How To Do And Interpret An Engine Compression Test (1990-2007 3.0L V6 Ford Taurus, Mercury Sable)

This article will take you thru' the test and interpretation of the cylinder compression results in a step by step fashion.

This is one of the most overlooked tests to find the root cause of a misfire code, rough idle or an engine miss or a blown head gasket.

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.

APPLIES TO: This tutorial applies to the following vehicles:

• 3.0L V6 Ford Taurus: 1990, 1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007.
• 3.0L V6 Mercury Sable: 1990, 1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005.

Symptoms Of Low Or No Engine Compression

In the majority of cases, engine compression problems will cause one of two problems:

• Engine no-start: The engine does not start.
• Cylinder misfire or rough idle: The engine starts and runs, but runs with a misfire (rough idle).

On OBD II equipped vehicles, low compression problems will usually set a misfire trouble code:

• 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.

You're also gonna' see one or more of the following:

• Bad gas mileage.
• Engine burns oil when accelerating the vehicle on the road.
• Rough idle (or misfire) that goes away once the engine is accelerated.

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:

TEST 1: Dry Engine Compression Test

To check the compression of each cylinder, we're going to need to remove the spark plugs. When you're removing them, it's super important to use a spark plug wire puller to disconnect the wires.

If you don't, what usually happens is the metal terminal inside the wire boot stays stuck to the spark plug when you yank it off. If that happens, you'll have to reattach it (possible—I've done it plenty of times—but a hassle) or replace the spark plug wire altogether.

To save yourself that headache, just use a spark plug wire puller. It's a simple tool, a real time-saver, and here's a link where you can grab one if you don't already own one.

One more thing: remove the spark plugs only from a cold engine. If the engine's been running, let it cool down completely. If you're in a hurry, set a fan on top of the engine to speed it up.

Do not remove spark plugs from a hot engine. If you do, the spark plug hole threads (on the cylinder head) can strip —and believe you me, that is a nightmare you don't want to deal with.

CAUTION: You'll be working around a cranking engine, so take all necessary safety precautions and keep your hands and any loose clothing away from moving parts.

NOTE: If you don't have a compression tester and need to buy one, take a look at my recommendations here: Which Compression Tester Should I Buy?

SPARK PLUG WIRE FIRING ORDER: Before you remove the spark plug wires, label them so that you'll be able to easily reconnect them to their proper original locations.

OK, let's get testing:

1. 1

   Disable the fuel system. The easiest way is by pulling out the fuel pump relay.
   
   This step keeps the PCM from spraying fuel into the cylinders while you run the compression test.

2. 2

   Disable the ignition system. If your Taurus/Sable uses a distributor, unplug the ignition coil. If it uses an ignition coil pack, disconnect the coil pack's electrical connector.
   
   Doing this prevents spark from firing at the plugs while you perform the test.

3. 3

   Take out all six spark plugs.
   
   NOTE: Handle them carefully. If one drops and hits the floor, its ceramic insulator can crack, and that spark plug will cause a misfire.

4. 4

   Screw the compression tester into the spark plug hole for cylinder #1.
   
   Tighten it by hand only—do not use a wrench or tool.

5. 5

   Have your helper crank the engine once the gauge is in place.

6. 6

   When the needle stops moving upward, signal your helper to stop cranking.

7. 7

   Write down the reading from the gauge along with the cylinder number.

8. 8

   Do the same for the remaining cylinders by repeating steps 4 through 7.

OK, let's see what your result are telling you:

CASE 1: 0 PSI on all six cylinders. This is bad news. The engine cannot run if every cylinder has 0 PSI compression.

All cylinders at 0 PSI usually point to one of these conditions:

1. Broken timing chain.
2. Blown head gasket.
3. Catastrophic engine damage.

CASE 2: One or more cylinders show low compression. A little variation is normal, but if any cylinder is more than 15% lower than the highest reading, that cylinder will misfire.

Thankfully, it's easy to confirm if those low readings are a real issue by doing a bit of math. Go to: Interpreting The Results Of The Engine Compression Test.

CASE 3: 0 PSI in one or more cylinders. Any cylinder with 0 PSI is "dead" and will cause either a misfire or a no-start condition.

The next step is to run a wet compression test. This will help you figure out if the loss is caused by worn piston rings or leaking valves. Go to: TEST 2: Wet Engine Compression Test.

CASE 4: Compression is even across all six cylinders. This is the result you want. It tells you the engine has good compression and rules out an internal mechanical fault.

If you're tracking down a misfire or no-start issue, this result means compression isn't the cause. You'll need to keep troubleshooting the fuel system, ignition system, or another area to find the problem.

Interpreting The Results Of The Engine Compression Test

Over time, as your Taurus or Sable racks up miles, the cylinders will wear and compression values will naturally drop. It doesn't happen all at once, but it does happen.

Now, uneven compression across cylinders isn't a problem as long as the variation is within 15% of the highest reading.

If TEST 1 shows that some cylinders are lower than the others, this section will help you confirm whether those numbers fall below that 15% threshold. If they do, those cylinders are causing a performance issue.

You've got two options to figure this out: do the math by hand with pen and paper, or use my low compression calculator. You can find the 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 engine compression test produced the following compression readings:

Cylinder	Pressure
#1	165 PSI
#2	95 PSI
#3	155 PSI
#4	175 PSI
#5	165 PSI
#6	175 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 showing 95 PSI, I can call it "dead" and confirm it's the source of the misfire.

To see if the lowest reading you got on your engine is still within a good range, you'll need to do the same calculation. Just remember to use the highest compression value from your test, not the one shown in this example.

Once you've identified the "dead" cylinder, the next step is to figure out what's behind the low compression. For that, go to: TEST 2: Wet Engine Compression Test.

TEST 2: Wet Engine Compression Test

To figure out why a cylinder has low or zero PSI compression, we can run a wet compression test.

This test simply involves adding about 2 tablespoons of engine oil to the cylinder that showed the low or zero PSI reading in the dry compression test.

The oil is gonna help us determine if the problem is with the piston rings or with the cylinder's intake/exhaust valves.

Here's how it works. If the piston rings are worn, the oil will help them seal better. When that happens, the compression value will go up —which tells us the low or zero PSI from TEST 1 is due to piston ring issues.

If the problem is with the valves, the oil won't help them seal. The compression reading will stay the same as in TEST 1 —and that's how we'll know the cylinder head valves are at fault.

So here's what we're looking for:

• If the compression goes up, the piston rings are the problem.
• If the compression stays the same, the issue is with the valves in that low compression cylinder.

OK, this is what you need to do:

1. 1

   Pour a little engine oil into the weak cylinder —the one that showed low or zero PSI during the 'dry' compression test.
   
   About 1 to 2 tablespoons is all you need.

2. 2

   Screw in the compression tester into that spark plug hole. Tighten it by hand only.

3. 3

   Once everything is ready, have your helper crank the engine while you watch the gauge.

4. 4

   You'll see one of two outcomes:
   
   1.) The compression reading climbs higher than before.
   
   2.) The compression reading doesn't change.

Here's how to read those results:

CASE 1: The compression jumped up. That's your confirmation the piston rings are worn out. The problem is coming from the bottom end (engine block) of your Ford Taurus or Mercury Sable.

CASE 2: The compression didn't move. That means the low reading is being caused by worn or damaged cylinder head valves in that cylinder.

More 3.0L V6 Ford Taurus Diagnostic Tutorials

You can find a complete list of 3.0L V6 Ford Taurus and Mercury Sable diagnostic tutorials in this index:

• 3.0L V6 Ford Taurus Index Of Articles.

Here's a sample of the diagnostic tutorials you'll find in the index:

• How To Test The Fuel Injectors (1991-1995 3.0L Ford Taurus).
• How To Test The Alternator (1994-1995 3.0L V6 Ford Taurus).
• How To Test The Fuel Pump (1990-2000 3.0L Ford Taurus).
• Constant Control Relay Module Pinout (1996-1997 3.0L Ford Taurus).