BEST GAS MILEAGE enhancers - spark plug reading

NORMAL - This is what a spark plug should look like, although the center terminal is worn. The insulator nose can be any color from a light tan to a light gray. The spark plug may have slight deposits with no negative effect on gas mileage or engine performance.

The engine is running normally, the air / fuel mixture is OK and the correct heat range spark plug is being used.

WHAT TO DO - Replace high mileage plugs with the same heat range and gap. The photo shows normal wear in the center electrode. A new plug would have square edges that helps the plug fire better and get better gas mileage.

OIL FOULING - The metal firing terminals of the plug are covered in fresh or cooked-on oil residue. The engine is consuming oil. There are three causes:

1. The engine is high mileage and oil is leaking past the piston rings or valve guides / seals into the combustion chamber.

The engine may still run OK, but gas mileage and emissions will suffer as the problem gets worse. Oil fouling can cause detonation, which can lead to severe engine damage. Trying a plug with a hotter heat range may work for a little while, but we don't recommend that for the average motorist.

WHAT TO DO - If the valve guides / seals are leaking, the cylinder head can be repaired. If oil is getting past the rings, the engine needs a rebuild. Better gas mileage is the least of the concerns here.

2. The transmission vacuum modulator diaphragm has ruptured, the PCV valve is clogged, or the fuel pump diaphragm is ruptured. Check these first, before you assume the worst about your engine. These problems could allow the engine to suck oil into the combustion chamber, causing the same type of oil fouling. Gas mileage and emissions will suffer.

WHAT TO DO - Replace the defective component and the spark plugs. Monitor plug condition.

3. The engine was recently rebuilt and the oil fouling took place while the engine was still in the break in process.

WHAT TO DO - The plugs should be replaced and the new plugs monitored for further oil consumption. Improperly rebuilt engines or incorrectly conducted break-ins can result in rings that never seated properly.

 

DETONATION / PINGING / PREIGNITION / SPARK PLUG KNOCK - The white ceramic insulator on the plug is often broken, or the plug has melted terminals. The dramatic increases in pressure and temperature inside the engine can also easily severe damage such as melted pistons, cracked cylinder heads, etc.

CAUSE - The air fuel mixture is being ignited before the spark ignites it, and the two flame fronts collide, producing an audible noise, like a clattering. There are a few causes:

1. Engine is running too hot.

WHAT TO DO - Check the cooling system and repair as needed.

2. Excessive internal carbon buildup. Stop and go driving, an overcooled engine, improper air / fuel mixture, or bad fuel can cause this. Unburned carbon stuck to any part of the combustion chamber can heat up during the compression stroke to the point where it ignites the air fuel mixture.

WHAT TO DO - Decarbonize the engine. If you don't know what this is, consult your mechanic.

3. Ignition timing is too far advanced. This will cause the cylinders to run too hot, combusting the air fuel mixture before the spark plug can light it.

WHAT TO DO - Adjust the ignition timing to factory specification as defined by your owner's manual.

4. Fuel quality is poor or the octane rating is lower than what the vehicle owner's manual calls for.

WHAT TO DO - Use higher quality fuel with the correct octane rating. If you need an octane rating higher than what is recommended, see cause #1.

5. Spark plug heat range is incorrect. The plug will run too hot, and may melt its terminals in addition to causing engine damaging detonation.

WHAT TO DO - Replace plugs with correct heat range.

6. Incorrect compression ratio.

WHAT TO DO - This is an engine design problem, and if the engine was recently rebuilt, the wrong parts may have been used.

 

CARBON BUILDUP ON THE PLUG - Visible dry, soft, black sooty deposits. Don't confuse this with the carbon buildup that sticks to valves and can cause detonation. The engine is not burning fuel properly or is using too much fuel. In either case, gas mileage is reduced.

CAUSE - There are quite a few causes for this problem. Plugs with the incorrect heat range, excessive stop and go driving, rich fuel mixture, sticking choke, clogged air filter, or an ignition system (cap & rotor, ignition timing) that needs servicing.

WHAT TO DO - Look at each one of the above causes and replace / service your vehicle as necessary.

ASH BUILDUP ON THE PLUG - Light brown or white deposits that may look like rust.

CAUSE - Usually due to fuel additives. Excessive amounts can cause a misfire, and buildup in a short period of time may indicate worn valve guides.

WHAT TO DO - Avoid fuel additives. Periodically check your spark plugs after you replace them to make sure the deposits do not come back. If they do, you may have to repair or replace your valve guides, that is cylinder head work.