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Check­ing engine cyl­in­der com­pres­sion

If your vehicle has been run­ning roughly or los­ing power, there may be a lack of pres­sure in one or more cyl­in­ders. To determ­ine wheth­er pres­sure is escap­ing from the engine, you need to check the com­pres­sion in the cyl­in­ders with a com­pres­sion gauge, which meas­ures the amount of pres­sure that the pis­ton exerts on the fuel/​air mix­ture before the spark plug fires the mix­ture. These gauges don’t cost much, and they’re easy to use. Some gauges screw into the spark plug open­ing, and oth­ers have to be held in place.

If there isn’t enough pres­sure, it’s escap­ing through one of the valve open­ings (because the valve is improp­erly adjus­ted or worn), down past the rings on the pis­ton or through a blown head gas­ket.

Here’s how to use a com­pres­sion gauge:

  1. Have someone sit in the driver’s seat with the engine off, the gear­shift in Park or Neut­ral, and the park­ing brake on.

  2. The next step depends on the type of dis­trib­ut­or you have:

    • On vehicles with dis­trib­ut­ors: Pull the big wire that leads to the coil from the cen­ter of the dis­trib­ut­or cap, and lean the met­al con­nect­or against an unpainted met­al sur­face as far away from the spark plugs as pos­sible.

    • On vehicles with dis­trib­ut­or­less igni­tions: Dis­con­nect the elec­tric­al con­nect­or at the igni­tion con­trol mod­ule. If you’re not sure what to dis­con­nect, ask a mech­an­ic.

  3. Dis­able the fuel injec­tion sys­tem so that gas­ol­ine mist won’t spray out of the spark plug holes and pos­sibly ignite.

    Remove the fuse labeled “Fuel Pump”; then start the car and let it run until it stalls from lack of gas­ol­ine.

  4. Label and remove the boots that con­nect each spark plug wire and each spark plug.

    If you get the plug wires mixed up, you can really screw up your engine.

  5. Remove all the spark plugs and lay them down in a clean place.

    Keep the labeled plugs in order to insure that you return each one to its ori­gin­al cyl­in­der when the time comes.

  6. Con­nect the starter switch to the bat­tery.

    If you have a remote starter switch, con­nect one clip to the battery’s pos­it­ive or “plus” ter­min­al, and the oth­er to small ter­min­al of the starter solen­oid.

  7. Insert the com­pres­sion gauge

    It should fit into the hole in the engine where the first spark plug screwed into the cyl­in­der.

    Checking compression.

    Check­ing com­pres­sion.
  8. If you don’t have a remote starter switch, have a friend turn on the igni­tion until the engine cranks over about six times. Oth­er­wise, press the but­ton of the remote starter switch.

    Be sure to keep the gauge plug firmly inser­ted while the engine is crank­ing. (The car won’t run because the engine has been dis­abled.)

  9. Look at the gauge and write down the read­ing, which will be in psi(pounds per square inch)., and then reset the gauge.

  10. Repeat these steps for each of the oth­er cyl­in­ders.

    Don’t for­get to reset the gauge and crank the engine each time.

  11. After you’ve tested each cyl­in­der, look at the read­ings.

    The highest and low­est shouldn’t vary by more than 15 per­cent. If one or more of the cyl­in­ders reads well below the rest, use a trig­ger-type oil can to send a good squirt of motor oil down the spark plug open­ing, and retest the com­pres­sion of that cyl­in­der with the gauge. If the read­ing is the same, the valves either are worn (and let­ting pres­sure escape) or are out of adjust­ment. If the read­ing rises dra­mat­ic­ally after you insert the oil, you prob­ably need new rings on the pis­ton in that cyl­in­der. If the pres­sure recor­ded by the gauges is less than 100 psi, the cyl­in­der def­in­itely isn’t mech­an­ic­ally sound.

  12. Replace each spark plug in the cyl­in­der it came from.

    Make sure that the igni­tion is off before you recon­nect the spark plug wires, and be sure to put the cor­rect spark plug wire boot back on each plug. Screw the plugs in by hand to avoid dam­aging the threads in the alu­min­um valve cov­er.

If the “Check Engine” warn­ing light comes on after you per­form a com­pres­sion test and doesn’t dis­ap­pear in a couple of days, have it reset at the deal­er­ship.

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