Spark plugs play a crucial role in your engine by igniting the fuel/air mixture to power the combustion process. But can you still drive with bad or failing spark plugs, and if so, for how long? This article will cover the risks of driving on bad plugs, how to spot telltale symptoms, and when to absolutely replace them.
Table of Contents
- Will Your Car Run With Bad Spark Plugs?
- Key Functions of Spark Plugs
- Key Signs Your Spark Plugs are Failing
- Risks of Continuing to Drive on Bad Spark Plugs
- Can I Just Replace One Spark Plug?
- How Long Can Spark Plugs Last?
- Warning Signs It’s Time For Replacement
- Is it Safe to Drive With Bad Spark Plugs?
- Steps to Take When Spark Plugs Go Bad
- Can Worn Spark Plugs be Cleaned or Gapped?
- Key Takeaways
Will Your Car Run With Bad Spark Plugs?
Generally speaking, yes a car can still run and be driven to some degree with worn out or fouled spark plugs. However, it can seriously impact performance and fuel efficiency and put additional strain on the engine.
Since each cylinder requires a spark for combustion, one or two bad plugs out of four, six or eight may only cause a subtle loss of power. But as more plugs degrade, the problems compound.
Let’s look at how issues like electrode wear, fouling, cracking, oil buildup and gap widening can affect spark plug function when they go bad.
Key Functions of Spark Plugs
To understand how bad spark plugs impact your engine, it helps to first look at what tasks they perform when in good working condition:
- Generate high voltage spark – Up to 30,000 volts jumps the electrode gap to ignite the compressed fuel mixture.
- Time the spark precisely – The plugs fire at exactly the right moment as the piston reaches the top of the compression stroke.
- Start combustion – The resulting explosion pushes the piston down with force to produce power.
- Repeat ignition cycle – Spark plugs continue igniting every cycle as the engine operates, up to several thousand times per minute.
When brand new, spark plugs perform these duties with precision timing and consistency. But as they wear over miles of use, their ability to spark properly degrades leading to a host of potential issues.
Key Signs Your Spark Plugs are Failing
Watch for these common indicators that your spark plugs need replacement:
- Hard starting – Taking longer cranking to turn over and start.
- Misfiring – Noticeable choppy acceleration and loss of power.
- Poor fuel economy – Reduced mpg as unburned fuel passes through.
- Rough idle – Engine shakes roughly at stoplights rather than idling smooth.
- Check engine light – P0300 random misfire trouble code.
- Fouling – Carbon/oil buildup on the electrodes insulating the spark.
- Visible damage – Cracked insulator, burned electrodes, excessive gap wear.
The sooner these symptoms arise, the faster you should address replacing the failing plugs.
Risks of Continuing to Drive on Bad Spark Plugs
Driving for long periods using degraded spark plugs can result in escalating issues:
- Worse performance – Acceleration, power and mpg will continue to suffer.
- Backfiring – Combustion happens unevenly in the exhaust rather than cylinders.
- Misfire damage – Unburned fuel and improper combustion heats and strains the catalytic converter.
- Preignition – Ignition happens too early causing engine knocking.
- Oil fouling – Built up oil on plugs causes insulation and misfires.
- Increased emissions – Unburned hydrocarbons pass through the exhaust.
The problems compound the longer you wait to replace them. Bad plugs can even cause permanent engine damage if left unchanged.
Can I Just Replace One Spark Plug?
When you first notice symptoms of a spark plug failure, your instinct may be to just replace the one visibly damaged plug. However, best practice is to replace the entire set if even one is worn out. Here are some reasons why:
- Even wear – Plugs age at similar rates so others are likely near failure too.
- Balance – A new plug with older ones can cause uneven combustion.
- Cost – Labor is the bulk of the cost, so might as well do the full set.
- Life spans – They are designed to be replaced at once every 30-60k miles.
Staggering the replacement over months just leads to the remaining old plugs failing sooner. For optimal engine performance and cost savings, a complete spark plug replacement is recommended.
How Long Can Spark Plugs Last?
With normal driving conditions, the standard maintenance guideline is to replace spark plugs roughly every 30,000 to 60,000 miles. But that can vary based on:
- Make/Model – Check the maintenance schedule in your owner’s manual.
- Driving Habits – Short trips and stop/go wear plugs out faster.
- Engine Type – Higher performance engines stress plugs more.
- Gasoline Quality – Lower quality or dirty fuel can accelerate wear.
- Oil Changes – Fresh oil helps reduce carbon fouling of plugs.
So while 30k-60k miles is typical, keep an eye out for any symptoms of spark plug degradation arising sooner than expected. Or periodically have them inspected by a professional technician.
Warning Signs It’s Time For Replacement
Watch for these more serious indicators your spark plugs absolutely need to be changed:
- Constant misfires – Repeated choppy acceleration.
- Extreme power loss – Sluggish acceleration at all speeds.
- Knocking/pinging – Rhythmic metallic knocking sound from engine.
- Failed emissions test – Due to increased hydrocarbon emissions.
- Visible damage – Severely cracked, fouled or corroded plugs.
- Illuminated check engine light – Specific cylinder misfire codes.
Don’t try to just drive through these more severe symptoms or engine damage can occur and the problem will just compound. Schedule spark plug replacement immediately if you notice any of these signs.
Is it Safe to Drive With Bad Spark Plugs?
While your car will generally continue running with some worn spark plugs, it is not considered safe to just keep driving as normal in the long run. Here are some key risks:
- Stalling – Bad plugs can cut out power entirely at inopportune times.
- Poor accelerator response – Worsening hesitation and sluggishness creates safety issues.
- Knocking/preignition – Abnormal combustion can damage the engine.
- Catalytic converter failure – Prolonged misfires overheat and ruin this expensive emissions component.
- Stranded – Eventual stalling leaves you stranded if all plugs fail at once.
So you can likely drive for awhile on some worn plugs, but the risks compound over time. Get them replaced promptly to ensure reliable and safe operation of your vehicle.
Steps to Take When Spark Plugs Go Bad
Follow this action plan when your spark plugs show signs of failure:
- Identify failing cylinders – Use an OBD2 scanner to pull any cylinder specific misfire codes.
- Inspect spark plugs – Remove them one by one to check for damage. Look for oil fouling, excessive gap wear, cracked insulator etc.
- Replace in sets – Install an entire new set of OEM spark plugs, even if only one is clearly worn out.
- Address other issues – If oil fouling caused the failure, repair any valve cover or piston ring leaks.
- Clear trouble codes – Use an OBD2 scanner to clear any check engine lights/codes after replacing the spark plugs.
- Follow up – Retest drivability in a week or so to confirm misfires are resolved after the new plugs have some use.
Taking a strategic approach by fully diagnosing issues and addressing the root cause, not just the symptom, will minimize the chances of repeat spark plug failure down the road.
Can Worn Spark Plugs be Cleaned or Gapped?
For spark plugs showing only minor fouling or gap widening, some DIY cleaning and adjustment can occasionally extend their life slightly before needing full replacement. But this is generally not recommended, as damaged plugs will often fail again soon after.
It’s far more reliable to just replace them, especially given how inexpensive spark plugs are relative to potential engine damage caused by bad ones. But here are a few possibilities:
- Cleaning – Use a wire brush to gently remove minor exterior carbon deposits. Don’t blast with compressed air which can damage internal components.
- Gapping – Carefully bend the ground electrode to re-gap worn plugs back to the proper spacing. Don’t over-gap.
- Readjust – Re-tighten plugs to the correct torque spec if loose.
Again, any visible damage or wear will likely warrant complete replacement. DIY cleaning and gapping should only be considered a very temporary fix in a pinch.
- Bad spark plugs can run but will cause poor performance.
- Watch for misfires, poor mpg, rough idle and hard starting.
- Driving long-term risksconverter failure and engine damage.
- For safety and reliability, replace worn plugs immediately.
- Always change as full sets rather than individual plugs.
So show diligence to replace worn spark plugs at the first sign of trouble. While an engine may continue to run on bad plugs for awhile, allowing their degradation to progress puts the health of your engine at risk. Addressing plug issues promptly improves performance, efficiency and safety.
Driving for short periods on slightly worn spark plugs is possible but ill-advised. Performance degradation, misfires, and fuel waste will occur and worsen over time. For optimal engine operation and health, replace bad spark plugs promptly. Watch for misfires, hard starting, rough idle and poor mpg as key indicators. Replace in full sets for best results.