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 two recommendations to you:

1) Which one to buy:  The engine compression tester that I have always used is the Actron CP7827 Compression Tester Kit. My only complaint about this engine compression tester is that it does not come with a case to store it in.

2) Where to buy:  You can buy an engine compression tester just about anywhere, but you'll end up paying more for it (especially at your local auto parts store). The above links will help you comparison shop. I think you'll agree it's the better way to save money on the compression tester!

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.