How To Do And Interpret An Engine Compression Test (1990-1995 5.0L V8 Ford Mustang)

In this tutorial, I'm going to explain how to do a compression test on the 1990-1995 5.0L V8 Ford Mustang and, more importantly, how you can interpret its test results to see if a compression problem is causing an engine no-start issue or a cylinder misfire.

All of the test steps are explained step-by-step so that you can quickly and easily figure out the engine performance problem you suspect is being caused by an engine compression problem.

APPLIES TO: This tutorial applies to the following vehicles:

• 5.0L Ford Mustang: 1990, 1991, 1992, 1993, 1994, 1995.

Symptoms Of Low Or No Engine Compression

Having done more engine compression tests that I can remember, I can tell you, matter of factly, that engine compression problems fall into one of two categories:

• Engine no-start: If enough or all of the cylinders have 0 PSI compression, the engine isn't going to start.
• Rough idle or cylinder misfire: This happens when one or more, but not all, of the cylinders have low or 0 PSI compression. And usually, the rough idle or misfire is only felt when the engine is idling. Once you step on the gas, the issue disappears.

Now, if the V8 engine in your Ford Mustang starts and runs, but it's dealing with an engine 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

To check the engine compression on your Ford Mustang's V8 engine, you'll need to remove the spark plugs. The one thing I strongly recommend you do is to disconnect the spark plug wires using a spark plug wire puller.

Otherwise, if you just pull the wire off of the spark plug, the spark plug wire's metal terminal will stay stuck on the spark plug.

Now, you can reattach the metal terminal back onto the wire, but it's a hassle that you're better off avoiding by using a spark plug wire puller. If you don't have one, this is the one I recommend:

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

I also want to mention two more things:

• Label the spark plug wires before you disconnect them, so you can put them back in the right firing order when you're done.
• 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.

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 chain.
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 5.0L 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

Depending on the wear and tear and mileage of your Mustang's 5.0 V8 engine, TEST 1 will have given you some cylinders that have a low compression value. Up to a certain point, this is normal.

Now, when it becomes a problem is when those low compression values are less than 15% of the highest compression value you recorded.

The cool thing is that we can do some simple math to figure out if those low compression values you got are causing an issue or not.

You can do this one of two ways. You can do it with pen and paper, and I'll explain how to in this section, or you can use my free online 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 5.0L 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 a nutshell, a low or zero compression value is going to be caused by one of two things:

• Bad cylinder head valves: The cylinder head valves of the low or 0 PSI compression cylinder are damaged or severely worn and can no longer seal the compression within the cylinder.
• Bad or worn piston rings: The piston rings are either worn or damaged and unable to seal the compression within the cylinder.

Thankfully, you and I can check to see which of these two are causing the low or 0 PSI compression value you got in TEST 1 by doing a wet compression test. The wet compression test simply involves adding some engine oil into the cylinder and checking its compression again.

This test will give us one of two results.

• Compression will shoot up: This lets us know that it's the piston rings that are causing the low or 0 PSI compression issue.
• Compression reading stays the same: In this scenario, this result lets us know that the issue is being caused by worn or damaged intake or exhaust valve in the low or 0 PSI 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 0 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 5.0L 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 5.0L V8 Ford Mustang Diagnostic Tutorials

You can find a complete list of 5.0L V8 Ford Mustang wiring diagrams and diagnostic tutorials in this index:

• 5.0L V8 Ford Mustang Index Of Articles.

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

• How To Test The Fuel Pump (1990-1995 5.0L V8 Ford Mustang).
• How To Test The Throttle Position Sensor (1994-1995 5.0L Ford Mustang).
• How To Test The MAF Sensor (1991-1995 5.0L V8 Ford Mustang).
• How To Test For A Blown Head Gasket (1990-1995 5.0L V8 Ford Mustang).