How to check the engine cylinder for a misfire

Written by chris stevenson
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How to check the engine cylinder for a misfire
Checking for a cylinder misfire requires a process of elimination. (Vintage Car Engine image by itsallgood from Fotolia.com)

Engines need to fire on all cylinders for smooth operation and optimum performance. A number of reasons account for a cylinder miss, or multiple cylinder misses. The cylinders require a measured amount of vaporised fuel, an electrical source to fire the fuel and the needed compression to supply the force to drive the pistons. Each cylinder has its requirements, and they all must work in combination to make the engine run smoothly and deliver the best fuel economy. Finding a cylinder misfire needs a little investigation, patience and a few basic tools.

Skill level:
Moderate

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Things you need

  • Owner's repair manual
  • Screwdrivers
  • Insulated plug wire pliers
  • Pen and paper
  • Tachometer
  • Ohm meter
  • Assistant
  • Test light
  • Compression gauge

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Instructions

  1. 1

    Place the vehicle in park or neutral with the emergency brake set. Raise the hood. Start the vehicle and listen for an engine that stumbles or chugs at idle. Step on the accelerator pedal and listen for pops or snaps. Look at the condition of the exhaust. Blue-white smoke will indicate excessive oil-burning, which will foul spark plugs. This constitutes a compression problem. If the exhaust smoke appears black or sooty, it indicates an excessively rich fuel condition, which points to dead spark plugs or a problem in the fuel delivery system. An engine that misses will shudder and vibrate.

  2. 2

    Shut the engine off. Hook up the lead from a tachometer to the negative side of the ignition coil, or to the negative side of the coil pack. Hook the other lead up to a metal ground source. Start the engine. Record the engine RPM (revolutions per minute) with paper and pen. For instance, the number might appear as 600 RPM.

  3. 3

    Use a pair of spark plug wires to remove a plug wire at the boot end on the spark plug. Notice any change in the RPM on the tachometer. There will be a definite RPM drop in a good cylinder. A normal drop for an 8-cylinder engine will be about 75 RPM. For a 6-cylinder it will be around 120 RPM. The drop will be around 75 RPM for a 4-cylinder engine.

  4. 4

    Replace the plug wire with the plug pliers and check the next cylinder. Record the number. Check each cylinder and record the tachometer number. The cylinder that does not drop in RPM will be the dead cylinder. If the RPM drop appears fractionally smaller than the rest, it would indicate a sporadic miss.

  5. 5

    Shut the engine off. Remove the plug wire at the spark plug location with plug pliers. Place the metal shank of a long screwdriver in the connector tip of the plug wire and hold it against a metal ground surface on the engine, such as the exhaust manifold or cylinder head.

  6. 6

    Have an assistant start the engine and let it idle. Look for a blue-white electrical arc jumping from the screwdriver shank to the ground source. An audible electrical snap should accompany the spark. No spark or snap indicates a dead plug wire or cap and rotor miss. Test all plug wires in a similar fashion. No spark, or a yellow intermittent spark, indicates no or inadequate voltage.

  7. 7

    Shut the engine off. Remove one plug wire from the spark plug and distributor location. Hook up the two leads of an ohm meter to each end of the plug wire connectors. Read the ohm meter resistance. A normal reading will indicate between 5,000 and 10,000 ohms of resistance, or the manufacturer's's specification according to your owner's repair manual. A readout that that goes to infinity means the wire has a break and can not carry voltage. Such a plug wire must be replaced.

  8. 8

    Shut the engine off. Disconnect the main coil wire, so the engine will not start. Disconnect one fuel injector wire from the fuel rail, if you have fuel injection. Place the sharp probe end of a test light inside the fuel injector wire connector. Have your assistant crank the engine over. If you see the test flicker rapidly, it means the injector has voltage going to it. Check each injector in this fashion. An injector head that does not flicker while the engine cranks denotes no voltage to that cylinder fuel injector.

  9. 9

    Use a spark plug socket and wrench to remove one spark plug. Look at the electrode on the end of the spark plug. It should have a yellow or brown colour to it. If it has heavy carbon build-up, a worn middle electrode, rust, black soot, or appears wet and smells of fuel it indicates the plug has shorted and no longer functions.

  10. 10

    Remove a spark plug with a socket and wrench. Connect a compression gauge to the spark plug hole; use either the screw-on type compression gauge or the pressure type. Remove the ignition coil wire from the distributor cap. Have your assistant crank the engine at least six times while you watch the needle on the gauge peak and stop.

  11. 11

    Note the reading on the compression gauge, registered in ft-lbs (foot-pounds). The compression number should not read any lower than your manufacturer's specifications in your owner's repair manual. Generally, any cylinder that reads 14-18kg below the rest, or specifications, will mean it lacks sufficient compression. Test each cylinder with the compression gauge in the same fashion and record all the numbers. Insufficient compression points to bad valves or position rings.

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