Proper compression is important especially since diesel engines run higher compression ratios. Low compression will throw other systems off, which can lead to false diagnosis and unnecessary component replacement. Cylinder leak-down tests can also be helpful in this type of diagnosis scenario. For this blog, we are going to focus on compression testing. 

What Causes Low Compression

Most of the time we hear "compression test" we automatically think about head gasket leaks. While a failing head gasket can cause low compression, other issues like cracked pistons, bent connecting rods, cracked blocks or cylinder heads, bent pushrods, leaking valves, or even a broken crankshaft can be factors as well. Some of these issues may not display any obvious symptoms. Let's see what it takes to check engine compression. These instructions may vary depending on the specific year of Duramax.

Required Tools for Testing

1. Glow Plug Bore Adapter (a.k.a Compression Gauge Adapter)
2. Compression Gauge 
3. Relay Puller (if applicable)

What to Watch for During a Compression Test

• Normal - Compression builds up quickly and evenly to the specified compression for each cylinder.
• Abnormal - Compression is low on the first stroke. Compression builds up with the following strokes, but does not reach normal. 
• Low - Compression is low on the first stroke and usually does not build up on the following strokes. 
• The minimum compression in any one cylinder is 300 PSI or 2069 kPa. There should not be more than a 50 PSI or 345 kPa difference between a cylinder and the average compression of all 8 cylinders.

How to Perform a Compression Test

1. Ensure vehicle batteries are in good condition and fully charged.
2. Operate vehicle until engine is at normal operating temperature.
3. Shut off engine.
4. Disable fuel delivery system by removing EDU/IGN 1 relay with relay puller tool. This may not be possible on certain model years. -If fuel delivery system cannot be disabled, a remote start switch will need to be used on step 10. Consult a certified repair shop if you are unsure otherwise engine damage may result!
5. Disconnect glow plug electrical connectors; remove all 8 glow plugs, make sure each is fully intact and not broken. -All 8 glow plugs must be removed from the engine during each cylinder test to obtain valid test results. If equipped with Ceramic style glow plugs, a Glow Plug Bore Reamer Tool MUST be used to clean the bores prior to installing the Glow Plug Bore Adapter. A broken glow plug could leave debris in the cylinder, debris MUST be removed before proceeding!
6. Prior to taking a compression reading, verify the cranking speed is greater than 140 RPM. If below 140 RPM, repair the slow cranking speed condition before proceeding.
7. DO NOT add oil to any cylinder during a compression test, engine damage may result!
8. Install Glow Plug Bore Adapter in the glow plug hole for the cylinder that is being checked.
9. Connect Compression Gauge to Glow Plug Bore Adapter.
10. Using the vehicle's starter motor or remote start switch, crank the engine for 6 compression strokes for the cylinder being tested. -DO NOT crank the engine using the ignition switch if the fuel system has not been disabled, engine damage may result!
11. Observe the Compression Gauge and note the readings as the test is being performed. 
12. Record the compression reading for the cylinder just tested.
13. Disconnect the Compression Gauge, remove Glow Plug Bore Adapter and move to the next cylinder. 
14. Repeat steps 8 through 13 for all remaining cylinders until completed. All 8 cylinders must be tested to obtain valid test results. 
    
15. When reinstalling, Ceramic style glow plugs must be discarded and replaced.

Example Readings

Let's take a look at the example results below. GM says the minimum compression in each cylinder should be at least 300 PSI and no more than 50 PSI difference of the average. 

Test Results:

• Cylinder 1: 300 PSI (70 PSI difference, higher than 50 PSI spec.)
• Cylinder 2: 380 PSI (10 PSI difference, ok)
• Cylinder 3: 380 PSI (10 PSI difference, ok)
• Cylinder 4: 380 PSI (10 PSI difference, ok)
• Cylinder 5: 380 PSI (10 PSI difference, ok)
• Cylinder 6: 380 PSI (10 PSI difference, ok)
• Cylinder 7: 380 PSI (10 PSI difference, ok)
• Cylinder 8: 380 PSI (10 PSI difference, ok)
• Average: 370 PSI (all readings must be within 50 PSI of this number)

Cylinder #1 read 300 PSI so it is at the minimum pressure and should be ok right? Unfortunately not. That is because the average reading of all cylinders is 370 PSI and the difference should not exceed 50 PSI. Since cylinder #1 has a 70 PSI difference, this leads us to believe there is an issue with that cylinder. Further diagnosis will be needed on cylinder #1 to determine the cause.

Conclusion

An engine may seem to run fine even though it has low or abnormal compression. The engine or fuel injection control module compensates fueling which can also mask these issues. This is where reading Fuel Injector Balance Rates can get us into trouble without proper diagnosis. As fueling is compensated, the balance rates go out of spec which leads most to think they have faulty injectors. If you are experiencing engine, fuel, and/or drivability issues, it is always a good idea to check engine compression to rule out possible underlying issues.