too much, you'll definitely see a rough idle condition or a misfire problem.
To find out if this low compression value or values are causing a problem, you've got to find out if they vary by more than 15% of the highest compression value you obtained from your tests.
Why? Because if the low compression value varies by more than 15%, then this cylinder is going to misfire and can be considered 'dead'.
You can do this one of two ways: You can calculate this 15% difference with pen and paper or you can use my low compression calculator. You can find the low compression calculator here: Online Low Engine Compression Calculator (at: easyautodiagnostics.com).
If you want to manually calculate the 15% difference, here's what you'll need to do:
- STEP 1: Multiply the highest compression value by 0.15 (this is the decimal value of 15%).
- STEP 2: Round the result to the nearest one (for example: 25.6 would become 26).
- STEP 3: Subtract the result (the number that was rounded) from the highest compression value.
- ANSWER: The result of this subtraction is the lowest possible compression value any cylinder can have.
Now, let me give you a more specific example: Let's say that my Ford Explorer produced the following compression readings:
| Cylinder | Pressure |
|---|---|
| #1 | 165 PSI |
| #2 | 95 PSI |
| #3 | 155 PSI |
| #4 | 175 PSI |
| #5 | 160 PSI |
| #6 | 165 PSI |
My next step is to do the following calculation:
- STEP 1: 175 x 0.15 = 26.25.
- STEP 2: 26.25 = 26 (rounded to nearest one).
- STEP 3: 175 - 26 = 149.
- ANSWER: 149 PSI. Any cylinder with this compression (or lower) value will misfire.
Since cylinder #2 is only producing 95 PSI, I can now conclude that it's 'dead' and causing a misfire.
To find out if the lowest compression value you got from your engine compression test is within a good range, you'll need to do the same calculation. Of course, you'll need to use the highest compression value you got and not the one in the example.
Once you've found the 'dead' cylinder, the next step is to find out what's causing the low compression value. For this step, go to: TEST 2: 'Wet' Engine Compression Test.
TEST 2: 'Wet' Engine Compression Test
You might be asking yourself, 'what's a wet compression test?'.
It's the same compression test you did before, with 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.
The engine oil that you're gonna' add to the cylinder will help determine if the low cylinder pressure or pressures you recorded previously are due to worn piston rings or worn cylinder head valves.
Depending on whether the compression pressure rises (on your compression tester) or not, you'll be able to say that the problem lies in the piston's rings or the cylinder head valves.
OK, this is what you need to do:
- 1
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. - 2
Install the compression tester onto the cylinder.
Do not use any type of tool to tightened the compression tester. Hand tight is fine. - 3
When all is set up, have your helper crank the engine.
- 4
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.
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 Ford Explorer (Aerostar, Mercury Mountaineer).
CASE 2: The compression value stayed the same. This test result tells you that the low compression in this specific cylinder is due to bad cylinder head valves.
More Ford 4.0L Ford Explorer And Aerostar Tutorials
You can find more Ford 4.0L Explorer (Aerostar and Mercury Mountaineer) test tutorials by going here: Ford 4.0L Index Of Articles.
Here's a sample of the test articles you'll find in the index:
- How To Test The Alternator (1991-2000 4.0L V6 Ford Explorer And Aerostar)
- How To Test The Fuel Injectors (1991-1996 4.0L V6 Ford Explorer And Aerostar).
- How To Test The Thermostat (1995-2003 4.0L V6 Ford Explorer And Mercury Mountaineer).
- How To Test For A Blown Head Gasket (1991-2010 4.0L V6 Ford Explorer, Aerostar, And Mercury Mountaineer).
- How To Test The Coil Pack (at: easyautodiagnostics.com).
If this info saved the day, buy me a beer!
