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How to: Compression test and specs

Discussion in 'Tips and Technical Info posted by Technician's' started by X-North, January 17, 2008.

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    1. X-North

      X-North Active Member

      July 16, 2007
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      I will make it short and easy to understand.

      #1 Start your engine so it go to normal operating temperature

      #2 Stop it and let it cool down for 10 minutes before removing spark plugs and very important you must remove them all before the test. Why? Suppose that you have a cracked head gasket between two cylinders when you will do the test you may not remark the lost of pressure because the cylinder to the next you are testing still have his spark plug to keep the pressure in the engine.

      #3 Disconnect the electrical wire that plug onto the coil pack. Never only unplug the spark plug wires because great injury or damage could happen if the 40,000volts spark hit you or a component of the truck

      #4 Do the test on each cylinder and be sure the hose is tight in the spark plug thread because a lost of compression could happen and causing a misreading of the results

      #5 Crank the engine so it could rotate 7 times and read the results.If you have time do it twice on each cylinder just to be sure you correctly did it.

      #6 The needle on the gauge should rise up quickly on the first rotation of the engine and continue to rise steadily. If at first rotation the needle stay almost the same or only goes up a little and after that it rise up normally then you have a ring problem.To test the rings pour a little amount of oil in the spark plug hole then replug the tester and make the test.If the first crank rise the gauge quickly then you are sure the rings are bad. Also at first rotation and each other after if the needle dont rise quickly then you have a valve problem.Maybe burned valve,deposits that make it not close correctly or craked head or even maybe blown head gasket.After the test of oil if no compression is present then you know it's not rings but valves,gasket pr cracked head.

      Ford specs tell 75psi maximum of difference between the highest and lowest cylinder reading and a minimum of 100psi in each cylinder.
      In exemple I just tested mine today because the engine had 130 500 miles.

      As you can see all my cylinders are far more than 100 psi and have only 20psi of difference between the highest and lowest reading.
      Last edited: January 19, 2008
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    3. BrooklynBay

      BrooklynBay Moderator & long time member. Staff Member Moderator Elite Explorer

      November 11, 2005
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    4. Kiliona

      Kiliona Active Member

      July 12, 2013
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      Good write-up!

      I have one quick thing to add, compression tests can be done in two ways,the first is a cranking test like you described and the second is a running test. I only mention this because running tests result in much lower pressures (higher rpm => higher velocity of the vapor => lower pressure) and so a person needs to be careful they read the correct spec, in this circumstance cranking not running. However the important thing usually is consistency between cylinders anyways.

      Also if a person has access to a good scan tool for their vehicle that's capable of doing key on engine running self tests the computer can perform it's own cylinder balance test (similar to a running compression test). This can save a lot of time as one wouldn't have to pull any spark plugs, not to mention it tests other things at the same time. May be a good place to start (and then if there is an indication of a problem move on to actually using a compression gauge and other forms of diagnosis). Also this is something I've done on my own first gen explorer (EEC-IV computer) but not on anything newer, I imagine it exists for all years but I could be wrong as I've only done it myself on a first gen.
    5. Drewmcg

      Drewmcg Elite Explorer

      August 27, 2015
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      This is an old thread, but I believe this statement is wrong. In the Ranger forum someone quoted a Ford tech manual for saying that the lowest cylinder must be within 75% of the highest cylinder (not 75 psi), and no cylinder less than 100 psi.
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