low compression?

Question

what does it mean when a compression test is done on a car and the result is low? possible causes are....

Answer 1

Low compression could be caused by bad piston rings, a bad head gasket, bad valves, or all of the above.

Answer 2

When a compress test is done, they remove one spark plug at a time and attach a pressure meter in place of the spark plug. When the engine runs, they can see how much pressure is generated. They do this with each spark plug. A low compression means something is broken and the spark is not igniting the fuel mixture.

If your car is using oil, then your piston rings are worn.
If you car is backfiring, running rough, you have valves not closing.
A car with bad compression is hard to start.
You can see a visible sign if the head gasket is blown

An engine needs to compress the fuel in order to ignite it. That is where the power that drives the engine. The explosion pushes the piston. One valve lets fuel and air into the piston, the other valve lets the exhaust gas out. If the valves don't seal, the gas mixture is released into your exhaust system, causing the car to backfire. The piston does not see the fuel mixture, it explodes in the muffler.

When you are losing oil, the oil is seeping up past the pistons rings. The oil is burnt with the gas mixture, usually you will see more smoke coming from your tail pipe.

Unless you are losing oil, it is normally valves not seating, not fully closing. Head gasket problems can be spotted by a mechanic.

Usually valve problems sound like a rough running engine, the vehicle will shake more than others.

Answer 3

It means the cylinder has a leak and is not holding pressure when the piston is pushed up into that cylinder.

Possible causes:
Broken pisron ring(s)
Hole or crack in piston
Cracked head or block
Boken head gasket
Extremely worn or scored cylinder wall
Stripped park plug threads (unlikely)
Valve or valve seat problem

Disassembly and inspection will easily find the cause. The best and most common scenario would be a head gasket needing replaced. Often this problem can be detected by the mixing of coolant and oil in either the radiator or crankcase. Occasionally you would smell antifreeze in the exhaust. Depending on the situation, milling of the head or block may be required.
The next "best" scenario would be a valve or head problem which would require a valve job be done or the head swapped.

Answer 4

It would help to know how low, what the readings were compared to what they should be. We used to put a squirt of oil into each of the cylinders and run the test on each again. If the pressures rise, your rings are bad or your cylinders are warn/scored. Usually, valve guides are warn causing poor sealing of the valves, thus low compression. Another, but remote chance, badly worn valve timing chain/belt.

Answer 5

Valve train wearing and compression rings worn

Answer 6

Worn out piston rings, leaking valves (blow-by). Result is low level engine performance and efficiency. Either an engine rebuild or replacement is likely in order.

Answer 7

Low on all cylinders or just one? Is it using oil? If it's using oil then it needs a rebuild, if it's not using oil have a different person do the test, it may have been just a bad test. If bad low on just one cylinder it will need to be worked on.

Answer 8

well you could have bad compression rings or maybe you need a valve job done.

Answer 9

Stuck valves, bent valves, broken valve springs, piston damage etc.

Answer 10

How much for a valve job