Purpose
To determine if compression is too low in any cylinder
NOTE: During this test the IDM shuts off the injectors so no fueling occurs.
NOTE: This test can only be done with the EST; MasterDiagnostics® software is required.
NOTE: This test is used in conjunction with the Injector Disable Test to distinguish between an injector problem or a mechanical problem.
The Relative Compression Test provides the difference between the fastest and slowest crankshaft speed during the power stroke of each cylinder.
As the engine is cranked, the IDM uses the cam and crank sensor signals to measure crankshaft speed, as piston reaches two points: Top Dead Center (TDC) compression and about 30 degrees after TDC compression.
When the piston approaches TDC, crankshaft speed should be slower because of compression resistance.
As the piston passes TDC, compression resistance dissipates and crankshaft speed increases.
At TDC compression, the cylinder reaches its highest compression and resistance to crankshaft rotation —
Crankshaft speed is the slowest. A cylinder with low compression will have less resistance to crankshaft rotation. Crankshaft speed will be faster than normal.
About 30 degrees after TDC, crankshaft speed should be fastest because compression has dissipated. On a
cylinder that has low compression, crankshaft speed will be close to, or less than crankshaft speed at TDC.
At TDC of each power cylinder, and about 30 degrees past TDC, the IDM collects data for crankshaft speed.
NOTE: If not cranked long enough to collect data, the EST will display 255. 255 represents an erroneous
rpm value The TDC value is subtracted from the value about 30 degrees after TDC and recorded for each cylinder.
Example: 200 rpm (30 degrees after TDC) – 180 rpm (TDC) = 20 rpm
The EST will display a value on the screen for each cylinder, as typified by the following example.
Compare the compression values of each cylinder with the other cylinder values.
A cylinder with compression lower than the other cylinders indicates a suspect cylinder. Test value of 18 for cylinder one indicates a suspect cylinder.
If a cylinder value is zero or a much lower than other cylinders and this cylinder is a non-contributor (identified in the Injector Disable Test), check for a mechanical problem.
Example
If TDC rpm is greater than rpm 30 degrees after TDC, the EST will display 0.
If the test value for a power cylinder is 0, the cylinder is suspect.
If the test value for a power cylinder is significantly below 15 rpm, the cylinder is suspect.
Test value 5 for cylinder 1 indicates a suspect cylinder.
Test value 0 for cylinder 6 indicates a suspect cylinder.
When the Relative Compression test is done, the EST indicates, stop cranking the engine, and will display
test values.
Test data displayed in this test should be compared with data collected from the Injector Disable test.
Tools
• EST with MasterDiagnostics® software
• EZ-Tech® interface cable
Procedure
NOTE: Batteries must be fully charged before doing this test. If multiple tests are necessary, use a
battery charger during this test; battery drain can be extensive.
NOTE: Read and be familiar with all steps and time limits in this procedure before starting.
1. Select Diagnostics from the menu bar.
2. Select Relative Compression Tests from the drop down menu.
3. Follow the messages at the bottom of the window.
• Turn the ignition switch to ON.
• Select Run.
• Crank engine for 15 seconds. (Another message may read Stop Cranking.) Do not turn ignition switch to OFF. If the switch is turned to OFF, test results will be lost.
NOTE: If test results are identical to previous test results, the current test failed and the previous results were displayed.
4. Interpret results.
• If a Relative Compression Test and Injector Disable Test identify a suspect cylinder, check for a mechanical problem.
• If a Relative Compression Test does not identify a suspect cylinder, but the Injector Disable Test does, replace suspect injector(s).
Possible Causes
• Broken compression rings
• Leaking or bent valves
• Bent push rods
• Bent connecting rods
• Loose fuel injectors
• Scored cylinder sleeve
• Piston damage
• Incorrect valve lash adjustment
