Replacing Worn Spark Plugs: Symptoms, Costs and Tips

replacing worn spark plugs

Do you ever think about your car’s spark plugs? For most drivers, spark plug maintenance falls way down the to-do list. But failing to replace worn spark plugs can cost you money at the gas pump and the repair shop.

Ignoring symptoms of bad spark plugs leads to reduced fuel economy, costly engine damage, and immediate risks like stalling on the road. Replacing old spark plugs before they fail saves you money and headaches.

This article will teach you to recognize 12 common signs that your spark plugs need replacement. We’ll also discuss average spark plug replacement cost and tips to extend the life of new spark plugs.

1. Difficulty Starting Your Car

Spark plugs provide the vital spark that ignites the air-fuel mixture in your engine cylinders. When spark plugs wear out over tens of thousands of miles, they can begin to misfire or fail to spark properly.

Misfiring or weak spark plugs mean your engine won’t start easily. You may need to crank the starter longer before the engine fires up. In severe cases with very worn spark plugs, the engine may not start at all.

Difficulty starting your car

Difficulty starting your car, especially on colder mornings, is a classic symptom of bad spark plugs. Replacing them can restore easy starting.

2. Reduced Gas Mileage

Your car’s spark plugs must fire properly to ensure full combustion of fuel in the engine. When the spark is weak, fuel does not ignite completely. This unburned fuel means wasted gas and reduced fuel economy.

If you notice your gas mileage decreasing over time, faulty or fouled spark plugs could be the problem. By restoring proper ignition, new spark plugs can improve your MPG and save money at the pump.

3. Engine Misfires

Does your engine ever stumble or misfire for just a moment? This sporadic misfiring is a tell-tale symptom of a spark plug problem.

Worn or damaged spark plugs may fail to provide a spark consistently in one or more cylinders. When this happens, you’ll feel a quick shudder or hear the engine miss a beat.

Engine misfires mean the air-fuel mixture is not burning properly. Driving with frequent misfires can cause catalytic converter and engine damage. Replacing faulty spark plugs immediately can help avoid costly repairs.

4. Hard Acceleration Issues

Do you floor the accelerator pedal only to be rewarded with sluggish acceleration? Does your vehicle lack the pep and power it had years ago?

When spark plugs are fouled or worn out, your engine may feel underpowered and struggle under acceleration. The weak spark limits the engine’s ability to burn fuel quickly and build power.

Installing a new set of spark plugs can often restore lost acceleration power. With clean and properly gapped plugs, your engine can burn fuel efficiently even under heavy load.

5. Power Loss When Climbing Hills

Worn spark plugs can’t ignite the air-fuel mixture as efficiently. This lack of power may be especially noticeable when trying to accelerate up hills or passes. The engine can bog down without the strong spark needed for maximum power production.

6. Decreased Engine Response

Over time, worn spark plugs will cause your engine to feel increasingly sluggish and unresponsive. Acceleration lag is common as combustion weakens. Even at steady speeds, the engine may seem to drag compared to the quick response you expect.

7. Rough Idling

Pay attention to your engine’s idle when stopped at a light. Does it run unevenly or rough? Rough idling is often caused by misfiring cylinders due to bad spark plugs.

Since spark plugs fire individually in each cylinder, a single worn plug can cause noticeable single-cylinder misfires at idle. As multiple plugs wear out, the idle gets progressively rougher.

Replacing the spark plugs can smooth out that rough idle and restore even combustion.

8. Check Engine Light

Modern cars have engine sensors that can detect misfires and ignition problems. If your check engine light illuminates, faulty spark plugs could trigger the error code.

The engine computer may log cylinder misfires, fuel trim issues, or sensor readings that indicate weak sparks. Scan for codes with an OBD2 scanner to pinpoint spark related faults.

In some cases, simply replacing worn spark plugs can turn off the check engine light after resetting the ECU.

9. Fouled or Damaged Spark Plug

When you remove your spark plugs, visually inspect their condition for signs of trouble. Look for:

  • Oil fouling – Black oily residue means oil is leaking past worn piston rings into the chamber. This coats the plug over time.
  • Carbon fouling – Dry black carbon deposits that hinder spark. Caused by rich fuel mixtures and short trips.
  • Wear – Rounded center electrode and excessive erosion from sparks. Normal with high mileage but reduces performance.
  • Corrosion – Rust, oxidation, and mineral deposits on plug terminals from moisture and blowby gases.
  • Cracking – Cracks in the ceramic insulator around the center electrode. Allows voltage leakage.

Any excessive fouling, corrosion, cracking, or wearing of the spark plug’s terminals and insulator indicates replacement is needed.

10. Strange Smells

Very rich and pungent exhaust smells can indicate unburned fuel is escaping from your tailpipe. While many issues can cause smells, lack of spark due to bad plugs is one possibility.

Without adequate ignition, fuel cannot combust fully in the cylinders. This unburned excess fuel makes it into the exhaust as unspent hydrocarbons, creating unpleasant odors.

Sniff test your exhaust after noticing any symptoms of bad spark plugs. Strong gasoline odors point to ignition issues like worn plugs, coils, or wires.

11. Failed Emissions Test

In areas with smog testing programs, worn out spark plugs are a common reason for failing your emissions test. Just like strange smells, this points to unburned fuel making it through the engine.

Without the strong consistent spark of new plugs, your engine misfires and allows hydrocarbons to slip into the exhaust and atmosphere. Emissions inspectors measure these high HC levels during a smog test.

One new set of quality spark plugs can get you passing again by restoring complete fuel combustion. Drive gently until your next test after replacing them.

12. It’s Time For Replacement By Mileage

Refer to your owner’s manual for the replacement interval for your vehicle’s spark plugs. In most modern engines, plugs are due for replacement every 30,000-60,000 miles. However, some vehicles may have different intervals. For instance, many Toyota models have a 120,000-mile replacement interval.

Higher performance plugs using precious metals like iridium and platinum often last up to 100k miles. But don’t exceed the interval even if other symptoms are not apparent yet.

Worn plugs running past their prime can increase long term engine wear and fuel consumption. Stick to scheduled replacement as basic maintenance.

Is It Safe To Drive With Bad Spark Plugs?

Driving with worn spark plugs is not recommended and can have consequences:

  • Increased risk of stall-outs, especially at idle and low speeds
  • Potential for engine misfires that overwork and damage the catalytic converter
  • Poor drivability from reduced power and acceleration
  • Decreased fuel economy and wasted gas
  • Further damage to ignition system components like the coils

While you can likely drive for short distances to the repair shop with bad plugs, it is not worth the risks of breakdowns and further engine damage. Get worn spark plugs replaced as soon as possible.

Know The Average Spark Plug Replacement Cost

know the average spark plug replacement cost

Okay, you’re convinced it’s time to replace your spark plugs. But how much does it cost to replace spark plugs? Here are the typical parts and labor costs:

  • Spark plugs themselves range from $2 – $100 per plug for quality aftermarket units, with iridium and platinum plugs at the high end. Check your manual for the number of plugs required.
  • Labor costs around $40 – $350 for most four and six cylinder engines. Expect closer to $400-500 for V6 and V8s. European and luxury vehicles often cost more.
  • Additional parts like plug wires, ignition coils, or boot kits can add to the cost of spark plug replacement.

So in total, expect to pay $100-700 for a full spark plug replacement job depending on parts and labor needed. Replacing them yourself can save big on labor costs.

What Causes Spark Plugs to Wear Out?

There are a few common causes of spark plugs wearing out over time and miles:

  • Normal wear – The spark plug electrodes erode over repeated sparking. The ceramic insulator can also develop micro-fractures. This is unavoidable even in perfect operating conditions.
  • Oil fouling – Oil leaking past worn piston rings and valves can coat the plugs over time, disrupting the spark.
  • Fuel fouling – Rich fuel mixtures, short trips, and excessive idling allow deposits to build up on plugs.
  • Overheating – Too hot of a spark plug heat range, lean fuel mixtures, and detonation can overheat the plug.
  • Corrosion – Moisture and combustion byproducts create corrosion on the plug terminals and housing.

How to Make Your New Spark Plugs Last?

Here are a few tips to extend the lifespan of new spark plugs:

  • Use the correct heat range specified for your vehicle to prevent premature fouling or damage.
  • Gap your new plugs properly before installation – usually 0.028 to 0.060 inches.
  • Avoid short trips and excessive idling to prevent fouling deposits on plugs.
  • Address oil or coolant leaks that can foul plugs over time.
  • Consider higher quality platinum or iridium plugs for increased durability.

Following scheduled replacement intervals and addressing any underlying issues will keep your new spark plugs firing properly for tens of thousands of miles. They are a small investment that pays off in engine performance and efficiency.

Don’t Ignore the Signs of Failing Spark Plugs

There you have the 12 most common symptoms of spark plugs needing replacement, plus an overview of typical costs. Pay attention for any of these warning signs:

  • Hard starting
  • Decreased gas mileage
  • Engine misfires
  • Sluggish acceleration
  • Rough idle
  • Check engine light
  • Fouled/worn plugs at inspection
  • Fuel smells in exhaust
  • Failed emissions test
  • Due for replacement by mileage
  • Power Loss When Climbing Hills
  • Decreased Engine Response

Addressing worn spark plugs promptly reduces major repair bills down the road. While costs vary, a full replacement of all your vehicle’s plugs averages $200-700. Considering how critical spark plugs are to engine operation and efficiency, this maintenance cost is money well spent every 30-60k miles.

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