How To Do And Interpret An Engine Compression Test (1990-1993 2.3L Ford Mustang)

In this tutorial, I'm gonna explain how to do a compression test —and more importantly, how to interpret the results.

With your test results, you'll quickly be able to tell if a compression issue is causing a rough idle, a misfire condition, or even an engine no-start problem.

All of the steps are laid out in a simple, step-by-step manner so you can pinpoint the issue right away on your 1990–1993 2.3L Ford Mustang.

APPLIES TO: This tutorial applies to the following vehicles:

• 2.3L Ford Mustang: 1990, 1991, 1992, 1993.

Symptoms Of Low Or No Engine Compression

It's been my experience, having diagnosed a ton of engine compression problems over the years, that a compression issue usually falls into one of two categories:

• Engine no-start: The engine cranks but doesn't start.
• Rough idle or cylinder misfire: In this case, the engine starts and runs, but it runs rough or misfires at idle RPM. This rough idle/misfire usually goes away as soon as you accelerate the engine.

Now, if the engine starts and runs but has a compression problem, you're going to see one or more of the following symptoms.

• Bad gas mileage.
• Blue smoke coming out of the tailpipe, most noticeable when accelerating the vehicle, since the engine is burning oil.
• 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

The 2.3L Ford Mustang comes equipped with a total of eight spark plugs. Four are on the exhaust manifold side, and the other four are on the intake manifold side of the cylinder head.

To do the engine compression test on your Mustang, you only need to remove the spark plugs on the exhaust side. If I were in your shoes, I'd also make sure the intake manifold side spark plugs are nice and tight, just to be sure they're sealing the compression in as best as possible during the test.

Now, before unplugging the spark plug wires, keep three things in mind:

• Label the spark plug wires so you can put them back in the right firing order when you're done.
• Use a spark plug wire puller to disconnect the wires from the spark plugs.
• Make sure the engine is cold or completely cooled down before removing the spark plugs. Pulling them out of a hot engine could damage the spark plug hole threads in the cylinder head.

Disconnecting the spark plug wires without a puller risks having the wires' metal terminals stay stuck on the spark plugs. If this happens, you can reattach them, but it's a bit of a hassle. You're better off just using a spark plug wire tool to avoid this complication.

If you don't have one, this is the one I use and recommend:

• Performance Tool W80519 Adjustable Spark Plug Boot And Wire Remover (Amazon affiliate link).

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?

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. You can do this by simply disconnecting the ignition coil pack's electrical connector.
   
   Doing this prevents spark from firing at the plugs while you perform the test.

3. 3

   Remove only the four exhaust side spark plugs (the intake manifold side spark plugs need to stay in place).
   
   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.

Alright, let's break down what the results are showing you:

CASE 1: 0 PSI on all four cylinders. This is the worst-case scenario. If every cylinder on your Mustang shows 0 PSI, the engine simply won't run.

When compression is at 0 across the board, it usually means one of the following:

1. A broken timing belt.
2. A blown head gasket.
3. Severe internal engine damage.

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

You can quickly confirm if those lower readings are a problem with some simple math. Head over to: Interpreting The Results Of The Engine Compression Test.

CASE 3: 0 PSI in one or more cylinders. Any cylinder on your 2.3L Ford Mustang that shows 0 PSI is essentially "dead" and will trigger either a no-start condition or a misfire.

The way forward is to run a wet compression test. That will help you confirm if the issue is due to worn piston rings or bad intake/exhaust valves. See: TEST 2: Wet Engine Compression Test.

CASE 4: Compression looks even across all four cylinders. This tells you the Mustang's engine has solid compression and rules out an internal mechanical failure.

If you're still chasing a misfire or no-start problem, this result means compression isn't the culprit. You'll need to move on to the ignition, fuel, or another system to troubleshoot what's really going on.

Interpreting The Results Of The Engine Compression Test

It's not uncommon on these 1990-1993 2.3L Ford Mustangs to see compression values that aren't exactly the same across all four cylinders, especially if the engine has high mileage.

Now, up to a certain point, uneven compression across cylinders isn't a big deal. But if the difference is more than 15% of the highest reading, you'll definitely have either a rough idle or a cylinder misfire when the engine is at idle RPM.

If the results of the dry engine compression test show that one or two cylinders have lower values than the others, then in this section, we're going to figure out if those cylinders are causing a performance issue.

To do this, we have two options. You can either do the math by hand with pen and paper, or you can use my handy low compression calculator, which you can find 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

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 on this 2.3L Ford Mustang is only hitting 95 PSI, I can call it "dead" and confirm it's the cause of the misfire.

To check if the lowest reading you recorded on your Mustang's engine is still within an acceptable range, you'll need to run the same calculation. Just be sure to use the highest compression value from your own test, not the number in this example.

Once you've nailed down which cylinder is "dead", the next step is figuring out what's causing that low compression. For that, head over to: TEST 2: Wet Engine Compression Test.

TEST 2: Wet Engine Compression Test

In the majority of cases, when you have one or more cylinders with low or 0 PSI compression, it boils down to one of two things causing the problem:

• Worn/damaged piston rings: The piston rings are worn out and not able to seal compression within the cylinder.
• Worn/damaged intake or exhaust cylinder head valves: The valves of the affected cylinder are severely worn and no longer able to seal in the compression the cylinder is producing.

Now, the cool thing is we can easily figure out which of the two it is by simply adding a bit of engine oil to the cylinder and checking its compression again. This type of compression test is known as a wet compression test.

And that's what we're going to do next to figure out what's causing that low or zero PSI compression result you got in TEST 1.

We can expect one of two things from this wet compression test:

• Compression goes up —This tells us the piston rings are the problem behind the low or 0 PSI test result.
• Compression stays the same —Then the issue is being caused by the cylinder head valves of that affected 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 make sense of those results on your 1990–1993 2.3L Ford Mustang:

CASE 1: The compression jumped up. That's your confirmation the piston rings are worn out. The low compression or 0 PSI issue is coming from the bottom end (engine block) of your Mustang.

CASE 2: The compression didn't move. That tells you the low or 0 PSI reading is being caused by worn or damaged cylinder head valves in that cylinder.

More 2.3L Ford Mustang Diagnostic Tutorials

You can find a complete list of 2.3L Ford Mustang wiring diagrams and diagnostic tutorials in this index:

• 2.3L Ford Mustang Index Of Articles.