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How To Test Your Compression

35K views 32 replies 23 participants last post by  ChrisH 
#1 ·
Compliments of Smaay:

http://www.newcelica.org/forums/showthread.php?t=227280

Smaay said:
I have been seeing alot of questions about compression and how to test, so here is a writeup i did.

Compression Test by Smaay

1. Open hood



2. Remove engine plastics with 5mm Allen Wrench.



3. Plastics removed



4. Reach behind engine to unplug power steering sensor






5. Remove the four 10mm bolts holding coils.



6. Unplug coils



7. Remove the two 10mm nuts holding wiring harness.



8. Lift wiring harness out of the way.





9. Pull all four coils.





Coils removed exposing sparkplugs


9.5 unplug the 4 injectors so they dont squirt.



10. Using 5/8 spark plug socket and suitable extension (6-8 in.) remove and inspect sparkplugs. They should be a copper color like pictured.



Image of compression tester purchased at Autozone/Pep Boys/Kragen/Napa/etc.



11. Screw compression tester into cylinder 1 hand tight.





12. Hold down gas and clutch at the same time, crank engine over about 6 times, until needle stops jumping on compression tester

http://www.youtube.com/watch?v=CkaUtDSdmF4

13. Record results.




14. Release air pressure from tester and repeat for each cylinder.





Straight from the Toyota Manual (BGB)

Compression pressure:
1ZZ-FE 1,500 kPa (15.3 kgf/cm2, 218 psi)
2ZZ-GE 1,400 kPa (14.3 kgf/cm2, 203 psi) or more
Minimum pressure:
1ZZ-FE 1,000 kPa (10.2 kgf/cm2, 145 psi)
2ZZ-GE 1,000 kPa (10.2 kgf/cm2, 145 psi)
Difference between each cylinder:
1ZZ-FE 100 kPa (1.0 kgf/cm2, 15 psi) or less
2ZZ-GE 110 kPa (1.1 kgf/cm2, 16 psi) or less

15. After test complete, put car back together in reverse order.

16. DONE! Have a beer to celebrate good results, or sulk in sorrows.


edit: this will throw a CEL for misfires. there is a fuse you can pull to prevent it but im not sure which one it is. but just unhook your negative terminal on the battery for 5 minutes and it will reset the CEL. Also the car might have a hard time starting, dont worry, just give it a few more cranks. It will start
Enjoy.
 
#2 ·


Im betting those coils have something to do with his CEL/misfire codes! Looks like they have been sitting in some water...

if you have a lab scope or graphing multimeter and an amp probe/clamp you can do an overall compression check in a few minutes without removing any parts. just graph the current ramp for the starter during cranking (pull fuse for fuel pump to prevent starting) with the clamp around the battery cable at the starter and sync the trigger off of number one cylinder .it wont tell you specific psi but all ramps will be even on a sound motor and if you have one ramp thats alot lower than the others, you can count in the firing order off of your sync to find which cylinder it is and do a real test on that one cyl. If your engine isnt running bad or other mechanical problems then this prob wont be much use to you, but it has saved me a ton of time on engines that are fairly hard to reach the plugs on and show signs of compression issues, since i didnt have to go looking at 11 other cylinders first (murphy's law will ensure the low cylinder is the last one checked, always!). This current ramp graphing method is also good if you have a low amp probe and need to check injectors when chasing rough running issues with no codes, since the electrical properties of coil windings and thier signature when open/closing/sticking are the same no matter who makes the injector.
 
#3 ·
if you have a lab scope or graphing multimeter and an amp probe/clamp you can do an overall compression check in a few minutes without removing any parts.
Wow... you just changed my opinion that most modern tech.s are merely parts replacers.
:clap:
 
#4 ·
of course jtj is the man!!
 
#5 ·
One thing that was skipped in the instructions.

When you are cranking the engine, open the throttle fully. It does make a difference in the pressure readings that you will get, and it will come up to full pressure much quicker (with less drain on the battery).

As noted above, I'd pull the fuse for the injectors rather than disconnecting the wring for them.

Also, check the engine manual for the car that you are going to do a compression test on. On some of them (for instance the "wasted spark system used on early Miatas), cranking the engine with the spark plugs disconnected will often burn out the coils requiring them to be replaced. That shouldn't be a problem in this case since the individual coils are removed (and on the Miata the proper way to do the test is to disconnect the separate coils).
 
#6 ·
One thing that was skipped in the instructions.

When you are cranking the engine, open the throttle fully. It does make a difference in the pressure readings that you will get, and it will come up to full pressure much quicker (with less drain on the battery).
Tim, I believe he covered that in step #12.

"12. Hold down gas and clutch at the same time, crank engine over about 6 times, until needle stops jumping on compression tester"
 
#11 ·
Maybe I missed it, but I didn't see battery voltage and engine temp mentioned. Both can alter your readings slightly.

Leak down tests will also help diagnose issues that compression tests may miss.
 
#12 ·
RASTA: WELL DONE!
JTJ: GOOD TIPS ON DIAGNOSIS!

auto techs are no longer 'greasemonkeys'.
many are highly trained and capable.
when it gets down and dirty on a subtle problem there's no substitute for training.
sam
 
#15 ·
TurboPhil and I just did a compression test on my engine...

140, 140, 140, 140 - like clockwork.

Funny thing is that it pulls like a freight train, doesn't burn any oil and the plugs looked fine (well a bit of detonation but no blown off tips or anything like that).

WTF!
 
#17 ·
I thought maybe the gauge was off too, so I hooked the gauge to my air compressor. The compression tester agreed within a psi of the pressure I had in the air compressor leading us to believe the tester is accurate... Perhaps we're over-looking something here, or perhaps Dark's compression rings are a bit toasty from running so lean for so long:shrug: If the latter is the case, it's fortunate forced induction tends to mask low compression:D As long as there' no oil blowing, we just need to up the boost--- once we can tune for it;)

Best,

Phil
 
#18 ·
Did you hold the throttle wide open while you cranked the engine? If you don't, the engine can't pull in enough air, and the compression readings will be low.

Is your car a "Throttle By Wire?" If so, does the ECU keep the throttle closed under the cranking conditions?

The fact that all the cylinders are reading consistent usually indicates that things are well, and the readings are low for some reason.
 
#19 ·
05 car and yes, the throttle was wide open... Tried both ways, thrttle closed and open- readings were the same... which is intuitive since all the other plugs were out and letting air into the plenum---An open spark plug hole creates a huge vacuum leak..

Best,

phil
 
#28 ·
Agreed.

The only way you can get a 140 pound reading that's correct is to have some serious blow by.

The car would be consuming oil.

Also, the instructions in this thread are for a "dry" compression test. One way to see if you're getting an accurate reading (if it;s really low) is to squirt a little oil into the cylinder through the spark plug hole. (not a whole lot)

The oil will help seal the rings.

If you really do have a blow by condition, as would be indicated by a 140 PSI reading, putting oil in the combustion chamber will bump up the reading by a considerable amount.

The funny thing is it's probably just the gauge.

I recently bought a 96 Cobra Mustang as a parts car. (needed to give the motor away. Long story for another thread.) The car was in a light front end accident, but the motor was totally fine. I paid a local "Performance" shop to perform a compression test prior to purchasing the car.

They came back and told me it was 85-90 PSI across the board.

If your car is below, or even too close to, 100 PSI, it generally won't want to start. It'll have to crank and crank and crank until it builds enough chamber pressure to actually start. This car started and ran just fine.

Knowing this couldn't be right, I talked the seller down a few hundred more bucks and brought the car home.

When I got home, I went to AutoZone and rented a compression tester. That tester gave me a higher reading (115, max), but it still wasn't right. One thing I noticed was that the fitting spun at the base where it should have been fixed in position.

On a lark, and taking the ever desirable opportunity to buy a new tool for the collection, I went to Sears and bought a brand new compression tester for like 70 bucks.

I dropped it in, screwed it down and cranked.

Every single cylinder was ABOVE 195 PSI.

The moral of the story: if the car starts on the first hit of the key and idles/drives fine, the compression should be okay. Make sure your gauge is working properly.

Hope this helps. :)
 
#22 ·
As per my other engine thread. I'll add my compression test results to this one:

190 - 185 - 180 - 190

This is with Mahle 9:1 pistons.

Also noted that with all plugs OUT, throttle open/closed made no difference in the readings. Clutch in/out also made no difference.

I think what could cause the biggest variance in readings is how well one secures the compression tube in the spark plug hole. Small dab of oil on the rubber seal is pretty good at ensuring a good seal and more accurate readings.

5 or 6 cranks worked well for me.

Rob.
 
#23 ·
Most of the old manuals say to stop at three pulses and use that reading. A concern would be if the first pulse is low, even though it gets up to pressure by the third or fourth. I'm lucky to have a compressor and leak down guage. The only way we measure for leaks in the chip manufacturing industry is by the leak rate over time. Did anyone mention to have the engine at running temp first?
 
#24 ·
Interesting, the procedures I've read and follow for compression testing are those provided by the manufacturer of the testing tool and/or service manual -- they suggest to never remove spark plugs when the engine is hot because of potential thread damage to the head -- hence why cold torque specs for spark plugs is only 13 ft/lbs.

Also, if the motor is too hot, it will tend to melt the compression testors rubber seal unless your compression tester doesn't use rubber seals and is of the type that can be torqued down.
 
#25 ·
Also, if the motor is too hot, it will tend to melt the compression testors rubber seal unless your compression tester doesn't use rubber seals and is of the type that can be torqued down.
A Viton O-ring is good for over 200 degrees C. A Teflon one is good for over 500. On my race car, I start with a hotter plug, warm the engine to temp, then change to the colder plug for the track. This eliminates the problems of fuel fouling. This is a common practice. Antisieze on the plug threads is good for even street engines.

P.S. work hard in Cupertino. I need my Apple stock back up to $150. Typed on my Powerbook, as my HP is half the speed.
 
#32 ·
Do not forget your choice of cams/cam timing/cam overlap all make a big difference in compression figures.
I built one engine with a static 10.5:1 compression, but with the high overlap cams, it measured at 8.5:1 at cranking RPMs, and with overstuffing, was 12.5:1 at peak RPMs.
 
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