Compression Test vs. Leak Down Test

[dug-e-fresh]

Which of these are more indicative of engine health? Should both be done as a cross functional check... or is one better than the other.

Just to note, I haven't had any problems, but just out of curiosity I would like to have my engine tested to see where I stand.

I know Pepboys sells a do-it-your-self tester for I beleive compression testing, is this thing worth the trouble? Should I just take the car to a professional shop and have one or both done?

Opinions, facts, ideas are welcome...

Thanks,
Doug

[AZScoobie]

Originally posted by dug-e-fresh
Which of these are more indicative of engine health? Should both be done as a cross functional check... or is one better than the other.

Just to note, I haven't had any problems, but just out of curiosity I would like to have my engine tested to see where I stand.

I know Pepboys sells a do-it-your-self tester for I beleive compression testing, is this thing worth the trouble? Should I just take the car to a professional shop and have one or both done?

Opinions, facts, ideas are welcome...

Thanks,
Doug

I like to do both and here is why...

During a compression test you are converting your cylinders into air pumps. You screw the tester into the spark plug hole and then crank the engine over (with ignition and fuel disabled and throttle wide open). The tester itself works like a tire pressure gauge. You end up with a max PSI rating of that cyl. This is is indicative of the compression ratio. A compression test is only useful when all cyl's are tested so you can compare PSI ratings. We know they should match, or at least be within 10-20% of each other or there is a problem.


Case in point: Years ago I had a watercooled VW I built for SCCA pro solo. After a season I performed a compression test. cyl 1-4: 110,120,90,40. Not good... the last cyl was way down on compression. Why? A compression tester will never tell you why. It will only tell you how much compression you have.

Next up, leak down test. A leak down test pumps compressed air into your cyl through your spark plug hole. You must rotate your engine until that cyl is top dead center (all the way UP with all valves closed). The tester has a readout and shows the % of air that is escaping out of that cyl. 10% is considered normal.. This is metal to metal and you can expect some leakage. My ratings where 8%, 5%, 12%, 40%. That last cyl that was down on compression was leaking 40% of the air I was pumping in. Not good. Where is the air going? First thing you do is run to the back of the car and listen at the tail pipe. If you hear a rush of air that means the air is escaping out the exhaust valves. If you hear air at the intake then you might have bent intake valves. Open the radiator cap and listen/look for air bubbles. If so you might have a blown head gasket. Next up, take off the oil cap and listed for air. If you do, then you probably have a ring to bore leak or a scuffed bore.

You start with a compression test. Keep in mind that a cold motor will have less compression then a warm motor. Dont pay attention to how high the reading is. All you care about is that all cyl's are close to one another. In the case of my old VW the exhaust valves where bent (ooops, over rev)and not sealing. I left it alone because the engine was burning no oil and it was making 76hp at the wheels which was good for a motor rated at 72hp at the crank.

HTH

CT

[WizardBlackWRX]

I would be worried if there was more than 10% variation between cylinders. Call a Subaru Tech; he/she can tell you what the spec is for the EJ20. It should be in the manuals as well.

[OAKOS Automotive]

Here is what the service manual says on page ME(DOHC TURBO)-22 of book 3.

Compression (350 RPM & Wide Open Throttle):
Standard: 138-166 psi
Limit: 121 psi
Diff between cylinders: 7 psi


Sincerely,
David Kearney
Oakos Automotive
www.oakos.com
sales@oakos.com

[dug-e-fresh]

Beautiful information!! So the compression specs are 152 +/- 14 psi...

Thanks a million!

However.... one question for Oakos Automotive:

You wrote that the test should be performed at 350rpms, WOT... how do you insure that the criteria is met?

Thanks again!

def

[OAKOS Automotive]

I would guess that this is the speed that the starter motor cranks the engine at. I am not sure about that though.

David Kearney
Oakos Automotive
www.oakos.com
sales@oakos.com

[AZScoobie]

Originally posted by dug-e-fresh
Beautiful information!! So the compression specs are 152 +/- 14 psi...

Thanks a million!

However.... one question for Oakos Automotive:

You wrote that the test should be performed at 350rpms, WOT... how do you insure that the criteria is met?

Thanks again!

def

All Spark plugs removed, Fuel and ignition disabled, and throttle open full. Crank the starter over until the gauge does not read higher. 350 RPM is the RPM you will get in this config.

CT

[SFastWRX]

Fuel and ignition disabled, as in, ignition fuse pulled and the green plug under the dash on passenger side unplugged?

I have the plugs out and the crank and cam position sensors and fuel pump (the green plug) unplugged, and even with the ignition fuse in there, it won't turn over. All I get is a click and a whine.

What exactly needs to be left connected/disconnected?

[AZScoobie]

Originally posted by SFastWRX
Fuel and ignition disabled, as in, ignition fuse pulled and the green plug under the dash on passenger side unplugged?

I have the plugs out and the crank and cam position sensors and fuel pump (the green plug) unplugged, and even with the ignition fuse in there, it won't turn over. All I get is a click and a whine.

What exactly needs to be left connected/disconnected?

Just pull the Fuel pump fuse and unhook all the coils. Sorry for not making that clear. You need to have all the plugs out anyways... Make sure your TB is Wide open. Might want to tie it back for now...

CT

[SFastWRX]

Pardon my blindness, but I can't find a fuse for the fuel injection - I looked in the fuse block under the hood and behind the change holder, but can't find it.

Plugs are out and I'm holding my foot on the floor.

Thanks for your help.

[AZScoobie]

Originally posted by SFastWRX
Pardon my blindness, but I can't find a fuse for the fuel injection - I looked in the fuse block under the hood and behind the change holder, but can't find it.

Plugs are out and I'm holding my foot on the floor.

Thanks for your help.

My car is not near...Sorry man.. At the time I looked it up and pulled it based on the location. The best I can do right now at 10PM is to suggest you search the forum for a listing of the fuses...

Worse comes to worse, Remove the plate in the pass side trunk and unplug the fuel pump connector. It takes about 5 minutes and is only some screws.

CT

[SFastWRX]

I think I'm OK with the green plug unplugged - my fuel pressure gauge is reading 0 psi, so I'll reconnect the cam and crank sensors and give it a try. I'm just a little nervous about the coils trying to fire, even though they're out. Should be OK with no fuel flowing, though.

Stand by for bad news - the insulators on the plugs from cylinders 1 & 3 are nasty black, while 2 & 4 look good.

[AZScoobie]

Originally posted by SFastWRX
I think I'm OK with the green plug unplugged - my fuel pressure gauge is reading 0 psi, so I'll reconnect the cam and crank sensors and give it a try. I'm just a little nervous about the coils trying to fire, even though they're out. Should be OK with no fuel flowing, though.

Stand by for bad news - the insulators on the plugs from cylinders 1 & 3 are nasty black, while 2 & 4 look good.

Prediction: 2 and 4 (looking good to you) are lean because of stock rails. 1 and 3 are being over fueled.

CT

[BamBooI]

Are you dug e fresh on a different name?