vypa

So i had another thread related to this posted earlier, but i have some useful information that i though maybe someone else could benefit from, so heres a new thread

First of all i had some idle surging or unstable idle which was worrying me.. along with a perceived loss of power (im fairly certain from the stock clutch is breathing its dying breath) To ease my paranoia i decided to do a compression test.

For those that dont know (probably very few) its a one (evidently old school) way to diagnose headgasket/piston/piston ring problems. Not as accurate as a leakdown test but may give you an idea of whats goin on. Since im sure there is someone out there who doesnt know how to do this I figured I'd try and offer some insight, but im no expert.

The basic procedure (someone correct me if im wrong here) is written below:

Now I'm not repsonsible if some how you destroy your car or your head or anything like that, and im no expert so take anything i say with a grain of salt but heres the basics. I'd obviously take the car to a mechanic if you get weridness but here ya go:

Test the car at OPERATING temperature (thank malloynx)

1. Disable Electronic Fuel Injection and Ignition by removing their respective fuse/relay from under the hood.

2. Disconnect all four spark plug wires and remove all four spark plugs.

3. Screw the compression tester into the head where you removed the spark plug. Hand tight. You can find a compression tester at your local auto parts store. I don't think they cost more than $50 or so but dont quote me on that.

4. Did you remember to disable the EFI and Igntion?

5. Get in the car and press the gas (and clutch if its a 5mt ) to the floor and attempt to start the car, it will fail of course, but hold the key until it turns over about 4 times.

6. Get back under the hood and read the gauge. Write down the reading you get (its in PSI)

7. Repeat 3-6 for all cylinders.

For the tC you should be reading between 145-190 psi (can anyone with a FSM verify my numbers? i dont have one.) The difference between any two cylinders should not be more than 15 psi

Low compression in one cylinder often means bad rings. If you can squirt a small amount of oil in there and it goes way up, thats probably the issue.

No compression in one cylinder could mean you have a hole in your piston.

Low compression in 2 adjacent (next to eachother) cylinders usually means you have a leaking or blown head gasket.

Thats about all i know how to determine from a compression test, anyone else have any more?

Anyways, to get down to it, my results were 180 180 174 182 so at least i kno im in pretty good shape, and this is in a 2005 ZPI turboed vehicle with a stage 1 running only 6psi for around 5k miles. The car has 25000 miles on it and im generally running around 11.5-12.3 AFRs. Plugs are NGK Iridiums, heat range 7, and they looked good when i pulled them out so im blaming my problems on the clutch, and the idle surging on the BOV.

Anyways any corrections or additions would be MORE than welcome. I got the basic procedure down from past experience, and the numbers from a friend at toyota a while back and am goin off memory so either could have failed me!

(credit must be given to malloynx on several corrections! thanks again!)

Whocares05050

good job

vypa

im just happy about the results! no toasted piston rings

Whocares05050

yet

malloynx

your a bit off the mark but i won't hold it against you. basically a compression test is like taking a ____ in the dark. ( your not going to hit your target) there ok for the shade tree fella cause the gauge is cheap. for us technicians we never rely on a compression test cause there too vauge. hell on new cars we don't even do a CLT. most new factory dia. equipment can handle this..

also please enlighten me on why doing a compression test cold? ohhh wait your right.... metal doesn't expand at operating temp and give a more accurate readings. ( get the sarcsm)

ALWAYS DO A COMP. TEST AT OPERATING TEMP. ask yourself,, do you drive around with a cold engine? then why the hell would you test it cold. do the laws of physics change when metal is cold?

vypa

hmm.. well anyways liek i said - no expert here but i have ALWAYS heard not to insert or remove spark plugs from an aluminum head when its at operating temperature.. is this incorrect? ill edit and make the corrections you noted though, as i know you are much more knowledgable on the subject than i am.