Compression Test
Posted: 03 Mar 2009, 18:49You need a compression gauge. You can get one of these for between $15 and $50 almost anywhere you can get auto supplies, including Canadian Tire, Pep Boys, and even WalMart. You don't need a particularly good or expensive one just to make sure you're in spec.
How to do a compression check:
1. Before you begin, make sure the battery is fully charged.
2. Pull the MPI (MultiPort Injection) fuse. This cuts power to the injectors so they don’t spray gas all over the place.
3. Unplug the coil connector. You could get a nasty shock if you don’t.
4. Remove all 4 plugs and examine them carefully for signs of a problem. If your valve seals are bad in any cylinder, the plug will be fouled to some degree. Signs of oil on the plugs could indicate a ring problem.
5. Screw the compression gauge into the plug hole
6. Press the gas pedal to the floor to open the throttle plate, and keep it at WOT (wide open throttle) for as long as you’re cranking (if you didn’t pull the MPI fuse, there would be a bit of a mess, now).
7. Turn the ignition to the ON position, and hold it there for a count of four revolutions (do not turn the key on-and-off four times). On each rotation, the gauge needle will jump slightly less than the previous one. It may take more than 4 revolutions to get a stable reading; crank for up to 10 seconds or until the needle stabilizes, whichever comes first.
Note: If your battery is weak, the engine will turn over slower and give you lower readings.
8. The last number the gauge jumps to is the compression for that cylinder.
9. Repeat for each cylinder
A "wet test" entails a bit of oil in the low cylinder...
1. Add one capful (1-2 tablespoons) of oil to the cylinder with low compression.
2. Let it sit for a minute to allow the oil to flow down and coat the sides of the cylinder and piston. If there are any gaps in the rings, the oil will seal them for the next few minutes.
3. Crank the engine for 4 revs, remembering to floor the throttle.
4. If the reading improved significantly (more than 30 psi), your rings are probably worn.
5. If there is little or no improvement, take it to a garage that can perform a leak-down test or perorm the test yourself to determine where the compression is escaping (intake, exhaust, crankcase, gasket, etc.).
6. Replace the plugs, preferably in the same cylinders they came out of. You may want to clean or re-gap them first, or put in new ones. A dab of anti-seize on the threads would be a good idea.
7. Remember to put the MPI fuse and coil connector back.
How to do a compression check:
1. Before you begin, make sure the battery is fully charged.
2. Pull the MPI (MultiPort Injection) fuse. This cuts power to the injectors so they don’t spray gas all over the place.
3. Unplug the coil connector. You could get a nasty shock if you don’t.
4. Remove all 4 plugs and examine them carefully for signs of a problem. If your valve seals are bad in any cylinder, the plug will be fouled to some degree. Signs of oil on the plugs could indicate a ring problem.
5. Screw the compression gauge into the plug hole
6. Press the gas pedal to the floor to open the throttle plate, and keep it at WOT (wide open throttle) for as long as you’re cranking (if you didn’t pull the MPI fuse, there would be a bit of a mess, now).
7. Turn the ignition to the ON position, and hold it there for a count of four revolutions (do not turn the key on-and-off four times). On each rotation, the gauge needle will jump slightly less than the previous one. It may take more than 4 revolutions to get a stable reading; crank for up to 10 seconds or until the needle stabilizes, whichever comes first.
Note: If your battery is weak, the engine will turn over slower and give you lower readings.
8. The last number the gauge jumps to is the compression for that cylinder.
9. Repeat for each cylinder
A "wet test" entails a bit of oil in the low cylinder...
1. Add one capful (1-2 tablespoons) of oil to the cylinder with low compression.
2. Let it sit for a minute to allow the oil to flow down and coat the sides of the cylinder and piston. If there are any gaps in the rings, the oil will seal them for the next few minutes.
3. Crank the engine for 4 revs, remembering to floor the throttle.
4. If the reading improved significantly (more than 30 psi), your rings are probably worn.
5. If there is little or no improvement, take it to a garage that can perform a leak-down test or perorm the test yourself to determine where the compression is escaping (intake, exhaust, crankcase, gasket, etc.).
6. Replace the plugs, preferably in the same cylinders they came out of. You may want to clean or re-gap them first, or put in new ones. A dab of anti-seize on the threads would be a good idea.
7. Remember to put the MPI fuse and coil connector back.