Compression Test: Camshaft Timing
A compression test is used in order to check and compare the compression of one cylinder compared to the others. An engines cylinders need a good seal between the piston rings and the cylinder walls and between the valves and their respective valve seats. Test each cylinders seal compared to specified compression and the other cylinders results with an engine compression tester. Either an engine analyzer or a manual compression tester may be used for this procedure. Review the manufacturers manual and technical service bulletins TSB's for specifications and any special test procedures.
The results of a compression test can be quite telling. The results shown below indicate the particular area that needs attention.
- Worn piston rings / cylinder walls: After running the first test squirt 2 oz. of oil into the cylinder and crank the rotate the crankshaft three more turns per cylinder. If the compression increases the piston rings are at fault.
- Burned valves: After injecting the oil in the cylinder the compression reading stays the same then one or more of the valves or valve seats are bad or not seating correctly. Low compression in one cylinder typically indicates a bad valve. Exhaust valves tend to burn due to the hot gases passing through while an intake valve is cooled by the incoming fuel. A burned valve is usually an exhaust valve.
- Faulty head gasket: A faulty or blown head gasket will leak compression between two adjacent cylinders. When the other cylinders are within specifications and two cylinders next to each other on the same bank are low suspect a faulty head gasket.
- Camshaft timing: When all of the cylinders are low and inserting oil into the cylinder does not increase compression the camshaft timing is likely off. The timing belt or chain can slip on the sprockets resulting in the staggered and low compression results shown in the image. Before replacing the rings in this engine check the camshaft valve timing.
- Hole in piston: A hole in the piston will result in no compression in the cylinder. Remove the PCV valve from its grommet. The blowby gases caused by this hole can be seen seeping through the PCV valve opening.
- Carbon buildup: Carbon buildup on the top of a piston will increase compression readings and can be seen with a probe inserted into the cylinder.
