The Volvo Parts, Accessories &
Performance Specialists Since 1963

Bosch Spark Plug FAQ

2010-05-03 - Courtesy of Boschusa.com

What is the function of a spark plug and why is it a vital engine component?

The function of the spark plug is to introduce the ignition energy into the combustion chamber and to initiate combustion of the compressed air-fuel mixture. It accomplishes this task by generating a spark between its electrodes, which then generates the required heat to ignite a smooth burn of the air/fuel mixture. The spark plug is an important factor in determining optimal performance and reliable functioning of an engine. It must permit reliable cold starting, it must guarantee that there is no misfiring during acceleration, and it must withstand the engine being operated for hours on end at maximum power. These requirements apply throughout the entire service life of the spark plug. For 100 years, Bosch Spark Plugs have been designed for optimal performance, reliable functioning, and to withstand extreme operating conditions. Today, the continuously developing spark plug is a key engine system component. It plays a major role in fuel economy, clean, efficient combustion, and the reliable operation of engines and catalytic converters.

What are the different types of spark plugs?

There is a large variety of different spark plug designs with more than 1,400 variations. Design variants and special materials such as platinum, silver, or yttrium allow Bosch spark plugs to be used in a wide variety of applications today. In all, Bosch has developed more than 20,000 different types of spark plugs over the last 100 years.

Why do we need to change spark plugs?

During operation, the spark plug is subjected to both wear and to fouling and should be replaced at regular intervals. In the course of its service life, the spark plug undergoes changes that increase the required ignition voltage. When the required voltage reaches a level that can no longer be compensated for by the voltage reserve, the result is misfiring. These changes can be caused by: Electrode wear Engine wear Abnormal operating conditions (pre-ignition, detonation, high oil consumption) Independent tests have shown that installing premium spark plugs such as Bosch’s Platinum+4 improves fuel mileage by as much as 4.8% over original equipment plugs. Platinum+4 uses race-tested technology including multiple electrode paths and more pure platinum to generate strong, consistent sparks.

How do I "read" a spark plug?

Reading the spark plugs provides valuable information on spark plug and engine operating conditions.

Normal Condition

Insulator nose grayish-white or grayish-yellow to brown. Engine is in order. Heat range of plug correct. Mixture setting and ignition timing are correct, no misfiring, cold-starting device functioning. No deposits from fuel additives containing lead or from alloying constituents in the engine oil. No overheating.


Sooted — Carbon-fouled

Insulator nose, electrodes and spark plug shell covered with velvet-like dull black soot deposits.

Cause: Incorrect mixture setting (carburetor fuel injection); mixture too rich, air filter very dirty, automatic choke not in order or manual choke pulled too long, mainly short-distance driving, spark plug too cold, heat-range code number too low.

Effects: Misfiring, difficult cold-starting.

Remedy: Adjust A/F mixture and choke device, check air filter.



Insulator nose, electrodes and spark-plug shell covered with shiny soot or carbon residues.

Cause: Too much oil in combustion chamber. Oil level too high, badly worn piston rings, cylinders and valve guides. In two-stroke engines, too much oil in mixture.

Effects: Misfiring, difficult starting.

Remedy: Overhaul engine, adjust oil/fuel ratio (2-stroke engines), fit new spark plugs.


Lead fouling

Insulator nose covered in places with brown/yellow glazing, which can have a greenish color.

Cause: Lead additives in fuel. Glazing results from high engine loading after extended part-load operation.

Effects: At high loads, the glazing becomes conductive and causes misfiring.

Remedy: Fit new spark plugs since cleaning the old one is pointless.


Pronounced lead fouling

Insulator nose covered in places with thick brown/yellow glazing, which can have a greenish color.

Cause: Lead additives in fuel. Glazing results from high engine loading after extended partload operation.

Effects: At high loads the glazing becomes conductive and causes misfiring.

Remedy: Fit new spark plugs since cleaning the old ones is pointless.


Formation of ash

Heavy ash deposits on the insulator nose resulting from oil and fuel additives, in the scavening area and on the ground electrode. The structure of the ash is loose to cinder-like.

Cause: Alloying constituents, particularly from engine oil, can deposit this ash in the combustion chamber and on the spark-plug face.

Effects: Can lead to auto-ignition with loss of power and possible engine damage.

Remedy: Repair the engine. Fit new spark plugs. Possibly change engine-oil type.


Center electrode covered with melted deposits

Melted deposits on center electrode. Insulator tip blistered, spongy and soft.

Cause: Overheating caused by auto-ignition. For instance, due to ignition being too far advanced, combustion deposits in the combustion chamber, defective values, defective ignition distributor, poor-quality fuel. Possibly, spark-plug heat-range value is too low.

Effects: Misfiring, loss of power (engine damage).

Remedy: Check the engine, ignition and mixture formation system. Fit new spark plugs with correct heat-range code number.


Partially melted center electrode

Center electrode has melted and ground electrode is severely damaged.

Cause: Overheating caused by auto-ignition. For instance, due to ignition being too far advanced, combustion deposits in the combustion chamber, defective values, defective ignition distributor, poor-quality fuel.

Effects: Misfiring, loss of power (engine damage). Insulator-nose fracture, possibly due to overheated center electrode.

Remedy: Check the engine, ignition and mixture-formation system. Fit new spark plugs.


Partially melted electrodes

Cauliflower-like appearance of the electrodes. Possible deposit of materials not originating from the spark plug.

Cause: Overheating caused by auto-ignition. For instance, due to ignition being too far advanced, combustion deposits in the combustion chamber, defective values, defective ignition distributor, poor-quality fuel.

Effects: Power loss becomes noticeable before total failure occurs (engine damage).

Remedy: Check engine and mixture-formation system. Fit new spark plugs.


Heavy wear on center electrode

Cause: Spark plug exchange interval has been exceeded

Effects: Misfiring, particularly during acceleration (ignition voltage no longer sufficient for the large electrode gap). Poor starting.

Remedy: Fit new spark plugs.


Heavy wear on ground electrode

Cause: Aggressive fuel and oil additives. Unfavorable flow conditions in combustion chamber, possibly as a result of combustion deposits. Engine knock. Overheating has not taken place.

Effects: Misfiring, particularly during acceleration (ignition voltage no longer sufficient for the large electrode gap). Poor starting.

Remedy: Fit new spark plugs.


Insulator-nose fracture

Cause: Mechanical damage (spark plug has been dropped or bad handling has put pressure on the center electrode). In exceptional cases, deposits between the insulator nose and the center electrode, as well as center-electrode corrosion, can cause the insulator nose to fracture (this applies particularly fro excessively long periods of use).

Effects: Misfiring, spark arcs-over at a point that is inaccessible for the fresh charge of A/F mixture.

Remedy: Fit new spark plugs.


How should spark plugs be tightened for best performance?

Bosch recommends when installing spark plugs to use a torque wrench and the correct torque in ft.-lbs. As listed below: 

  • 10mm With Gasket: 8-11 Ft.-Lbs
  • 12mm With Gasket: 12-15 Ft.-Lbs
  • 14mm With Gasket: 19-22 Ft.-Lbs
  • 14mm Tapered Seat: 12-15 Ft.-Lbs
  • 18mm With Gasket: 20-23 Ft.-Lbs
  • 18mm Tapered Seat: 14-17 Ft.-Lbs

If a torque wrench is not available, hand tighten the plug until it is seated in the cylinder head. Spark plugs with gaskets should be tightened an additional 90°. Spark plugs with tapered seats should be tightened an additional 15°.


Are Bosch Spark Plugs pre-gapped at the factory?

Most Bosch Spark Plugs are factory pre-gapped for popular vehicle applications. The gap measurements are indicated either on the box, or by the part number located on the spark plug shell.

  • X suffix = .044"
  • Y suffix = .060"
  • Z suffix = .080"

Gaps should never by adjusted on Platinum+4 or Platinum2. Bosch Platinum+4 and Platinum2 have factory pre-set gaps, and should never be gapped by the installer.

Bosch Platinum and Super Spark Plugs also have factory-set gaps. For most plugs, the setting is shown on the plug package. These gaps are correct for the most popular applications of these plugs. There are applications, however, for which the gap setting has to be adjusted according to the vehicle manufacturer’s specifications. To avoid damage to a spark plug in the process of adjusting the gap, it is important to follow these guidelines:

 Widen gap using gap tool
To widen the electrode gap, use a tool that only pulls back the ground electrode, without applying pressure to the center electrode. The tool must not be wedged between the electrodes as that may cause damage to the insulator nose.

Decrease gap by tapping on hard surface

To close the electrode gap, carefully tap the plug, electrode first, on a hard surface, as shown in the illustration.

What is detonation?

Detonation or "knocking" is uncontrolled combustion with a very steep rise in pressure. It is caused by spontaneous ignition of the fuel mixture, which has not yet been reached by the ignition spark. As the high-pressure waves hit the walls of the combustion chamber, their impact produces a metallic knocking sound. Failure to recognize and deal with knocking will inevitably lead to serious engine damage.

What is heat range?

The spark plugs’ heat range is an index of its capacity to dissipate thermal energy. The different characteristics of automotive engines regarding operating load, compression, engine speed, cooling, and fuel make it impossible to run all engines with a standard spark plug. The same spark plug may get very hot in one engine type, but may reach only a relatively low temperature in another. In the first case, the air-fuel mixture would ignite on the glowing parts of the spark plug projecting into the combustion chamber (pre-ignition) and, in the second case, the insulator tip would soon become so badly fouled by combustion deposits that misfiring would occur. To ensure that the plug runs between the desired temperatures, plugs with different heat capacities were developed. The so called “heat range”, which is assigned to each spark plug, is used to characterize these heat dissipation capacities. A plug with a low heat range number (e.g., 2-4) indicates a cold plug that quickly dissipates heat to the engine block and cooling system, while a high code (e.g., 7-10) indicates a hot plug that retains heat. By properly selecting the heat range of the plug, it ensures that the plug will operate between the plug’s designed operating range of 500-900 degrees Celsius. In this range, the spark plug will be self-cleaning, yet will not be hot enough to pre-ignite the air/fuel mixture.

Temperature curves of spark plugs with different heat-range code

Spark plug with high heat-range code number ("hot plug"). large Insulator nose area absorbs much heat, low heat dissipation.Spark plug with medium heat-range code number. Insulator nose area smaller than "hot plug".  Lower heat absorption, better heat dissipation.Spark plug with low heat-range code number ("code plug"). Smaller insulator nose area absorbs little heat. Very good heat dissipation through short thermal conduction path.

How does Bosch test engines to determine the heat range and what heat range should I use?

To ensure optimal performance in your car, Bosch has tested each vehicle model and the various engines available for that model listed in the Spark Plug catalog or our web-page. Using a special spark plug with a thermocouple built in, Bosch engineers find the hottest cylinder in an engine and then test various spark plugs in that cylinder until the ideal plug is found. This ensures that you will always get the best performance possible.

I want t use Bosch Spark Plugs, but have modified my engine by adding a performance system.
Can I still use the plug listed in the catalog for my vehicle?

If you have modified your engine with a performance system such as a turbocharger, supercharger, or nitrous, the recommended spark plug may not be correct. You may need to use a plug with a colder heat range to compensate for the additional heat generated by the engine. Bosch recommends that in these situations, you consult a qualified engine builder for assistance in determining the appropriate heat range.

How often do I need to change my plugs?

As a rule, Bosch recommends that you follow the replacement interval listed in your vehicle owners manual, however we highly recommend that you inspect (read) your spark plugs annually and replace as required, to insure optimum engine performance.

Do Bosch Spark Plugs meet original equipment requirements?

Yes! Bosch Spark Plugs meet or exceed warranty requirements of all vehicle and engine manufacturers, and when properly installed in accordance with the recommended application, will not adversely affect the emission control system of any vehicle.

What about vehicle manufacturer warranties?

Consumers are sometimes told by an automobile dealer’s service writer or mechanic that a brand of replacement spark plug cannot be used in the consumer’s vehicle during the warranty period. The claim is made that use of the brand will “void the warranty”, with the statement or implication that only the original equipment brand of spark plugs may be used. This, of course, tends to cast doubt on the quality of the replacement spark plug. That claim is not true. Under the Magnuson-Moss Warranty Act and general principles of the Federal Trade Commission Act, a manufacturer may not require the use of any brand of spark plug (or any other article) unless the manufacturer provides the item free of charge under the terms of the warranty.

So, if the consumer is told that only the original equipment spark plug will not void the warranty, they should request that the OE spark plug be provided free of charge. If they are charged for the spark plug, the manufacturer or dealer is in violation of the Magnuson-Moss Warranty Act.

Does the Bosch part number have meaning to help determine the specifications of the plug?

Yes!   Click here for designation codes.

How does platinum improve a spark plug?

Perhaps the greatest innovation in spark plug design over the years is “pure platinum” technology, combined with multiple electrodes. As early as the 1960’s engineers recognized the value that platinum added to automotive plugs: it provides improved corrosion and erosion resistance and imparts a longer performance life than the standard copper core center electrode plug design. The use of platinum not only broadens the heat range beyond that achieved with a copper core center electrode design, but also provides extremely consistent performance, a reliable spark and a longer performance life.

What are the advantages of surface air gap plugs such as Platinum+4 and Platinum+2?

To validate the improved performance of surface air gap plugs with a pure platinum core and multiple ground electrodes, extensive testing was conducted in the laboratory and in the field:

The first test measured the energy transfer efficiency of the four-electrode platinum plug compared to conventional and other premium plugs with a single ground electrode. This certified laboratory test measured the additional pressure generated by the spark discharge in a pressure chamber. Higher pressure equals a higher energy transfer by the spark. The test showed that the four-electrode platinum plugs transferred more energy to the air/fuel mixture resulting in improved driveablility and lower emissions.

As a spark plug wears via erosion and corrosion, the gap between the center electrode and ground electrode gets larger. The larger the gap, the more voltage is required to produce a spark, until the capability of the ignition system is exceeded and the plug misfires. It was found that the multiple ground electrode, pure platinum center electrode plug reduces gap erosion and increasing voltage requirements over time.

Roush Industries, an independent test lab, measured the fuel consumption for the four-electrode platinum plug, as well as, that for original equipment and other premium single ground electrode plugs. Fuel savings up to 4.8% were found in the four electrodes, platinum core plugs. Tests were conducted in accordance with Federal Test Procedure 75FTP and HFE.

Tests for carbon fouling were simulated in a cold cell by repeatedly cooling, starting and operating the engine without letting the spark plugs reach their self-cleaning temperature. This cycle does not let the spark plug reach its self-cleaning temperature and allows the formation of carbon deposits. The tests found that the four ground electrode spark plugs had up to 33% better cold restart reliability than conventional plugs.

Does technology used in racing plugs find its way into Bosch Premium plugs?

YES! Racing, as it has for 100 years, provides a great developmental impetus and test bed for spark plug durability and performance, and what is learned at Indy or Darlington, or Le Mans helps improve the spark plugs installed in today’s cars or trucks. From the first Indy 500 won by Ray Harroun’s Marmon Wasp to the last 17 Indy 500 winners, and many in between, each have used Bosch spark plugs. Many premium passenger car spark plugs use multiple ground electrodes and precious metals, such as platinum, which are a direct result of their development and testing in long distance racing.

Click here for more details on how racing improves the breed of passenger car spark plugs.Click here for more details on how racing improves the breed of passenger car spark plugs.

What if I do not feel a difference after I have Bosch Premium Spark Plugs installed?

Bosch guarantees that you will feel improved performance in your vehicle when using a spark plug of the Bosch Platinum Series (Platinum+4, Platinum2, and Platinum). However, if you do not experience quicker starts, smoother acceleration, and improved fuel efficiency, you can return the plugs with a dated sales receipt and Bosch will refund the purchase price of the spark plugs.


  • Multi Ground - Plugs with multiple ground electrodes are called multi ground. These plugs also use surface gap spark patterns and the multiple ground electrodes increase service life.
  • Surface Gap - Surface gap technology is when the ground electrode is to the side of the center electrode. There can be no ground electrode (racing applications), multiple ground electrodes (PT+4 and PT2), or a single ground electrode. These spark plugs have improved efficiency and a higher probability of igniting a lean air/fuel mixture.
  • Resistor - Many spark plugs have a special conductive glass seal between the center electrode and the terminal stud. This seal acts as a resistor in the plug which reduces the transmission of pulses of energy to the ignition cables. These pulses can cause Radio Frequency Interference (RFI) with electrical components in the car. For some newer cars, resistor plugs are required for effective communication between the plugs and the electronic ignition.
  • Radio Frequency Interference (RFI) - When the spark plug creates a spark, a high frequency burst of energy is created. If this energy was to travel through the ignition wires, it could cause interference with other sensitive electronic devices, such as the radio or electronic control units. The resistor in the spark plug reduces this energy before it causes interference with other electrical components.
  • Fouling - Fouling occurs when a plug becomes contaminated with fuel, oil, or other contaminates that prevent the plug from generating a spark. Most plugs today are designed to reduce fouling and become self-cleaning when they reach 500 degrees Celsius. However, short trips, low speed driving, improper spark plug heat range (too cold), improper timing, or an oil leak in the combustion chamber can cause a spark plug to become fouled.
  • Bridging - Over time, contaminants can build on the surface of the spark plug if it does not reach self-cleaning temperature often. These buildups can grow between the electrodes until they are actually connected by a bridge of contaminants. This will often cause misfire.
  • Flashover - Occurs when the spark does not jump between the electrodes within the combustion chamber, but instead jumps between the metal shell and the terminal on top of the plug. This will always cause a misfire since the air/fuel mixture will not be ignited. Bosch spark plugs have a ribbed insulator to prevent flashover.
  • Quenching - The purpose of a spark plug is to introduce enough heat into the combustion chamber to initiate a smooth burn of the air/fuel mixture. Quenching occurs when that heat generated by the spark is reabsorbed back into the ground electrode, the center electrode, and the ceramic insulation. To reduce quenching and ensure an efficient transfer of heat, Bosch Platinum+4 and Platinum2 spark plugs use surface gap technologies.