An engine compression test, is the most practical way, to learn about the mechanical condition of your engine.
Because, doing a engine compression test, will help you locate; potential internal problems, affecting engine performance.
Running rough or losing power, there may be a lack of engine compression, in one or more cylinders.
So, internal engine damage can be detected, by checking engine compression.
- Leaking valves.
- Leaking piston rings.
- Excessive carbon buildup.
- Broken Valve Spring.
- Blown head gasket.
- Worn camshaft.
- Bent pushrods.
- Broken timing belt or chain.
- Hole in piston.
There are some other causes of low engine compression but, these are by far the most common.
You should be aware of these problems; so you can make an informed decision when investing in repairs. As a rule, most engines should have, 140 to 160 lbs. of cranking engine compression. Also, there should be no more than 10% difference, between any of the cylinders.
Checking Engine Compression
So, to check engine compression manually with a gauge, all the spark plugs must be removed. The ignition coil must then be; disabled or the high tension lead grounded. If the engine has a distributorless ignition; the ignition coils must be disabled; to prevent them from firing.
The throttle must also be held open. The engine is then cranked for a few seconds; while a compression gauge is held in a spark plug hole. The maximum compression reading is noted; then the process is repeated; for each of the remaining cylinders.
Consequently, a cylinder leakdown test, needs to be performed, if low compression is found.
This will assist in diagnosing, what is going on inside of the engine.
Cylinder Leak Down Testing
This test pinpoints specific leakage. This test uses a set of pressure gauges with a regulating device and can, quantify the percentage of leakage. However, this is a static test that takes more time to perform; compared to a regular compression test.
First Engine Misfire Codes
Most drivers are first alerted there may be a problem; is by seeing engine misfire codes ( P0300 – P0312 ). So, if you see any codes the first things to check for are, fuel and ignition problems. If these do check out ok, the next step is confirm proper compression.
What Can Cause, Low Or No Engine Compression
Low Engine Compression, In One Cylinder
If readings are very low in one cylinder, it is highly probable that; internal engine damage exists such as:
- The piston could have, a broken connecting rod or a hole in it.
- There could be a stuck, burnt or leaking valve.
- There could be a, broken valve spring or a bent push rod.
- The camshaft has, excessive wear and is not opening the valve(s).
- If the compression is low or zero on two adjacent cylinders; it would indicate a leaking gasket.
- There is a weak sealing surface at the head to block mounting area; which basically means a bad head gasket.
- There is a broken camshaft, in an area that operates valves for two adjacent cylinders.
Low Engine Compression, In All Cylinders
Consecutive low compression in all cylinders; could mean that the problem of fuel washed cylinders exists. So, this means that the engine has had, too much fuel introduced into it. As a result, all of the oil has been washed off the cylinder walls. The oil creates a sealing effect; between the piston and ring assemblies and the cylinder walls of the engine block. This is common with an engine that has, a ‘flooding’ problem.
If the engine seems to run normally, but is weak and puffs smoke, it could have worn piston rings. In either of these events, squirt a little oil into each cylinder; then repeat the compression test. If the compression dramatically increases, then you have found the problem(s). If the compression readings do not change, then it would indicate a timing problem.
No Engine Compression In One Cylinder
Dropped Valve Seat
If a valve seat cracks it will allow hot gases to leak; burning both the valve seat and the valve. Most cylinders heads are made of aluminum and expand at a different rate; compared to the metal valve seat.
This variance in the expansion rate can cause, the seat to fall out of the head. Once this has occurred the cylinder will have no compression; as the air escapes into the valve port. Once detected, the cylinder head must be removed, replaced or repaired.
Broken Valve Spring
A valve spring is responsible for closing the intake and exhaust valves, once the camshaft has opened them. Over time valve springs can become brittle and break. As a result, allowing the valve to hang open, which will allow the compression to leak out.
Valve keepers are two half moon pieces of metal that; lock into the valve retainer holding the valve in place. If these pieces become dislodged, they can fly out of the retainer. Consequently, allowing the valve to drop into the cylinder contacting the piston.
The head of the valve, seals against the valve seat. When these valves fail, the head can come apart from the stem. The head of the valve, will drop into the cylinder. This will allow compression to leak from the cylinder; while causing extreme damage to the piston and cylinder head.
Piston Damage Or Hole
The piston may fail due to, excessive heat in the combustion chamber. So, a burned piston will typically have a melted appearance; or a hole burned completely through the top of the piston. Aluminum can only withstand so much heat, and when it gets too hot, it melts. The underlying cause, is usually detonation and/or pre-ignition.
No Engine Compression, In All Cylinders
Broken Timing Belt or Chain
Every car engine needs a timing belt or chain; to keep the camshaft in correlation with the crankshaft. When these parts fail the camshaft stops turning; which causes the intake and exhaust valve not to open and close. Without the camshaft rotating, the engine cannot make compression.
If a camshaft breaks, it will stop the camshaft from turning; much like a broken timing belt or chain.
Low Or Zero Readings, In Two Adjacent Cylinders
This usually happens if there is a blown or weak head gasket. Another possibility, is a broken camshaft in the area that operates valves, for two adjacent cylinders.
Finally, the engine compression could be too high, in one or more cylinders. As a result, this would be an indication of, excessive carbon buildup in the engine.
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