Repair Topics / Engine Compression

• What is Compression?
• Why does my engine have low power output?
• Why does my engine stall at idle?
• Why does my engine crank over but not start?
• How does a timing belt work?

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Car Engine Compression Questions and Answers

Question: I read how to fix my problem (or at least find out what the problem is) on your site but what do you mean when you say a cylinder leak down test must be done?

Answer: After doing a compression check, if one or more of the cylinders tests low, then a cylinder leak down test will help you determine the cause of the weak cylinder. Special equipment (leak down gauge) is required which can be purchased at most auto parts stores. With all the spark plugs removed, rotate the engine so that the cylinder to be tested is on top dead center and ready to fire. This will assure that the valves are closed. Install the leak down gauge in the spark plug hole and apply air pressure. The gauge will register the pressure drop indicating the percentage of leak. In addition, by listening at the exhaust pipe or air intake system or the oil breather you may hear escaping air, which means one of the components, is leaking. Example: air out of the exhaust pipe means the exhaust valve is leaking. By removing the radiator cap and observing the coolant, you can tell if there is a blown head gasket and or a cracked head if there are bubbles.

Question: 1998 Olds Cutlass Ciera mileage: 100,000.  I recently did a compression test on my car because of poor running and stalling problems, and because we wanted to determine if the car's engine performance was good enough to keep a little longer before getting other things fixed.  We got the following readings, and would like to get your comments/opinions on them.   Don't know official cylinder number but will give them from left to right: #1 - 152;  #2 - 165;  #3 - 152;  #4 - 149.  Did notice that the last cylinder mentioned, jumped up to about 90 psi real quick like the others, but then took a little longer to get to 149.  Any comments or suggestions?  Are these acceptably balanced numbers or do I have a serious problem looming?  Thanks for your help.

Answer: Your numbers are very respectable for a car with 100,000 miles.   From what you have told us, it sounds like the internal engine is in good shape.

Question: 2002 Chevy S-10 Blazer mileage: 78,200.  I took a set of compression reading the other day, and need some help analyzing them.   According to what I've read, these compression readings look good, but I've yet to see an actual number that would be acceptable.   I just see "lowest cylinder must be 70% of highest, 100 psi min" does that mean Anything that falls into that range is fine.  Anyway, here's the readings (took 3 readings each cylinder, averaged them...all 3 readings were within +/- 5 psi): Left bank Cyl 1: 163psi, Left bank Cyl 2: 144psi, Left bank Cyl 3: 146psi, Right bank Cyl 1: 160psi, Right bank Cyl 2: 130psi, Right bank Cyl 3: 146psi.  I then added 3 squirts of engine oil to Rt bank Cyl's 2 and 3: Rt. bank Cyl 2 w/oil: 135psi, Rt. bank Cyl 3 w/oil: 140psi.  Other helpful info may be that when I pulled spark plugs, Cyl 2 on both sides were coated with heavy black dry (carbon?) deposits, and Cyls 3 on both sides were coated with a good amount of oil.  Cyls 1 on both sides looked to be ok.  Any help on what all this means would be greatly appreciated!

Answer: Although the compression readings are not perfect, they are respectable for an engine with 120,000 miles.  The two cylinders that you added oil to tell us that the rings are in good condition but the valves may have slight leakage.  Oil and carbon on spark plugs indicates to us that your valve stem seals may be allowing oil to get into the cylinders and foul the plugs.

Question: 2004 Honda Accord Vtec/ 4 cylinder mileage: 56,000.  Have a problem with the engine; mechanic says that I only have 30 psi in first cylinder, 90 psi in second, and next two are fine.  What do you think is wrong?

Answer: Have your mechanic perform a cylinder leak-down test.  This will pinpoint whether your problem is rings, valves or head gaskets.

Question: 1999 Jeep Wrangler mileage: 98,900.   My engine is blowing oil out around the distributor shaft, oil dip stick, oil pan, and up into the air filter. A compression check shows approximately 160 PSI, dry to 175 PSI wet.  All vacuum lines intact and pulling approximately 20" HG including through the PCV valve.  Would this be a piston and ring problem or just the head (valve seals) or something else?

Answer:  You may have piston ring problems.  The PCV valve can only handle so much blow by, the excess will result in the problem you are experiencing.   Try a cylinder leak down test to check the rings for blow by.

Question: 1968 Chevy Corvette 327/350,  87,000 miles. What is the proper procedure to conduct a compression test?

Answer:  Remove all eight spark plugs. Remove and secure 12-volt wire from ignition coil. Place compression gauge in spark plug hole. Crank engine approximately 5 to 10 seconds. Gauge reading should be be between 135-155 pounds.

MORE: Engine compression questions

Repair Topics / Engine Compression