Next on the list for Project 17 is checking the cylinder compression before attempting an engine start. Cylinder pressure readings can tell us much about our engine's health — if the piston rings are weak or broken, if the cylinder bore is damaged, if the valve seating is compromised, if the head gasket is leaking. In other words, if everything is okay or not.
There's an even more comprehensive check called a leak-down test, where constant air pressure is applied to the cylinder via the spark plug hole while a separate gauge tells us if that pressure is being held within the cylinder. The advantage of a leak-down test is that if a leak is detected it can be pinpointed by listening to the exhaust pipe or intake tract for the telltale hissing sound of escaping air. The leak-down test is common in aircraft maintenance and requires specialized equipment...which I don't have.
But the simpler compression check is still a viable check for me, as I don't know the history of these projects. One of the three critical ingredients to combustion is compression, so if I find a cylinder with low or no compression, well....Houston, we have a problem. To Honda's credit, in my years of checking these engines I have never found one with compromised compression readings.
There's an even more comprehensive check called a leak-down test, where constant air pressure is applied to the cylinder via the spark plug hole while a separate gauge tells us if that pressure is being held within the cylinder. The advantage of a leak-down test is that if a leak is detected it can be pinpointed by listening to the exhaust pipe or intake tract for the telltale hissing sound of escaping air. The leak-down test is common in aircraft maintenance and requires specialized equipment...which I don't have.
But the simpler compression check is still a viable check for me, as I don't know the history of these projects. One of the three critical ingredients to combustion is compression, so if I find a cylinder with low or no compression, well....Houston, we have a problem. To Honda's credit, in my years of checking these engines I have never found one with compromised compression readings.
The process is quick and simple and only requires an inexpensive compression tester, shown at right. The tool will provide adaptors to fit different spark plug hole sizes and threads. Rubber o-rings provide sealing with only hand pressure used to seat the adaptor into the plug hole.
A proper compression test is done on a recently run (warm) engine — that's not possible for me as my projects haven't been run yet, but the test will still tell me if we have adequate compression.
A proper compression test is done on a recently run (warm) engine — that's not possible for me as my projects haven't been run yet, but the test will still tell me if we have adequate compression.
With all the spark plugs removed and the kill switch "off" (or the plug wires grounded to prevent damage to the ignition system) screw the tester into the plug hole and connect the gauge. Crank the engine over with the starter till the pressure reading stops advancing — that's your reading. Repeat for the remaining cylinders while recording your readings.
While specific values are sometimes provided in service manuals, the bottom line is that all the cylinders should have similar readings, with the lowest vs. highest within about 10% of one another. My typical readings for the VFR are around 150 psi (cold engine). In this case, my readings were 160-197 psi. I don't apply the 10% rule here, again, because this engine hasn't been run in many years. Once the engine has some run time, the readings will be different and probably very close to one another.
The reason my readings were high is due to the fact that I'd added oil to the cylinders before testing (to protect the cylinders and rings). Adding oil is a technique used to isolate the reason for low pressure readings — if adding oil increases your reading, then the cause most likely lies with the piston rings (the oil is helping the rings achieve a seal), as opposed to valve seating being the cause. In this case, I'm happy with these readings — they tell me that it's OK to move forward with this project.
While specific values are sometimes provided in service manuals, the bottom line is that all the cylinders should have similar readings, with the lowest vs. highest within about 10% of one another. My typical readings for the VFR are around 150 psi (cold engine). In this case, my readings were 160-197 psi. I don't apply the 10% rule here, again, because this engine hasn't been run in many years. Once the engine has some run time, the readings will be different and probably very close to one another.
The reason my readings were high is due to the fact that I'd added oil to the cylinders before testing (to protect the cylinders and rings). Adding oil is a technique used to isolate the reason for low pressure readings — if adding oil increases your reading, then the cause most likely lies with the piston rings (the oil is helping the rings achieve a seal), as opposed to valve seating being the cause. In this case, I'm happy with these readings — they tell me that it's OK to move forward with this project.
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