How do you perform a compression check??

juiced_94gt · Sep 17, 2007

I have the compression tester but how do i do it?? and what numbers should i be looking for on a 1k mile motor?? what will doing this test eliminate as far as problems that could be bad with the car??

BlackVert · Sep 17, 2007

see this:

http://forums.stangnet.com/showthread.php?t=654806&highlight=compression+test+results

it is pretty straight forward.

cut off the fuel pump using the disconnect switch in the trunk

attach a battery charger (keeps the battery from running down)

for each cylinder, remove the spark plug and screw the tester in

crank the car over about 6 times or so

the gauge should read 150+ psi. my engine was around 150-160, but newer engines i think will be higher, more like 180 or i think

they should all be within 5% of each other or so

go to the next spark plug

that is the basic procedure. i'm sure others will chime in to fill in details i have missed.

for a "wet" test, you would squirt some oil into the cylinder before screwing in the tester and cranking the engine. if the rings are worn, a wet test would result in higher compression.

a low number in one cylinder indicates like a stuck valve or broken ring or something along those lines. a low number for 2 adjacent cylinders indicates a possible gasget problem

good luck with it and post the numbers.

juiced_94gt · Sep 17, 2007

thanks blackvert

kck6894 · Sep 17, 2007

When you do the test you need to make sure you remove ALL the spark plugs before you do the test (this helps free the motor up so it will spin easier to do the test). You also need to make sure you open the throttle all the way. If can't make compression if it's not getting any air. Do the test warm also. Start the car let it warm up and then do the test. You don't need do the test hot but do it after the car was started and warmed up. If you don't drive your car all the time take it out and run it good and hard before you do the test too. Also if you have a cylinder that does not produce the kind of number you want, check it again. You can check the same cylinder twice in a row and get a different reading both times. Good luck

BlackVert · Sep 17, 2007

no prob. i remember also someone saying you should disconnect the coil too

i used an egg carton to keep the spark plugs with the right cylinders ... it just helped keep them organized a little better

Dino Dino Bambino · Sep 18, 2007

Just remember to number the plug wires with sticky labels so that you'll remember which cylinder each one goes to. Alternatively, you can number each terminal on the distributor cap with a permanent marker. It'll save you a lot of headache and you won't cross the wires to the wrong cylinders. If you disconnect the coil lead, handling the plug wires during the compression test won't become an electrifying experience.

91LX_5L · Sep 18, 2007

when i did mine long ago (well the shop) they wrote down 100/95/100/100/95/100/99/100 seems low as hell or mabye they were measuring it some other way?

Dino Dino Bambino · Sep 19, 2007

They might have quoted relative percentages in which case, yours are healthy.

kck6894 · Sep 19, 2007

91LX_5L said:
when i did mine long ago (well the shop) they wrote down 100/95/100/100/95/100/99/100 seems low as hell or mabye they were measuring it some other way?

The funny thing about compression tests is that if you talk to 10 different guys they will tell you 10 different ways to do it. 100lbs of compression would be low, but on the same not it's still enough for and engine to run on. The main thing is they are within 5 psi or even 5%. That's all you're really looking for.

final5-0 · Sep 19, 2007

+1 on the readings being within 5% of each other

Here is a short list of things that could make the readings lower
or higher when comparing to another members combo

Size of combustion chamber
Thickness of hg
Piston design
Block deck surface at oem specs
Head deck surface at oem specs
Camshaft specs
Ring seal to cylinder wall effectiveness
Valve to valve seat effectiveness
and
there are certainly others

Grady

parchisi · Sep 19, 2007

make sure to disconnect the fuel. My buddy with a 4.6 burned his car down doing a compression test.

MLC Stang · Sep 19, 2007

juiced_94gt said:
what will doing this test eliminate as far as problems that could be bad with the car??

juiced_94gt,

If you're looking to do some general troubleshooting to find out what might be wrong, then running a cylinder balance test is a much better option than just doing a compression check.

The car's computer will run a cylinder balance test for you. It runs right after the KOEO and KOER self tests that are built into the computer. It turns off each cylinder one at a time and measures the RPM drop. If any one or more cylinders cause an RPM drop of a certain percentage less than the rest of the cylinders there is something wrong with that cylinder and the computer will indicate that in the test results. If a "bad" cylinder is indicated by the test, you will then have to investgate possible causes for that particular cylinder's problem, e.g. bad spark plug or wire, bad injector, bad compression, etc.

You can search this site or the web to find info on how to run these tests that are built into the car's computer.

PS) The cylinder balance test can be run a second time right after the first time. The second time the computer tightens the RPM drop specs for determining if a cylinder is bad, i.e. it's tougher to pass the test the second time.

If your car passes both times, I would think that your problem is not cylinder malfunction related.

Good luck,

Danman765432 · Sep 19, 2007

so what were your results????

kck6894 · Sep 19, 2007

final5-0 said:
+1 on the readings being within 5% of each other

Here is a short list of things that could make the readings lower
or higher when comparing to another members combo

Size of combustion chamber
Thickness of hg
Piston design
Block deck surface at oem specs
Head deck surface at oem specs
Camshaft specs
Ring seal to cylinder wall effectiveness
Valve to valve seat effectiveness
and
there are certainly others

Grady

you forgot the brand of gum he's chewing while doing the test!!

On a serious not it may not be a bad idea to also do a cylinder leak down test also. There are times where you can have good compression but have tons of leak past the rings.

How do you perform a compression check??

juiced_94gt

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BlackVert

juiced_94gt

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kck6894

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Dino Dino Bambino

91LX_5L

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kck6894

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final5-0

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parchisi

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MLC Stang

Danman765432

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