Doing a compression test, is a good way to measure the condition of, the rings, cylinders, and valves.
So, if your engine is running rough or is lacking power, consider doing a compression test.
Also, it is important to perform a compression test every time a tune-up is done, as part of preventative maintenance.
You really don’t need special skills, to do a compression test. But, you do need to know, how to understand your results properly.
By performing a compression test, internal engine malfunctions; such as bad valves, piston rings or excessive carbon buildup can be detected, before they cause irreparable damage.
Be aware, that late model engines, are mostly made of aluminum and therefore, are prone to spark plug thread damage.
This is very common, when you remove a spark plug, on a hot engine.
So, before warming up the engine, remove the spark plugs and add a dab of, anti-seize compound to the threads. Install and tighten the spark plugs, to the torque listed, in your vehicle repair manual, using a torque wrench. Because, this will make it easier, to remove the plugs next time around.
How To Do A Compression Test (Dry)
- Start with a warm engine, the ignition and fuel disabled, and all spark plugs removed.
- Above all, wear protective clothing and gloves, while working around a hot engine.
- While doing the test, the throttle and choke plates, must be fully open, for an accurate test.
- Have your helper, fully depress the accelerator, while he is cranking the engine.
- Avoid any flames around the engine, during the test.
- First of all, connect the tester to a cylinder and turn the engine over, 6 to 8 compression strokes.
- You will hear the cranking speed slow, as the tested cylinder, comes up on it’s compression stroke.
- Note how fast the compression increases and write down the highest reading.
- Test all cylinders the same way, with the same number, of compression strokes.
What Do The Results Tell You
There is more than one school of thought on, what the compression, of the average engine should be. So, as a general rule, a compression of 135 PSI or better, is excellent. On the other hand, a compression of 85 PSI or lower, is extremely bad. Firstly, the best situation, is that all cylinders give the same or close to, the same reading.
In addition, that reading should be, above 135 PSI. Uneven readings are not uncommon on high mileage or worn engines. Also, the difference between, the highest and lowest readings, should be no more than 20%. So, as long as the lowest reading is, 100 PSI or better, then the engine is decent.
Consequently, greater differences between cylinders, indicate worn or broken rings, leaky or sticky valves or a combination of all.
While watching the gauge, during the cranking, you should have noticed, the way each cylinder pumped up. Usually, a cylinder will produce, about 40 PSI on the first stroke and 35 PSI, on each additional stroke.
Problem cylinders, may have trouble pumping up and may increase, by only something like, 10 PSI per stroke.
It may be possible to crank these cylinders enough times, to come close to, the other cylinder’s total PSI. This is why we try to crank, all cylinders the same amount. Consequently, poor rings, usually cause this condition. Be aware that a cylinder suffering from excessive oiling, even from bad rings, can yield high compression test results. Other symptoms, may give you a clue to the problem. (a smoking engine)
There are some variables that affect the readings, obtained from compression testing:
- Cranking speed
- Worn camshaft lobes
- High performance long duration profile camshafts
How To Do A Compression Test (Wet)
Many automotive books describe, a dry and a wet compression test together. Usually these tests, must be viewed together to isolate the trouble, in cylinders or valves. To perform the wet test, a tablespoon of engine oil, is poured into the cylinder, through the spark plug hole.
So, you squirt the tablespoon of engine oil, into a cylinder, that is reading low. Crank the engine, two revolutions or so to spread the oil, then retest the cylinder. If the compression comes up markedly, 40 PSI or more, the trouble is, poor ring to bore sealing.
If compression, doesn’t increase much, about 5 PSI, then the problem, is probably with the valves. In addition, it could also be, pulled head studs or a, warped cylinder head.
What Does All This Info Tell You
- Compression builds up quickly and evenly, to the specified compression for each cylinder.
Piston Rings Leaking
- Compression is low, on the first stroke. Consequently, compression builds up with the following strokes, but does not reach normal. And, your compression improves considerably, when you add oil.
Your compression, is low on the first stroke. And, compression usually does not build up, on the following strokes. Furthermore, your compression does not improve much, when you add oil.
Two adjacent cylinders, have lower than normal compression and injecting oil into the cylinders, does not increase the compression. Most often, the cause may be, the head gasket leaking between the cylinders.
So, testing compression, is really nothing more than a way to figure out, which cylinder has a problem. And, if you do find any issues, the next step would be, doing a cylinder leak down test. A cylinder leak down test, is similar to a compression test. It also tells you, how well your engine’s cylinders are sealing. But, instead of measuring pressure, it measures pressure loss.
TIP: You can also do a, A running (dynamic) compression test.
Thank You !