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How To - Compression Test; compression test
Topic Started: Nov 26 2009, 08:07 AM (24,501 Views)
Johnny Mullet
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Hi-Tech Redneck

In order to determine if a rough running engine has internal issues, then a compression test is highly recommended. This is a very simple test and you can usually rent or borrow a compression tester to get your readings. You could also purchase a tester fairly cheap if you shop around. Here is what a typical compression tester looks like..........

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To do this test, you will need an assistant to help you. It is best to perform this test with the engine warm, but if it will not start, then you have no choice but to test it cold. First off, you need to remove the coil wire and then remove all the spark plugs. Be sure to mark your wires so you do not get confused. Find the appropriate adapter in your compression test kit to match the spark plug thread and insert it into the #1 cylinder. You just thread it right into the cylinder by hand until it's snug.

After you have the tester inserted, make sure your car is out of gear or in Park (Auto trans) and the parking brake is set. Have your assistant hold the gas pedal all the way to the floor (WOT - Wide Open Throttle) and begin cranking the engine over for a count of 5 to 7 seconds. Record your readings from the gauge and repeat this on all cylinders. If you do not have an assistant, do like member Jittney does and use a tube of lipstick or whatever you can find to hold the throttle open.........

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Now add one teaspoon of engine oil into the cylinders and repeat the compression test one cylinder at a time. This will give you the "wet" readings and determine if you have worn rings.

The compression readings should be no less than 156 PSI on all cylinders and they should not vary much between the others. Here is an example of a low compression reading.........

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A healthy reading would look like this............

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A really healthy engine will have around 200 PSI of compression. Please note that these engines will run with lower compression numbers, but will not run correctly or get the expected fuel mileage.
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iamgeo
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Is that a fact....or just a rumor?

Quote:
 
Now add one teaspoon of engine oil into the cylinders and repeat the compression test one cylinder at a time. This will give you the "wet" readings and determine if you have worn rings.

Please explain, for those that do not know, how this will determine if the rings are worn.
Edited by iamgeo, Nov 26 2009, 09:50 AM.
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Johnny Mullet
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Hi-Tech Redneck

Compression numbers should typically go up when the oil is added to the cylinder. If not, then the oil is leaking through the rings.
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Choppydrivesageo
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Would compression readings go up on a healthy (200psi) engine when you do the wet test? If so what is the acceptable amount of increase?
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dimetrodon


Hmmm. When I have done compression tests in the past...I have never removed ALL the plugs at once. I removed only the plug for the cylinder I was testing. I wonder if this will make any difference?

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Johnny Mullet
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Hi-Tech Redneck

You can do it one at a time if you wish. Each cylinder is measured individually anyway regardless of whether plugs are all out or in.
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billy508
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billy508

I pull all the plugs to compare them and see how they are firing.ie wet, fouled, electrode wear. When all plugs are pulled the engine spins faster with less load on the starter. Also you can go back and recheck each one with a little less effort. But if I am pretty sure I know which cylinder is dead, I just pull the one plug a do a quick check. No compression=shows over. :banana :banana :banana
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dimetrodon


"When all plugs are pulled the engine spins faster with less load on the starter."

That might make a significant difference. I'll try it that way next time.
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Metro_caon
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Johnny Mullet
Nov 26 2009, 08:07 AM
Have your assistant hold the gas pedal all the way to the floor (WOT - Wide Open Throttle) and begin cranking the engine over for a count of 5 to 7 seconds.
Why does the pedal has to be all the way down to the floor? What difference does this make on the test if it's only halfway down?
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Murf 59
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I may be wrong. But I think its access to the air. It will be able to draw all the air it needs to give an accurate test.
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mwebb
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FOG

as Mr Murf says ...

VE or
Volumetric Efficiency is best with no restriction to air flow so that the cylinder s have an UNrestricted path to atmosphere at local ambient barometric pressure as much as possible during the test .
when the piston s traveling down with intake valve open , a lack of pressure or vacuum is created , the air at relatively higher barometric pressure rushes in through the open intake valve to equalize / fill the lower pressure inside the cylinder .

the throttle plate is a restriction to flow and a huge restriction to flow when closed .
hence the term
"suction throttling loss"

so
measuring compression with out completely or as completely as possible filling the cylinders during the test will result in incorrect test results
Edited by mwebb, Jan 8 2010, 01:33 PM.
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Coche Blanco
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Troll Certified

It is also possible for your car to run well, and smoke. Mine has 190+ on all cylinders, but smokes like a dog when cold, and smokes when warm.
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BillP
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Coche Blanco
Jan 8 2010, 03:40 PM
It is also possible for your car to run well, and smoke. Mine has 190+ on all cylinders, but smokes like a dog when cold, and smokes when warm.
Valve guide seals?
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Coche Blanco
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BillP
Jan 8 2010, 06:53 PM
Coche Blanco
Jan 8 2010, 03:40 PM
It is also possible for your car to run well, and smoke. Mine has 190+ on all cylinders, but smokes like a dog when cold, and smokes when warm.
Valve guide seals?
I'll tell you when I rebuild.
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Bad Bent
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Facetious Educated Donkey

"It is "easier" to replace the whole head, it is "better/cheaper" to do the valves. "
...or so I've heard. :thumb
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