how to do a compression check?
Joined: Mar 2000
Posts: 43,187
Likes: 43
From: Littleton, CO USA
Car: 82 Berlinetta/57 Bel Air
Engine: L92/LQ4 (both w/4" stroke)
Transmission: 4L80E/4L80E
Axle/Gears: 12B-3.73/9"-3.89
The technique is detailed in any decent repair manual, but here's a Reader's Digest version:
1st, you need a compression gage. The screw-in type are a little easier to use by yourself.
Remove all of the spark plugs. Disconnect the power wire to the distributor/coil.
Hold the throttle wide open. With the gage installed in one cylinder, crank the engine over with the starter for 3 or 4 compression strokes (a remote starter switch or helper good here, especially if your gage isn't the screw-in type). Record the reading for that cylinder, repeat for remaining cylinders.
If readings are low, repeat the tests having put about a tablespoon of oil in the cylinder before the test (this is known as a "wet" test). If readings go up significantly, you've likely got bad rings.
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82 Berlinetta, orig V-6 car, now w/86 LG4/TH700R4. 2.93 limited slip. Cat-back from '91 GTA, ZZ3 intake, Accel HEI SuperCoil. AMSOIL syn lubes bumper-to-bumper. Daily driver, work-in-progress (LG4 w/'87 LB9 block, ZZ3 cam, ported World 305 heads, Hooker headers & y-pipe, 3" Catco cat & 3" cat-back, Spohn SFCs).
57 Bel Air, my 1st car. '66 396, 9.7 CR forged TRWs, Weiand Action+, Edelbrock 1901 Q-Jet (Holley 3310 on the way), GK 270 cam, Magnum rockers, Jacobs Omnipack, 1-3/4" Hedders & 3" Warlocks, TH400 w/TCI Sat Night Special conv & Trans-Scat shift kit, MegaShifter, 3.08 8.2" 10-bolt w/Powertrax, AMSOIL syn lubes bumper-to-bumper. Idles smooth @ 600 RPM in D. Best 15.02/95.06 @ 5800' Bandimere (corrected 13.93/102.4 @ sea level).
1st, you need a compression gage. The screw-in type are a little easier to use by yourself.
Remove all of the spark plugs. Disconnect the power wire to the distributor/coil.
Hold the throttle wide open. With the gage installed in one cylinder, crank the engine over with the starter for 3 or 4 compression strokes (a remote starter switch or helper good here, especially if your gage isn't the screw-in type). Record the reading for that cylinder, repeat for remaining cylinders.
If readings are low, repeat the tests having put about a tablespoon of oil in the cylinder before the test (this is known as a "wet" test). If readings go up significantly, you've likely got bad rings.
------------------
82 Berlinetta, orig V-6 car, now w/86 LG4/TH700R4. 2.93 limited slip. Cat-back from '91 GTA, ZZ3 intake, Accel HEI SuperCoil. AMSOIL syn lubes bumper-to-bumper. Daily driver, work-in-progress (LG4 w/'87 LB9 block, ZZ3 cam, ported World 305 heads, Hooker headers & y-pipe, 3" Catco cat & 3" cat-back, Spohn SFCs).
57 Bel Air, my 1st car. '66 396, 9.7 CR forged TRWs, Weiand Action+, Edelbrock 1901 Q-Jet (Holley 3310 on the way), GK 270 cam, Magnum rockers, Jacobs Omnipack, 1-3/4" Hedders & 3" Warlocks, TH400 w/TCI Sat Night Special conv & Trans-Scat shift kit, MegaShifter, 3.08 8.2" 10-bolt w/Powertrax, AMSOIL syn lubes bumper-to-bumper. Idles smooth @ 600 RPM in D. Best 15.02/95.06 @ 5800' Bandimere (corrected 13.93/102.4 @ sea level).
Supreme Member
Joined: Jul 1999
Posts: 18,457
Likes: 16
From: Loveland, OH, US
Car: 4
Engine: 6
Transmission: 5
Yes, if you have FI, it would be a good idea to unplug the injectors. If you have a carb you don't have to do anything to it.
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"So many Mustangs, so little time..."
ICON Motorsports
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"So many Mustangs, so little time..."
ICON Motorsports
Supreme Member
Joined: Jan 2001
Posts: 1,978
Likes: 0
From: PA
Car: 88 Firebird WS6
Engine: 350 TPI
Transmission: T56
Axle/Gears: 3.42
No, if you have FI the computer goes into 'clear flood' mode and shuts the injectors down above 80% throttle.
If you are doing the check in very cold weather, disconnect the CSI plug on the front of the intake manifold or remove the 3A crank fuse, if your car is so equipped, but since it's pretty warm out I wouldn't bother.
If you are doing the check in very cold weather, disconnect the CSI plug on the front of the intake manifold or remove the 3A crank fuse, if your car is so equipped, but since it's pretty warm out I wouldn't bother.
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