How To Do A Compression Test (2003-2014 V8 Chevrolet Express, GMC Savana)

In this tutorial I'm gonna' explain how to do a compression test on the 4.8L, 5.3L, 6.0L V8 engines of the Chevrolet Express and GMC Savana van.

I'm also going to explain how to interpret to your test results so that you can find out if your compression test results are pointing to a problem.

APPLIES TO: This tutorial applies to the following vehicles:

Chevrolet:

• Express (1500) 5.3L: 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013, 2014.
• Express (2500) 4.8L, 5.3L, 6.0L: 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013, 2014, 2015, 2016.
• Express (3500) 4.8L, 6.0L: 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013, 2014, 2015, 2016.

GMC:

• Savana (1500) 5.3L: 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013, 2014.
• Savana (2500) 4.8L, 5.3L, 6.0L: 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013, 2014, 2015, 2016.
• Savana (3500) 4.8L, 6.0L: 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013, 2014, 2015, 2016.

Symptoms Of Low Or No Engine Compression

It's been my experience, that engine compression problems usually fall into one of two categories.

1. The compression problem causes the engine to not start.
2. The compression problem allows the engine to start but the engine runs with a misfire or a rough idle problem.

To be a bit more specific, when a compression issue causes the engine not to start, it's because all of the engine cylinders are not producing compression.

On the other hand, if only one cylinder has a low/no compression issue, the engine will start but you'll have a bonafide misfire problem on your hands.

Here are the specific symptoms you'll see in each case:

Engine Starts But Runs With A Misfire:

1. Also known as an engine miss, rough idle condition: Usually caused by very low compression in one or more cylinders (but not all eight).
2. Check engine light on with misfire codes:
   • 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.
   • P0307 Cylinder #7 Misfire.
   • P0308 Cylinder #8 Misfire.
3. Bad gas mileage: This is caused by the simple fact that the engine is NOT running with all 8 cylinders.
4. Engine pollutes more: Low engine compression will cause the air/fuel mixture to not burn correctly. This means that unburned fuel escapes into the exhaust. If the cylinder has no compression, all that raw fuel is being sent into the exhaust.

The Engine Cranks But Won't Start:

This usually is caused by having 4 or more cylinders with no compression. When this happens, you'll see:

1. The engine cranks very fast: This fast cranking speed is very noticeable.
2. All eight spark plugs are getting spark: This tells you that the no-start condition is not caused by a fault in the ignition system.
3. All eight fuel injectors are spraying fuel: You can confirm this with a Noid light test. You can confirm this, although indirectly, by removing the spark plugs and checking to see if they are fuel soaked (fuel fouled).
4. Fuel pump is working: Fuel pressure output is within specification.
5. The most common causes of no compression on 4 or more cylinders are:
   • Blown head gasket(s).
   • Broken timing belt.
   • Engine threw a rod.

OK, let's get testing.

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 some recommendations to you:

TEST 1: 'Dry' Engine Compression Test

To get the most accurate test results from your compression test, it's important that you test all 8 cylinders (and I hear you, it's gonna be a pain in the neck).

The illustration above will help you to identify the engine cylinders.

When you remove the spark plugs, inspect them closely to see if they have any visible damage or show signs of the engine burning oil inside the cylinder.

IMPORTANT: You'll be working around a cranking engine, so you have to be careful and stay alert at all times. Think safety all of the time!

IMPORTANT: If the engine has been running for any length of time, let it cool down completely before removing the spark plugs! You risk of damaging the spark plug threads in the cylinder heads by removing the spark plugs from a hot engine.

This is what you'll need to do:

1. 1

   Disable the fuel system. You can do this by removing the fuel pump relay from its place on the under-hood fuse/relay box.
   
   This will prevent fuel from being injected into the cylinders as you crank the engine.

2. 2

   Remove all of the spark plug wires and remove all four spark plugs.
   
   As your taking the spark plugs out, be careful and don't drop any of them on the floor, or you could cause the spark plugs ceramic insulator to break, and this will cause a misfire!

3. 3

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

4. 4

   When the tester is set up, ask your helper to crank the engine. Your job is to keep your eye on the compression tester's gauge.

5. 5

   Once the needle on the gauge stops climbing, have your helper stop cranking the engine.

6. 6

   Write down the compression value on a piece of paper. Include the number of the cylinder this reading belongs to.

7. 7

   Now repeat steps 3 thru' 6 on the other 7 cylinders.

Let's take a look at what your test results mean:

CASE 1: Low or no compression in 4 or all cylinders. This test result indicates a serious internal problem.

The most common issues would be:

• Blown head gasket(s).
• Broken timing chain.
• Engine threw a rod.

CASE 2: Low compression in one or a few 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.

The next step is to do some math to find out if this low compression value is within a normal parameter or not. Go to: Interpreting Your Compression Test Results.

Interpreting Your Compression Test Results

For the most part, engine cylinders tend to wear evenly. But it's not uncommon in high-mileage engines, or engines that received little maintenance, to have uneven wear across the cylinders.

Up to a certain point, the uneven wear won't cause a problem and you won't notice any driveability problems.

It's only when the low compression cylinders are producing a value of less than 15% of the highest compression value (the engine is producing) that you'll have a misfire problem occurring in those cylinders.

To find out if the lower compression value is causing a problem, you need to find out if it's lower than 15% of the highest compression value you got.

You can do this (figuring out the 15%) in 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 I got the following compression readings:

Cylinder	Pressure
#1	165 PSI
#2	95 PSI
#3	155 PSI
#4	175 PSI
#5	175 PSI
#6	170 PSI
#7	175 PSI
#8	170 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

There is a way to find out if the low/zero compression value you got in TEST 1 is due to a problem in the cylinder's piston rings or in the cylinder's intake/exhaust valves.

And it simply involves adding about a tablespoon or two of engine oil to the cylinder with the low compression value.

The cylinder's compression is tested once again to see if the value increases or stays the same.

If the value increases, then you can conclude that the problem is due to bad piston rings. If the value does not increase, then you can conclude that the problem is in its 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.
   
   You don't have to add a lot of oil. 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.

Let's take a look at what your test results mean:

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 Chevrolet Express (GMC Savana).

CASE 2: The compression value stayed the same. This confirms that the low compression problem of the affected cylinder is due to worn or damaged cylinder head valves.

More Chevy Express/GMC Savana Articles And Tutorials

You can find a complete list of articles and diagnostic tutorials for the V8 Chevy Express and GMC Savana here:

• GM 4.8L, 5.3L, 6.0L Index of Articles.

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

• How To Test For The Fuel Pump (2008-2013 V8 Chevy Express, GMC Savana).
• How To Test The CKP Sensor (2008-2013 Chevrolet Express, GMC Savana).
• How To Test For A Blown Head Gasket (2003-2013 V8 Chevy Express, GMC Savana).
• How To Diagnose An Engine Cranks But Does Not Start Problem (2003-2007 V8 Chevy Express, GMC Savana).