Compression test questions

Hi,

I was thinking of getting a compression test kit and doing a compression test on all cylinders just to check health and maybe do a leak down test if any abnormalities are found in a cylinder.

Has anyone here done this before on their car? Are the sparkplugs easily accessable/removable for this type of test or are they a bitch to get to? Any other components/tools recommended to do the job easier?

They're not very easy to reach - that's the "charm" of subaru Anyway - to make things a bit easier you can remove the airbox on one side and water tank on another.
Rather don't bother with a push-in type of plugs - use the gauge with screw-in fitting, to make sure the pressure is not lost through some leaks while you try to keep the gauge's plug inside the spark-plug socket - with not very easy access to it.

Anyway - while gauge is fitted, make sure yet that coil-pack is disconnected, not trying to make a spark. I hope you know the rest?

FYI i have now finally done a compression test on engine while changing the spark plugs - the job is a real PITA! It's a nightmare and costly to find the right tools for the job (the spark plug changing more than the compression test). Anyway results were OK and give me certain peace of mind.

Got 138, 132, 138, 142 for PSI values. That's going around from closest to driver anti-clockwise to furthest away. No idea why the driver side front piston is slightly less than the others and I repeated this cylinder just to be sure and got exact same result. Also engine was warm when doing test (I was burning the back of my hands off block earlier getting plugs out). And this was opening throttle (foot to floor) and seven cranks per cylinder. Do these values look normal enough to people?

p.s. one thing I found when doing this job was that there was NO bolt holding the coil pack of the driver closest cylinder in there at all. Don't know why - must be whoever did this job before forgot to put it back in - so the only thing holding coilpack in place was the rubber boot that holds onto sparkplug. I wonder what affects this could have on engine operation .

Also anyone know where I can get a replacement bolt? I tried B&Q and Halfords and neither have those type of bolt (seems to be M8 about 35mm long only half threaded).

I haven't put everything back in and started engine yet until I get replacement bolt.

I know this is an old thread, but I was thinking of proactively check the compression on my 04 STi Type UK. I don't know what are the exact number to consider it a good or bad, but I definitely know it will not be a simple task.

I found a video online of this test being done, but it's from a US spec, so not sure if I should really use it as a reference?

https://www.youtube.com/watch?v=wOUf3JYLu_c

Any feedback from more knowledgeable colleagues is welcome.

Skipper555 said:

I know this is an old thread, but I was thinking of proactively check the compression on my 04 STi Type UK. I don't know what are the exact number to consider it a good or bad, but I definitely know it will not be a simple task.

I found a video online of this test being done, but it's from a US spec, so not sure if I should really use it as a reference?

https://www.youtube.com/watch?v=wOUf3JYLu_c

Any feedback from more knowledgeable colleagues is welcome.

Click to expand...

It is very easy. Take all the coils out. Put in the compression tester, foot flat on accelerator and start/turn over the motor for 3 sec or more. Do it in your sleep

scoobySteve said:

It is very easy. Take all the coils out. Put in the compression tester, foot flat on accelerator and start/turn over the motor for 3 sec or more. Do it in your sleep

Click to expand...

Thanks, but what numbers should I use as a guide for good results? 150psi?

Take plugs out disable fuel and spark

Taken straight from the Subaru manual

Skipper555 said:

Thanks, but what numbers should I use as a guide for good results? 150psi?

Click to expand...

Depends on model. And if built before. HG thickness used etc.

But on your model yes 145-150psi.

Being equal all round is what you are looking. Test on a warm engine.

Sent from my SM-N910U using Tapatalk

120

you can sometimes see as low as 120 psi .. and still run ok..

You need to have the engine as hot as you can bare to work on... and have a really good battery or power source.

Ah, I see my old thread was resurrected!

Great timing because I was just thinking a few days ago to redo the compression tests and see how it is 7 years later! Probably worse... but I have more serious problems at the moment

Looks like I will have to redo these compression tests and also probably do (or get done) a leak-down test.

Getting new problem now... randomly occurring misfire error P0301 on cylinder 1...

...and its not the coil-pack or spark-plug because I already swapped both of them from cylinder 1 to cylinder 2 to see if the code changed to misfire cylinder 2 instead...

...and after a few drives... it still came back as P0301 cylinder 1 misfire...

...so must be something in cylinder itself...

I'm thinking head-gasket... since I do sometimes notice white smoke from exhaust and a lot of air bubbles in coolant when I "de-air" and refill it using the coolant funnel filler I have.

I just came back here so I could remember the compression numbers from last time... and seems to match since cylinder 1 was lowest compression that long ago also .ie 138, 132, 138, 142 for PSI values and cylinder 1 is the 132 one.