North American Turbocoupe Organization

How to do a Compression Test
andrewjs18 Offline
How to do a Compression Test
by Pete Dunham

The engine should be warmed up to normal operating temp before the test. The car battery should be in very good condition and fully charged. Advice, now is the perfect opportunity to do a quick tune-up with a fresh new set of plugs and wires, cap and rotor.

Tools needed:
  • Compression Tester (type the screws into the spark plug hole)
  • Spark plug wrench
  • approx 6" length of 3/8" rubber hose/tubing or old spark plug boot (not required)
  • motor oil (whatever you are using in the car)
  • other hand tools for removing the IC
  • clothes pins or tape and a pen to mark each spark plug and wire
  • advice... have at least one spare plug on hand incase you crack one during removal

Perform the following steps:

  1. FIRST AND FOREMOST, take a air compressor or shop vac and clean the area around each spark plug recess BEFORE removing the plugs in order to prevent any dirt/sand from dropping into the cylinder. If you have oil and dirt caked around/inside the recess, take a long screw driver (or similar) and scrap it clean, then vacuum it out or use compressed air to remove the rest. Also clean the area around the IC and throttle body hoses.
  2. Remove the I/C or the hose from the IC or compressor outlet tube to the throttle body, and block the throttle plate open. If you remove the IC, be sure to cover the turbo inlet hole with a towel or cup. Do NOT stuff a rag in the hole, lay it over it.
  3. MARK EACH SPARK PLUG AND WIRE with clothes pins or tape and a pen so you can return them to the proper location when reinstalling them. Don't laugh, there has been quite a few people that have reinstalled the wires in the wrong order thinking they could remember.
  4. Remove all 4 spark plugs. Use the kind of compression gauge that screws into the spark plug holes for best results. Perform the test on one cylinder at a time.
  5. Install the Compression tester into the first hole, then crank the engine over with the starter and allow 5 to 6 compression strokes on each cylinder. Ideally you should hope to see 120 to 150+ in each cylinder. The Ford spec is that the lowest cylinder should be 75% of the highest. I think that's way to loose. It would allow 140 in the highest cylinder (for example) and 101 in the lowest cylinder. Personally I would want to see all cylinders within 10 to 15 psi.
  6. Repeat the test at least two times (preferably three times) on each cylinder in order to get an accurate average on each.
  7. If you get a low reading on a cylinder, try adding about a tablespoon of oil to each cylinder. If the readings go up significantly the second time around, you know the rings are weak/bad. If the compression is low and doesn't go up significantly in round 2, then compression is leaking past the valves or the cam lobes are worn significantly. A lower compression reading in one or two adjacent cylinders can be sign of a blown head gasket.
  8. Once your have tested each cylinder, reinstall the plugs and wires in the proper order. TIP: place a spark plug in the 6" length of rubber hose (or old spark plug boot) then install the plug. This gives you more to hold onto and helps get the plug in those hard to reach spots and makes it easier to get the plug started in the threads.

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