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Diagnosing Problems That May Be Caused by a Weak Spark

Dan Ferrell writes about DIY car maintenance and repair. He has certifications in Automation and Control Technology and Technical Writing.

A weak spark, or no-spark, problem may originate within one or more common ignition system components:

  • Worn or damaged spark plug wire insulation. At high temperature, this problem can produce bucking or misfires.
  • Dirty or worn spark plug. This can cause the car to be hard to start or to misfire.
  • Worn or damaged ignition coil. A faulty primary circuit (from the coil to the power source) can cause the engine to stall under load. A faulty secondary circuit from coil to spark plug can cause the engine to misfire under load.
  • Cracked spark plug porcelain can cause a misfire or bucking under load.

Other symptoms that may point to a weak spark plug include:

  • rough idle
  • poor fuel economy
  • poor acceleration
  • exhaust pop sounds

However, these symptoms are not necessarily caused by a weak spark plug; you may be dealing instead with a fuel system issue or poor compression in one or more cylinders. So you need to determine, before you do any work, whether you have a weak spark or no spark.

The next sections will help you determine a weak spark and potential problems affecting ignition spark.

It is recommended to have on hand the vehicle repair manual for your particular vehicle make and model. If you don't have this manual yet, you can buy a relatively inexpensive copy through Amazon. Haynes manuals include step-by-step procedures, images, and electrical diagrams for diagnostics, maintenance jobs, troubleshooting tasks, and parts replacement projects. So you can recoup your investment in a short time.

Index

1. How to Do a Quick Ignition System Check

2. How to Check for Spark, Voltage Leaks and Resistance

3. How to Interpret Diagnostic Trouble Codes

4. How to Read Spark Plugs

5. How to Deal With a Weak Spark

6. Resources

diagnosing-weak-spark-problems

1. How to Do a Quick Ignition System Check

If you have noticed a mild problem with misfires or a no-start condition, but the engine still cranks, this quick test (for vehicles with a crankshaft position sensor) can lead you to the source of the problem:

When cranking or starting the engine:

If the tachometer doesn't work or seems unstable (engine misses), check the ignition system primary circuit:

  • coil primary winding
  • crankshaft position (CKP) sensor
  • pickup coil
  • Ignition control module (ICM)

If the tachometer seems stable, check the ignition system secondary circuit:

  • coil secondary winding
  • spark plug wires and coil wire
  • distributor cap and rotor
  • ground between distributor or ignition module and engine
  • spark plugs

Consult your vehicle repair manual to verify the configuration of your particular ignition system and components. Also, refer to the Resources section at the bottom of this post for more help.

diagnosing-weak-spark-problems

2. How to Check for Spark, Voltage Leaks and Resistance

Diagnosing spark, leaks and resistance are common tests you can do when dealing with ignition system problems. These tests are closely linked to ignition system basic maintenance procedures.

2.1 Checking Ignition Spark

If possible, use an adjustable spark tester for this test.

  • Unplug the spark plug wire from the spark plug you need to test. On coil-on-plug engines you may need an inline tester for this.
  • Set your spark tester for 30,000 or 40,000 volts.
  • Connect the spark tester to the end of the spark plug wire.
  • Ground the spark tester to a metal bracket, engine bolt or battery ground (the terminal marked with the "-" sign).
  • Crank or start the engine.

The spark on the tester should be a bluish-white color; an orange or red color indicates a weak spark.

If you have a weak spark, you may be dealing with a:

  • bad spark plug wire
  • bad distributor cap or rotor
  • faulty ignition wire or coil
  • ignition module or ignition driver in the powertrain control module (PCM)
  • bad coil-on-plug coil

depending on your ignition system configuration. Consult your vehicle repair manual.

The following video shows you how to use a spark tester.

2.2 Diagnosing Voltage Leaks and Resistance

Spark plug wires wear out after years of operating under extreme temperatures (hot, cold), engine vibration, and contamination from oil, grease and coolant.

The wire can leak voltage or increase resistance (on carbon-core wires) and interfere with ignition coil voltage.

Check your spark plug wires for visual damage like cuts, chafing, and breaks that might leak voltage to ground.

Quick test for leaking ignition voltage:

  • In a spray bottle, mix a bit of salt or liquid soap and water.
  • Start the engine.
  • Spray a mist of water on the distributor cap, spark plug wires and spark plugs area.
  • A voltage leak in any of theses areas will cause an electric arc.

Spark plug wire resistance check:

The typical lifespan of a spark plug wire is around 50,000 miles. That's when internal resistance (carbon-core wires) begins to increase.

You can use an Ohmmeter to check your spark plug wires' resistance.

  • Set your multimeter to the Ohms function and select a high range
  • Connect the multimeter probes to each end of a spark plug wire

Most plug wires will have a resistance of 12,000 Ohms per foot. Consult your vehicle repair manual.

If you need help checking your spark plug wires, head over to the "Resources" section at the bottom of this post.

2.3 Alternative Test for Plug Wire Leaks

  • Connect a test light to a metal bracket, engine bolt, or battery ground (the one marked with the "-" sign).
  • Start or crank the engine.
  • Move the test light probe along the length of each spark plug wire.

If the test light blinks at any point, that wire is leaking voltage.

One of the first checks you want to do early in your diagnostic procedure is to download potential trouble codes (DTCs) from your computer memory.

diagnosing-weak-spark-problems

3. How to Interpret Diagnostic Trouble Codes

One of the first checks you want to do early in your diagnostic procedure is to download potential trouble codes (DTCs) from your computer memory. You want to do this whether the check engine light (CEL) is on or not. Often, a pending code can give you a clue to the problem. If you don't have a scanner, your local auto parts store can download trouble codes for you.

P0300 misfire code series:

A specific code like P0301, which signals a misfire on cylinder one, may point to a:

  • bad spark plug wire
  • fouled spark plug
  • faulty ignition coil (coil-on-plug [COP] systems)

If necessary, check the spark plug chart in your vehicle repair manual to read for spark plug potential problems.

If there are no other codes present besides a P030(0+) code:

You may be dealing with an ignition issue or fuel system issue. Check for:

  • faulty ignition coil
  • dirty fuel injector

However, keep in mind that a weak coil, injector, or dirty injector may not trigger a CEL.

Other DTCs Present:

If you find a misfire code along with a fuel injector or ignition coil related code, check the fuel injector first.

diagnosing-weak-spark-problems

4. How To Read Spark Plugs

Removing and inspecting spark plugs can give important clues about what type of problem, if any, is affecting your engine and what to do about it.

  • Blistered: indicates your engine is running too hot
  • Worn electrodes: replace with new spark plugs
  • Electrode gap too wide: adjust gap
  • Fouled plugs: indicate worn valve guides, piston rings, excessive fuel
  • Cracked plugs: damaged, must be replaced

Check the spark plug chart included in your vehicle repair manual.

A spark plug can give you clues about potential engine problems.

A spark plug can give you clues about potential engine problems.

diagnosing-weak-spark-problems

5. How to Deal With a Weak Spark

Dealing with a weak spark in one cylinder is usually not too difficult to diagnose. However, a random weak spark that results in random misfires is another story.

Either condition may cause damage to components and result in expensive repairs if not fixed on time.

Primary Circuit Problems

An electrical "open" or low resistance in a coil primary circuit can lead to:

  • high temperature ignition module operation and damage
  • weak spark
  • no spark

An electrical short in the primary circuit can lead to:

  • excessive current through coil
  • damage to ignition module
  • weak spark
  • hard-starting condition
  • hesitation
  • misfires during acceleration

Secondary Circuit Problems

An electrical "open" or high resistance in the secondary circuit can lead to:

  • damage to the ignition control module (ICM)
  • weak spark
  • no spark

An electrical short or low resistance in the secondary circuit can lead to:

  • weak spark

Weak sparks allow unburned or partially burned fuel to flow into the exhaust system. Eventually, this will damage other components like the oxygen sensor, catalytic converter, and EGR system (through excessive carbon buildup).

Resources For Diagnosing The Ignition System

This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.

© 2021 Dan Ferrell