Changing Spark Plugs? Do This 2-Minute Borescope Check While They're Out
Quick answer: When the spark plugs come out, every cylinder is open for inspection. Read each old plug for clues, then slide a borescope through the plug port to check the piston crown, cylinder walls and valve edges. It takes about two minutes per cylinder. You'll catch oil burning, coolant leaks, scoring and debris before the new plugs seal it all back up. Do it on a cold engine, which is when plugs should come out anyway.
Key Facts
- Spark plugs are normally changed on a cold engine, which is also the only safe time to insert a borescope.
- A 6.2mm ROVE probe fits typical car spark plug ports, including deep coil-on-plug wells.
- Plug deposits tell you what's happening; the borescope shows you where and how bad.
- Blow debris out of the plug wells before removing the plugs, so nothing falls into the cylinders.
A plug change is one of the few jobs where the engine opens itself up for free. Most people swap the plugs and button it up. Two extra minutes per cylinder turns routine maintenance into an engine health check.
Step 1: Read the old plugs
Lay the plugs out in cylinder order. The plug that looks different from the others is the cylinder to look at first.
| Old plug looks like | What it often means | What to look for with the borescope |
|---|---|---|
| Light tan or gray deposits | Normal combustion | A baseline: note the carbon level on each piston |
| Dry black soot | Rich mixture or weak spark | Heavy carbon on the crown and valve edges |
| Wet, oily deposits | Oil getting into the cylinder | Oil sheen on the walls, wet carbon on the crown |
| White or blistered insulator | Running hot or lean | Damage at the piston edge, unusual discoloration |
| One plug much cleaner than the rest | Possible coolant entering that cylinder | A steam-cleaned piston compared with its neighbors |
Step 2: The 2-minute borescope look, cylinder by cylinder
- Confirm the engine is cold and can't start. Keep the ignition off and the key away from the car.
- Insert the probe straight down the plug port until the piston crown fills the screen.
- Set the LEDs low. Level 1 or 2 shows carbon texture better than full brightness on shiny metal.
- Sweep the walls. Steer around the bore and look for vertical scratches or a wet sheen.
- Angle up toward the valves if the port allows. Look for chipped edges and heavy deposits.
- Check for debris. Anything that fell in during plug removal must come out before the engine runs.
- Save a photo of each piston at the same LED level, so next time you can compare like for like.
Step 3: Install the new plugs right
Check the gap if your plugs need it, start each one by hand so it can't cross-thread, and torque to the vehicle maker's spec. A spark tester is still worth using if you suspect a weak coil. It checks the ignition side; the borescope checks the mechanical side.
Frequently asked questions
Can something fall into the cylinder when I remove a spark plug?
Yes. Grit and old debris in the plug well can drop in. Blow the wells out with compressed air before you remove the plugs, then check each cylinder with the borescope afterward.
What if I see debris in a cylinder?
Don't start the engine. Small grit can sometimes be removed with a thin vacuum hose; anything larger or stuck needs a mechanic. The borescope is for finding it, not retrieving it.
Will a borescope fit down a deep coil-on-plug well?
Usually, yes. The 6.2mm probe fits typical car plug ports, and the 1.6m probe easily reaches the bottom of a deep well.
How much carbon on a piston is normal?
A thin, even layer of carbon is normal. What matters is a cylinder that looks different from the others: heavier, wetter or much cleaner.
Do I need a spark plug tester as well?
If you suspect a weak coil or misfire, yes. A spark tester checks the ignition; the borescope checks the cylinder.
How often should I do this check?
Every time the plugs come out. Saved photos give you a history you can compare against at the next change.
Can I do this on a motorcycle or small engine?
Yes, with the engine cold. See our engine borescope guide for motorcycles, boats and small engines.
Which ROVE fits this?
For a straight look at each piston crown, the ROVE RS2 does the job. If you want the walls and valve edges too, the ROVE RS4 sweeps the whole bore from the plug port with its 360° joystick. Both save each cylinder to the included 32GB microSD card.
About ROVE
ROVE is a U.S. brand headquartered in Orlando, Florida, and the team behind ROVE dash cams. Its borescope line includes the RS2 (two-way articulation) and the RS4 (four-way, 360° articulation): standalone 1080P inspection cameras with a 6.2mm IP67 probe, 1.6m reach and a built-in 4.3-inch screen, backed by a one-year warranty.
