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Collection of compression test stuff


chiplee
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We're going to lose alot of good info if we don't start collecting what gets posted in the VM forum so I went ahead and collected as much stuff as I thought was important on compression testing since there's no FAQ for it.  Reply with any recommendations for addition that you see fit but it's almost too big for a single post as is.  Thanks to Shelby especially but to anyone who's input contributed.  I didn't make a list of credits or anything, sorry.

 

Compression Test Procedures

and rules of thumb

Concepts and definitions: (things to understand before you start)

  • A compression tester is a one way valve.  Its stability is no indication of engine condition, only its reading.  You could come back 8 days later and it should be right where you left it.

[*]A compression test and a leak down test are two different things and require different guages.  Anyone can do a compression test in 10 minutes with a $20 guage, but usually a shop will have to do a leak down test which is used to pinpoint the trouble you’re having.[*]Whenever the compression guage jumps and the readings fall back it means the gauge is leaking. Redo the tests , it's kind of common for the schrader valves too go bad, due to dirt or carbon getting into them. This is why it's a good idea to always watch the guage when doing a compression test.  The highest pressure achieved should always hold on the gauge , if not then it's not a true reading.  Redo the test and keep and eye on the guage, if it doesn’t hold, replace the schrader valve. This valve is inside the end of the tester that screws into the head.  It looks just like one for a tire stem but they are diff , most parts store have them, don't get tire schrader valves.[*]When your engine is idling, pay attention to idle vacuum on your aftermarket gauge.  Engine idle vacuum is directly related to the engine's compression, and the cylinders being even. Watching the vacuum gauge can clue you in to uneven compression across the cylinders, or at least if one or more is missing; any miss of the spark plugs will cause a drop in the indicated vacuum. An unsteady or bouncing gauge reading at idle indicates something wrong with a cylinder's compression or spark plug; this also could indicate a vacuum leak in a single cylinder or two cylinders, more so than all 4.[*]The specifications given below pertain to engines with a stock cam.  As a general observation, rather than an actual specification, upgrading to a 284 cam has resulted in compression test results of approximately 120psi per cylinder on an otherwise healthy engine.  A 292 may bring compression as low as 105-110psi.  I get about 107psi per cylinder with my 292 from Schnieder.[*]If you suspect coolant ingestion, run the test with the radiator cap off and coolant over filled.  This could reveal a pressure leak into the cooling system.

Procedures:

1.  Go for a drive and get the engine to normal operating temperature.  A little movement on the temp guage is not what I’m talking about.  Get it hot like it normally runs, and ensure the battery is strong enough to crank the engine between 250 and 400rpm for at least a minute of total cranking time.

 

2.  Remove all four plugs and inspect them. Bad valve seals will show up as slightly fouled plugs.  Ring damage might show up as oil on the plugs.

 

3.  Disconnect the primary injector clip. (both if you don’t know which is which)

 

4.  Remove the coil wire from the distributor, not from the coil or spark could jump to electrical components in that area.  

 

5.  If you suspect cylinder head or gasket damage, crank the engine with all plugs out to clear  cylinders of water or other debris.  Watch your face.

 

6. With the throttle wide open and the tester screwed into a cylinder, crank at least 5 to 7 times.  The time to stop cranking is when the guage stops moving up.

 

Specifications:

Starion and Conquest (all years)  2.6L G54B

 

[table][tr][td]Compression ratio--------------------------------7.0[/td][/tr][/table]        

[table][tr][td]Standard value-----------------------------------142psi[/td][/tr][/table]

[table][tr][td]Service limit--------------------------------------113psi[/td][/tr][/table]

[table][tr][td]Maximum difference------------------------------14psi[/td][/tr][/table]

7.  If any of the cylinders has compression pressure and/or pressure difference that exceeds limits, add a small amount (teaspoon) of engine oil through the spark plug hole, allow a moment to spread evenly and repeat step (6) on that cylinder.

 

Possible causes for various compression test results

 

Note:  Information gathered from past posts on SQC.com and from the factory repair manual.  Special thanks to Shelby117, but if you see something in here that you wrote, then thank you too.  Keep the shiny side up.

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Thanks... I'll look into the cold cranking idea but I'm pretty sure you would only want to crank cold when you have no other choice (like when the engine doesn't run).  There's nothing wrong with doing the test cold, but you would almost have to do a wet test to get the same numbers you get with a hot test, since it's oil that seals the rings from below when you do a hot test.  Check step 6 for the crank duration, but consider the fact that rotation speed plays a big role in required number of revolutions so you really just want to crank it until the guage stops going up.
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  • 3 weeks later...

quick question.........are you supposed to use the little extension that you get with the compression kit? you know, the one that screws into the spark plug hole and attaches to the compression hose. just want to be careful, because the instructions say something about hitting the piston if it is used on wrong car. thanks

-chris

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  • 4 weeks later...
  • 1 year later...
3.  Disconnect the primary injector clip. (both if you don’t know which is which)

 

4.  Remove the coil wire from the distributor, not from the coil or spark could jump to electrical components in that area.  

 

3. pre 87 remove both injector clips

 

4. Ground coil wire attach a  jumper (16 gage wire with alligator clip on each end) to the coil wire and a good known ground.

 

You can also make use of a remote starter switch or a Hose on the compression checker that is long enough to reach all plug holes and tape to the windshield on the drivers side ( for one man operation)

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