| RELYING ON RELATIVE COMPRESSION TESTING | | | | pressure increase will be present each time a piston |
| TO ASSESS ENGINE ISSUES: | | | | heads for bottom dead center. |
| This test uses the digital storage oscilloscope (DSO) | | | | If the rings are sealing properly, the peaks will be |
| and a high amp clamp to measure starter amperage | | | | uniform and the loss of compression you found has |
| draw at a time base fast enough to see the impact | | | | to be caused by the top end. |
| each cylinder has on that draw. | | | | SECONDARY IGNITION PATTERNS TO SLOVE |
| 1. Synch the pattern to a given cylinder and use the | | | | ENGINE MYSTERIES: |
| firing order of the engine to identify weak cylinder(s). | | | | This takes some skill, but you can master it. |
| A weak cylinder takes less effort to spin through | | | | 1. Look at the average of the firing lines (firing |
| than a strong one does, and that means it also takes | | | | voltage demand) for all the cylinders. |
| less current, and that is displayed on the scope's | | | | It's normal for the firing line of an individual cylinder to |
| screen. | | | | vary as conditions vary in the combustion process. |
| 2. Modern pressure transducers allow relative | | | | Any cylinder that has a consistently lower firing line is |
| compression tests to be done via the exhaust | | | | suspect. |
| pressure pulses leaving the tail pipe. Cylinders that are | | | | 2. Watch the spark line closely. No compression in a |
| healthy will pass equal pressure pulses through the | | | | cylinder will not only have a lower firing demand, but |
| exhaust that the sensor can detect. A weak cylinder | | | | the spark (or burn) line will be flat and longer than the |
| will show up as a weaker peak in the pattern. | | | | others. |
| 3. The pattern can be synched to help identify the | | | | NOTE: When a valve doesn't seal, the pressures built |
| bad cylinder(s). Here, though, the pulse displayed will | | | | up in the cylinder have a path to escape through. It |
| be offset from the originating cylinder. When viewing | | | | shows up in the pattern as a series of peaks in what |
| these patterns, look for anomalies rather than | | | | should otherwise be a relatively smooth slope. Some |
| specifics. Some exhaust designs cause one bank to | | | | turbulence at higher engine speeds is normal, though, |
| have a longer route to follow than the other, and this | | | | so don't confuse the two. |
| can reduce the amplitude of the pattern on all the | | | | By following the three tests outlined above, you |
| cylinders for that bank. | | | | should be able to gain a better understanding of the |
| NOTE: If there is a weakness, the pressure | | | | problems the engine may be experiencing. With luck, |
| transducer can determine if the fault is in the valve | | | | you should be one step closer to providing your |
| train or cylinder. | | | | customer with the auto repair information they need. |
| 1. Be sure to disable the fuel and ignition systems to | | | | Rely on Motor Age for all your automotive news and |
| avoid both over fueling the cylinders and accidental | | | | industry updates. Motor Age has been in publication |
| engine start. | | | | since 1899, and today reaches over 140,000 |
| 2. Remove the oil dipstick and put a rubber hose | | | | automotive service industry professionals, providing |
| over the tube, connecting the sensor to the tube. | | | | them with high-quality auto repair information, body |
| 3. Crank over the engine and watch the pattern. A | | | | shop news, collision repair news, and much more. |