DIY Compression Test?
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Joined: Mar 2016
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From: Memphis, Tn
DIY Compression Test?
I’m about to do plugs and coils on my 60k miles 997.2 C4S. I’ve done plugs on a few cars but I don’t know anything about compression tests. The gauges look to be $50? So why not buy a set and do it while I’m down there?
First, is this overkill? I tend to overthink things. Or is this a good idea?
Second, can I mess up and hurt my car in any way? Some things are above my paygrade and I try to be realistic.
Engine has to be warmed up though... And I gotta take hot mufflers off...hmmmm
First, is this overkill? I tend to overthink things. Or is this a good idea?
Second, can I mess up and hurt my car in any way? Some things are above my paygrade and I try to be realistic.
Engine has to be warmed up though... And I gotta take hot mufflers off...hmmmm
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From: Clemson, SC
On the 997 platform, there's no real need to do a compression test. If a cylinder (or more) have a compression issue, you'll see misfire codes from it. Now if you're chasing the cause of misfires and that's why you're doing plugs and coil packs, then might be worth the extra effort to do that but if the car is running well and the plugs and coil packs are purely a PM or service interval procedure, you can skip the compression test.
When doing a compression test, you don't want the car to be able to start, just to crank. On old school cars, you would just pull the main coil wire to the distributor, and the fuel pump fuse. On the 997 with individual coil packs, there isn't a coil wire to pull, so I guess all you'd be able to do is cutoff the fuel supply. Maybe pull the fuel pump fuse with the car running until the engine dies, then do the plug change and compression test, but that means you're going to run the cylinders lean right before it dies and I'm not sure that's good for the motor. One advantage to this method is that you'll get higher and more accurate compression readings as the motor will be warm, the hydraulic valves pressurized, and the piston rings sealed to the cylinder bores with oil film.
Maybe someone else knows specifically the best way to do this on a 997, but again, is likely unnecessary.
When doing a compression test, you don't want the car to be able to start, just to crank. On old school cars, you would just pull the main coil wire to the distributor, and the fuel pump fuse. On the 997 with individual coil packs, there isn't a coil wire to pull, so I guess all you'd be able to do is cutoff the fuel supply. Maybe pull the fuel pump fuse with the car running until the engine dies, then do the plug change and compression test, but that means you're going to run the cylinders lean right before it dies and I'm not sure that's good for the motor. One advantage to this method is that you'll get higher and more accurate compression readings as the motor will be warm, the hydraulic valves pressurized, and the piston rings sealed to the cylinder bores with oil film.
Maybe someone else knows specifically the best way to do this on a 997, but again, is likely unnecessary.
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From: Momence, IL
Unless you have a problem you are trying to diagnose, I wouldn't worry about a compression or leakdown test. The first test we will do typically is a manometer test to measure crankcase vacuum at idle both hot and cold. That will give us a good overall indicator of engine health.