At this point, a vastly oversimplified explanation of engines is in order. In an engine, there are hollow cylinders (6 in our case). They are arranged in banks (2 in our case, and yours too if you’ve ever seen ‘DOHC’ on your car anywhere). These cylinders have valves that open and close to let in fuel and air and discharge exhaust. Each cylinder has a piston that moves back and forth inside it and its function is to, among other things, work in concert with the valves to control the density of gasses in the cylinder. The orchestration of this mechanical dance is called timing and it controlled by a timing belt mounted on timing cogs which control cam shafts running though the cylinder banks.
A compression test is a commonly performed diagnostic procedure which provides a lot of information about the state of the engine and how well it’s doing its job of producing compression.
So we did one.
As this is a blog about our experiences learning to work on cars and not a how-to, I’m skipping over all the irrelevant stuff about how a compression test is done (per cylinder: remove the spark plug, insert a special compression testing tool in its place, disable ignition, and turn the engine over for about 5 seconds) and the equally tedious recounting of all the stuff we had to disconnect and/or remove in order to get access to the spark plug banks. The tutorial on how to do this ran 16 pages* and it still didn’t account for the effects California’s special emission standards on engine design. We had some extra mystery hoses that we couldn’t identify for a while.
Normal compression for this car is spec’d between 115 and 150 psi per cylinder. On a new car, we’d expect compression across all the cylinders to be at the high end of that range, with maybe a 10% psi variance from cylinder to cylinder, if that. If one cylinder were to drop more than about 10 psi from the group, we’d know something was wrong with that cylinder and we’d start investigating the spark plug more closely and worry about malfunctioning values.
Front bank compression (psi): 180 178 182
Rear bank compression (psi): 92 93 88
What we can read from this is that all the cylinders within a bank are extremely consistent with each other but the banks themselves are wildly different. So, looking at these results, it’s very possible that there’s nothing wrong with the engine itself and we just need to adjust the timing.
Except for one thing.
This engine is classified as an interference engine, meaning that during normal operation, the pistons and the valves occupy the same space at different times. It is only by the grace of highly advanced mechanical engineering that they do not attempt to occupy the same space at the same time.
Oh, and timing.
The timing belt is the only thing preventing a piston from shoving itself as hard as it can straight into a valve. So if the timing belt were to break or become ill-adjusted somehow, the engine might bend all its piston rods or shatter all its valves or both. The failure mode is, to put it lightly, catastrophic.
So in that context, these compression results were terrifying. On the one hand, the internals of the valves may well be fine. On the other hand, the timing is way off (and steadily getting worse, because the timing belt seemed to slip a tooth every time we accidentally touched it). So in our quest to determine the state of the engine, we might have accidentally made things a whole lot worse.
*Just to get to the spark plugs. Not even to do the compression test.
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