How To Check Engine Compression (GM 3.5L, 4.2L)

In this tutorial, I'll show you how to do an engine compression test on your 3.5L (5 cylinder) or 4.2L (6 cylinder) equipped GM pickup or SUV.

Why check the engine compression? Well, because piston rings and cylinder head valves do wear out and when they do, you'll have one of two things on your hands: 1.) a misfire condition (miss at idle or at acceleration) or 2.) a no-start condition.

The only way to find out if the internal engine components are OK or worn out (piston rings and cylinder head valves) and causing any of the two above conditions is to do an engine compression test.

To make the long story short, the engine compression test is a test that you can do (without having to take your pickup or SUV) to the shop. In this tutorial, I'll show you how to do both a dry and wet engine compression test and then interpret the results.

Symptoms Of Low Engine Compression

'Low compression' in one or several engine cylinders will have a direct impact on the way the engine Idles and the way the engine accelerates. 'No compression' on all cylinders and the engine won't start, it'll Crank but Not Start.

Here are other symptoms of low or no compression that you'll see:

1. Rough idle.
2. Check engine light (CEL) on with one or several of the following diagnostic trouble codes (DTCs):
   • P0300, P0301, P0302, P0303, P0304, P0305, P0306.
3. The spark plug (of the cylinder with Low or no compression) will always get fuel fouled.

No compression in all of the cylinders will result in a cranks but does not start condition. The symptoms you'll see will be:

1. No Start.
2. Everything else works, for example:
   • The fuel pump will activate, so you'll see fuel pressure at specification (if testing with a fuel pressure gauge).
   • All of the COP coils will spark.
   • If the COP coils are sparking, then this indirectly proves that the crank sensor is OK too.
   • The PCM will still activate all of the fuel injectors.
   • The spark plugs (of the cylinders with no compression) will always get fuel fouled.

What Tools Do I Need To Test The Engine Compression?

The most important tool that you're gonna' need is a compression tester. If you don't have one, you can rent one from your local Auto Zone or O'Reilly Auto Parts Store.

If you're gonna' buy one and want/need to save some bucks, buy it online and below is where I recommend you buy it.

Since the COP ignition coils and the spark plugs need to be removed, you'll need some of the following basic tools:

1. Ratchet wrench.
2. 5/8'' spark plug socket.
3. Extensions for the ratchet wrench.
4. Motor oil (for the 'Wet' compression test part).

In case you're needing to buy a compression tester and want to save money by buying it online, you can shop here:

What Is Engine Compression?

In a nutshell, engine compression refers to the fact that the air that the engine breathes in is compressed (after it's been mixed with fuel).

As you may already be aware, at a certain point in the piston's compression cycle, the compressed air and fuel is ignited by a spark from the spark plug.

Taking a few steps back, and looking at the entire picture, each engine cylinder needs 3 things to run optimally:

1. Air.
2. Fuel.
3. Spark.

If any one of these three is missing from the mix, then that engine cylinder will not produce power (or not enough). This then leads to a misfire condition if one or two cylinders are affected or to a no-start condition if several or all cylinders are affected.

And so the point of all of this is to let you know how important engine compression is, because engine compression belongs to the 'Air' part of the above mix of air, fuel, and spark.

If the engine has low to no compression, then you'll have a misfire or a no-start condition.

TEST 1: Dry Engine Compression Test

Although it isn't always possible (the engine may not start), you should do the compression test with a slightly warmed up engine.

In case you're wondering why, it's because metal expands with heat and this expansion may have an effect on your compression tester readings.

Now, this isn't a super critical thing, so if your particular pickup or SUV doesn't start, don't worry about this.

Before you start, take a look at the whole article and familiarize yourself with all of the steps. Please remember to always think safety first, since you'll be working around a cranking engine.

OK, to get this show on the road, I'll first explain the test steps. At the end of the test steps, you'll find two possible test results that will help you to interpret your specific test results. Let's get started:

1. 1

   Disable the fuel system. You can easily do this by simply: Removing the fuel injector fuse or disconnecting the fuel pump relay.
   
   This step is important because it will prevent fuel from being injected into the cylinder as you do the compression test, so don't skip it.

2. 2

   Remove all of the spark plugs. As your 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!

3. 3

   Install the compression tester. Thread the engine compression gauge into the spark plug hole for the number 1 engine cylinder. Hand tighten the compression gauge only! Do not use any type of tool to get it tight.

4. 4

   When ready, crank the engine as you observe the needle on the compression tester's gauge. Once the needle on the gauge stops climbing, have your helper stop cranking the engine.
   
   Write down the compression value. Record this compression reading on a piece of paper. Include the number of the cylinder this reading belongs to. Now repeat steps 1 thru' 6 on the other cylinders.

5. 5

   Interpret the results. After testing all cylinders and having written down all of your compression test readings, now you need to interpret the results.

OK, let's interpret the results of your compression test:

CASE 1: If you got a reading of 100 PSI or less (less being 0 PSI) on all of the cylinders you tested then you've got serious engine mechanical problems.

It's absolutely rare for the engine to get to the point of 0 PSI compression on all cylinders, but if it were to, this would usually means a broken timing chain.

What is common, if you have a very high mileage engine, is low compression across the board. Low usually means anything under 120 PSI (although the service manual says 100 PSI is the minimum). If your engine has reached this point, it's also smoking from the quart of oil it's burning every few days. The other symptoms you'll see is that the idle will be very rough.

CASE 2: One or two cylinders gave a low compression value. This might be normal, since each cylinder will not give the exact same compression value.

What is NOT normal is if the pressures vary by 15% or more. That's right, the individual cylinder compression readings of each engine cylinder can not vary more than 15%. To find out, let's go to: Interpreting Your Compression Test Results.

Interpreting Your Compression Test Results

As engines rack up miles and wear over time, the cylinder compression readings will usually start to vary from one another.

Most of the time, if the difference is small, you won't notice any change in how the engine runs.

But when that gap becomes too wide, you'll end up with a rough idle or a misfire issue.

The way to know if that low reading is actually causing trouble is to check if it's more than 15% lower than the highest compression number you measured.

There are two simple ways to do this: you can work it out by hand with a quick calculation, or you can save yourself the math and use my online tool here: Online Low Engine Compression Calculator (at: easyautodiagnostics.com).

If you'd rather figure it out manually, here's how you can calculate the 15% difference yourself:

• 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 the vehicle I'm working on produced the following compression readings:

Cylinder	Pressure
#1	165 PSI
#2	95 PSI
#3	155 PSI
#4	175 PSI
#5	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.

With cylinder #2 only showing 95 PSI, it's clear that this one isn't pulling its weight —it's 'dead' and the reason for the misfire.

To check if the lowest compression number from your test is still within an acceptable range, you'll need to run the same calculation again. Just remember to base it on the highest reading from your own test results, not the numbers from this example.

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

TEST 2: Wet Compression Test

A 'Wet' compression test will help you to find out if the low cylinder pressure or pressures you recorded in the 'Dry' compression test are caused by worn piston rings or worn cylinder head valves.

What you're gonna' do is add about 2 tablespoons of oil to the cylinder (that recorded no or low compression in the previous test) and then check its compression once again.

What will happen is that if the low compression values is due to worn piston rings, the compression value will go up from the previous one you got doing the Dry compression value.

If the compression value does not go up (from the previous one), then you'll know that the problem lies in the cylinder head valves.

OK, this is what you need to do:

1. 1

   Add a small amount of engine oil to the cylinder that reported low compression or no compression in the 'dry' compression test.
   
   NOTE: The amount should be about 1 to 2 tablespoons of oil.

2. 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. 3

   When all is set up, have your helper crank the engine.

4. 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.

5. 5

   Repeat steps 1 thru 4 on any other cylinder with low or 0 PSI compression.

Let's take a look at your test results:

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.

CASE 2: The compression value stayed the same. This confirms that the problem is in the cylinder head valves.