How to do a B9 compression test - SB9T.com
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Old 12-31-2018, 04:54 PM   #1 (permalink)
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Default How to do a B9 compression test

Just got this 06 TriBeCa and had the wrong plugs in it. While I got all the plugs out I'm doing a compression test. This car doesn't seem to have a cable attached at what I believe is the throttle, but looks like an electrical servo. Thought it was Mabel a TPS. So I floored the foot feed while cranking the motor. Got what might be low readings about 180 except #3 which is 145. Is there a trick to doing this? There was a misfire code for #3 which is Where I found one of 2 autolite plugs. I bench tested all the plugs and that one is completely dead. The 4 NGK's weren't the right ones. They are the type for a 3.6 engine. Anyway just looking for tips on doin the compression test. I,m an old fart so talk slow. (Autolite- seriously?)
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Old 12-31-2018, 05:40 PM   #2 (permalink)
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These cars and just about everyone available by now have a drive by wire throttle body, so there is no direct connection from the throttle body to the pedal. The good news is they rarely give problems and they have been in use by Subaru since about 2005 without much change.

I am not sure if opening the throttle is strictly necessary especially if you relieve fuel presssure first by pulling the pump fuse and letting the engine clear the fuel in the line. I don't recall doing it when I did my compression check and mine came out to 175 psi for all cylinders at around 150k miles on clock. I would do the plugs and check the coils all out and then put it back together and see how it runs. Unless you have a head gasket issue or valve issue the pistons and rings rarely give out on these motors.

You did right by switching to OEM plugs for sure.
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Old 12-31-2018, 06:49 PM   #3 (permalink)
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Thank you
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Old 12-31-2018, 07:05 PM   #4 (permalink)
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Is there a way to bench test the coils? Or what?
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Old 01-01-2019, 03:27 AM   #5 (permalink)
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You have everything in service manual Index of /Auto/Japan/Subaru/Tribeca

But if you have low compression in one of cylinders, replacing spark plugs or ignition coil wont help to bring up compression. Your next step should be measuring valve clearance.
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Old 04-06-2019, 10:15 AM   #6 (permalink)
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Ok, I'm revisiting this problem. It's been throwing a #3 misfire code off & on for months. I have been erasing the code but know that's not a fix. On vertical engines we would squirt some oil into the combustion chamber to see if that brought the compression up,indicating ring trouble. If it doesn't help the valves would be suspect. Or gasket.

On a flat engine I doubt the oil would get around the whole ring. And with the frame so close to the plug access it was hard to get oil inside. I'm sure it didn't.

Any suggestions?

Thanks
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Old 04-07-2019, 04:51 AM   #7 (permalink)
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The next logical step is a leakdown test, that will tell you where you are leaking compression from.
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Old 04-08-2019, 08:22 AM   #8 (permalink)
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Thank you. Good idea. I'll give it a try.
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