Chapter 5

Chapter 5

Factors Affecting Engine Performance

Chapter Introduction

Upon completion and review of this chapter, you should understand and be able to describe:

  • The common engine mechanical causes of improper engine performance.
  • The components that seal the combustion chamber.
  • The effects of combustion chamber sealing problems on engine performance.
  • Fuel volatility and its effect on engine performance.
  • The octane rating and explain its effects on combustion.
  • The different forms of abnormal combustion.
  • The potential causes and effects of abnormal combustion.

Terms To Know


Proper engine performance requires that the engine is mechanically sound and that its support systems are functioning as designed. One of the most important tasks in engine repair is determining the precise cause of the concern. Before you make engine mechanical repairs, you must gather as much information as possible. There are many different reasons why an engine can perform poorly. You will need to understand the requirements for proper engine performance before you can successfully diagnose the reasons for performance failures. In this chapter, we will look at the areas of the engine that can affect performance and create a need for mechanical repairs.

Components as common as spark plugs can cause an engine to perform as though it has a “dead” cylinder(s). Problems that occur related to the combustion chamber may require special test procedures.

Smoke coming from the tailpipe is never a good sign (Figure 5-1). Some causes may be relatively simple and inexpensive to rectify, while others may require a complete engine overhaul or replacement. You will need to understand the possible causes of engine smoking to correctly diagnose the cause of the unwanted emission of smoke.

Figure 5-1

A lot of thick blue smoke can require serious engine mechanical repairs or relatively inexpensive and simple cures. Your task will be to properly diagnose the cause to make an effective repair.

5-1Spark Plugs

Spark plugs are the workhorses of the ignition system. All the buildup of electrical energy of the coil(s) comes to a head at the spark plug gap (Figure 5-3). The electrodes of the spark plug must provide an adequate electrical path. If the spark is well timed and strong enough, it jumps the gap with a powerful burst. Then the conditions within the combustion chamber determine whether the spark will ignite the air-fuel mixture well enough to foster good combustion.

Figure 5-3

The spark plugs must be functioning well for the engine to provide good performance.

Inspecting the spark plugs is a method you will use to gather information about how the engine is operating. You may use this during a routine tune-up, when there is a drivability concern, and as part of gathering information about an engine mechanical failure. Spark plugs can give you tremendous insight into what has been occurring within the combustion chambers. When you have a drivability concern, one of the first areas to begin investigating is the spark plugs. This can help identify if one or more cylinders are acting up or if all cylinders are affected. Spark plugs can also help guide your diagnosis toward a fuel, ignition, or mechanical issue, for example.

Author’s Note

Many technicians will use nothing but OEM (original equipment from the manufacturer) spark plugs on late-model vehicles. Today’s PCMs and their monitoring systems are very sensitive; using aftermarket spark plugs is a very common cause of drivability problems and customer complaints. Vehicle manufacturers work closely with spark plug manufacturers, and both spend a lot of time and money designing the spark plug that will deliver the perfect spark for the individual combustion chamber. Many aftermarket companies produce excellent spark plugs. Unfortunately, they are usually designed to fit several applications. This does not guarantee that they will operate exactly like original equipment in each vehicle. In some cases you will find that aftermarket spark plugs will not cause major issues. If you do find quality aftermarket spark plugs that perform properly in an engine, it is appropriate to use them. Many technicians have a spark plug brand they have come to trust and use it on many, if not all, applications successfully. You will have to make this judgment call as a professional technician.

5-2Combustion Chamber Sealing

Proper combustion is the key to the engine’s power production. Fuel must be delivered in the correct quantity; a strong spark must be ignited at precisely the right instant, and the combustion chamber must be full of a fresh charge of air. Then the combustion chamber must be properly sealed for the engine to be able to harness the power of combustion. The spark plug, the engine valves, the head gasket, as well as the piston, rings, and cylinder walls seal the combustion chamber (Figure 5-4). Each of those components must be functioning properly, or the piston strokes cannot achieve their optimum result and engine performance will deteriorate.

Figure 5-4

A leak through any of these combustion chamber sealing points will lead to engine performance problems.

On the intake stroke, the engine creates a vacuum so a charge of fresh air can enter into the cylinder. If any of the components that form a seal for the combustion chamber are leaking, adequate vacuum level will not develop. Reduced vacuum levels can affect the ratio of air and fuel, causing the cylinder not to contribute its share of power (Figure 5-5). The more air an engine can take in, the more power it can put out. If an exhaust valve is leaking, residual pressure will be present in the cylinder when the piston moves down on the intake stroke. Exhaust from the exhaust manifold will join with the fresh air from the intake manifold and dilute the charge. These effects will create significantly rough engine operation and a loss of engine power. Any other combustion chamber leaks would have a similar effect on the intake stroke.

Figure 5-5

The engine needs a strong vacuum from the cylinder to pull air through the intake runners and into the cylinder.

The compression stroke should develop somewhere between 125 and 200 psi of pressure while the engine is cranking. This is a widely generalized estimate; always refer to themanufacturer’s specifications when evaluating a specific engine. Compression of the air-fuel mixture adds heat and pressure to the mix to make it more combustible. If compression is low, the spark may not be able to develop a strong flame front. Or the flame may start but sputter out as it hits loosely held areas of cooler air and fuel. This would cause incomplete combustion or even a total misfire. A misfire is when combustion does not take place. Any leaks past the valves, head gasket, piston, rings, and cylinder walls would lower the amount of compression a cylinder can develop.

Variable valve timing (VVT) can be used to lower the emission of . It does this by closing the exhaust valve later and opening the intake valve earlier, thereby increasing a valve overlap, and causing exhaust gas to dilute the incoming intake charge.

Leaks in the combustion chamber reduce the power of combustion. All the force of the expanding gases should be exerted on the top of the piston. This pushes the piston down and turns the crankshaft through the connecting rod. A cylinder has the potential to develop roughly 2,000 lbs. of force on top of the piston. If one quarter of that force leaks past worn piston rings and cylinder walls, the driver will definitely notice that lack of power. This is a typical scenario of a worn, high-mileage engine. The engine could also leak those forces past a burned intake or exhaust valve (Figure 5-6). The power loss would be evident, but the consumer would also likely notice a popping noise in the intake or exhaust, depending on which valve(s) had failed.

Figure 5-6

A burned valve will significantly reduce the power output of an engine and cause rough running.

5-2aRing and Cylinder Wall Wear

Piston rings will normally develop wear as the engine accumulates miles. Out of the box they have very sharp edges; these wear down over time and compromise their ability to seal against the cylinder walls (Figure 5-7). When piston rings wear, they may allow combustion gases to leak down into the crankcase. They may also let oil track up the cylinder walls into the combustion chamber, where they are burned. This is one of the primary causes of bluish smoke exiting the tailpipe. It is called oil consumption when oil is burning in the combustion chamber. It is important to remember that some oil consumption is considered normal. The piston’s rings will not completely seal all of the oil from the combustion chamber. The amount of oil consumption (oil burning) varies with engine load, driving style, type of oil, and amount of wear of the engine. One of the primary processes of an engine overhaul is replacing the piston rings and refurbishing the cylinder walls.

Figure 5-7

The cylinder walls must be in excellent condition to allow the rings to seal the combustion chamber properly.

5-2bValve Wear

Intake and exhaust valves can wear in a few different ways. The most dramatic problem they encounter is when they burn. Valves generally burn when they are open during combustion and exposed to the extreme combustion temperatures. This can happen when the valve springs become weak and cannot close the valves at the proper time, particularly when the engine is spinning at high speeds. When the engine is revving high, the momentum of the valve opening is greater and a slightly weak spring may not be able to close the valve at the proper time. With the valve face and margin exposed to combustion temperatures, the valve will rapidly burn.

The valve faces and seats can also become pitted from the corrosive by-products of combustion or can develop carbon buildup on the seats (Figure 5-8). In either case, leakage past the valve face and the seat can occur. Again, once combustion temperatures can leak past parts of a valve, the parts tend to burn rapidly.

Figure 5-8

A pitted valve face allows leakage that will eventually cause the valve to burn.

Misadjusted valves can also cause valve leakage and burning. If a valve is adjusted too tightly, it will be held open longer than it was designed to. This can cause a reduction in performance through poor sealing of the combustion chamber during the appropriate strokes. In extreme cases, it can also cause the valves to burn if they are exposed to excessive temperatures.

A Bit of History

As recently as the mid-1980s, it was not uncommon for engines to require valve reconditioning as early as 60,000 or 75,000 miles. Now due to advances in materials and machining, most of today’s engines can run at least 150,000 miles before requiring valve service.

5-2cHead Gasket Damage

When a head gasket leaks, it can present a whole host of different symptoms. In the context of combustion chamber sealing, a failure usually results in a leak between two adjoining cylinders. This will lower the compression and combustion of both cylinders significantly. This will cause rough running and a lack of power. Combustion gases can also leak out into the cooling system. This can cause the cooling system pressure cap to release pressure and coolant. The most common symptom of a blown head gasket is coolant leaking into the combustion chamber. Another common symptom of head gasket failure is the presence of coolant in the oil. The oil dipstick will show signs of coolant mixing with the oil, and it will look foamy and brownish, like a coffee milkshake. In these situations, a complete engine rebuild may be required. This burning coolant causes clouds of white, sweet-smelling exhaust to exit the tailpipe (Figure 5-9).

Figure 5-9

A blown head gasket often results in a cloud of white smoke.

There is a significant difference in cost, labor, and technique, depending on what problems exist with combustion chamber sealing. It will be your job to recognize the possible causes of low performance, in order to offer the customer responsible repair options with a realistic estimate.

5-3Fuel and Combustion

When combustion does not occur normally, severe engine damage can result. It is important that you are aware of the causes of abnormal combustion. When you repair or replace an engine with a catastrophic failure, you need to find the source of the problem so it doesn’t happen again (Figure 5-10). The fuel that customers use can have a significant impact on their engine performance and durability. Your customers may ask your advice about what type of fuel to use and why. You will also see drivability problems caused by fuel issues affecting combustion.

Figure 5-10

This diesel head gasket has multiple steel layers; yet it did not hold up to the pressures and stresses of the combustion chamber and engine when it overheated due to a cooling system malfunction.

5-3aOctane Rating

The octane rating of a fuel describes its ability to resist spontaneous ignition or engine knock. Engine knocking (detonation) or pinging (preignition) results when combustion occurs at the wrong time or at the wrong speed. Low octane fuel can be a cause of preignition and detonation. If combustion begins before the spark, for example, combustion pressures may try to push the piston backward at the end of the compression stroke. This results in a rattling noise from the piston. This is called preignition, and the sound is often described as pinging. Fuel is generally available for automobiles as regular, 87 octane; mid-range, 89 octane; or as premium, 92 octane or 93 octane (Figure 5-11). The higher the octane number, the greater its resistance to knock. Octane is tested in two ways, by the research method and by the motor method. The advertised octane rating is the average of the two ratings. You will often see this described on the pumps as:

Octane =RON+MON


Figure 5-11

Make sure your customer is using the correct octane fuel to prevent abnormal combustion.


Fuel volatility is the ability of the fuel to vaporize (evaporate). The Reid vapor pressure (RVP) defines the volatility of the fuel. The RVP is the pressure of the vapor above the fuel in a sealed container heated to 100. The higher the pressure of the vapor, the greater the volatility of the fuel. This means that the fuel will more readily vaporize.

Fuel volatility is adjusted seasonally in many parts of North America. A higher volatility fuel is allowed in the winter to help the engine start when it is cold. The fuel vaporizes more easily during compression rather than puddling along the cool walls of the combustion chamber. A fuel with very low volatility will cause hard starting and rough running at start-up. A lower volatility fuel is used in the summer to reduce the amount of evaporative emissions and to prevent vapor lock in the fuel lines. A fuel with very high volatility used in the summer months can cause extended cranking times. This occurs because the fuel actually boils in the fuel lines, allowing air in the fuel stream after the vehicle is shut off. When the vehicle is restarted, it takes a significant time for the pump to develop fuel pressure.

5-3cCombustion Issues

Combustion is the chemical reaction between fuel and oxygen that creates heat. It is a closely controlled burning of the air and fuel. Spark ignition occurs before top dead center on the compression stroke. The hot, compressed, air-fuel mixture is ignited, and a flame front develops. For normal combustion to occur, the air-fuel mixture must be delivered in the proper proportions and mixed well, the spark must be timed precisely, and the temperatures inside the combustion chamber must be controlled. The flame can then move quickly and evenly (propagate) across the combustion chamber, harnessing the power of the fuel as heat. Pressure builds steadily as the gases expand from heat. The peak of this pressure develops around  ATDC to push the piston down on the power stroke (Figure 5-12).

Figure 5-12

Normal combustion allows a flame front to travel rapidly across the cylinder and develop the peak force of combustion at about 10 degrees after top dead center.

5-3dAbnormal Combustion

Normal combustion takes about 3 milliseconds (3/1,000 of a second). One form of abnormal combustion, detonation, is more like an explosion, occurring as fast as 2 millionths of a second (2/100,000 of a second). The explosive nature of detonation can potentially cause engine damage. Preignition is a form of abnormal combustion when part of the compressed air-fuel mixture ignites before the spark. Engine misfire is another form of abnormal combustion; it causes the engine to run poorly and lack power.


Preignition means that a flame starts before intended spark time. This premature ignition can happen when a hot spot in the combustion chamber autoignites the fuel. A flame front develops and starts moving across the chamber (Figure 5-13). Then the spark occurs, and the normal flame front develops. When these two flame fronts collide, a pinging or knocking is heard. Preignition causing pinging can lead to the more damaging detonation by overheating the cylinder. Hot spots in the combustion chamber, the wrong spark plug, cooling system problems, lean air-fuel ratio, low octane fuel, over-advanced timing or metal edges sticking out from the head gasket are some of the reasons preignition can occur. Preignition can lead to engine damage such as melting the center of pistons. But preignition is less likely to cause major engine damage than detonation. It is also less likely than detonation to cause loud spark knock before it causes engine damage.

Figure 5-13

The preignition process is started when an uncontrolled flame front is started too soon. (A) An uncontrolled flame front is started before the spark by a hot carbon deposit. (B) The spark plug is fired. (C) The flame fronts collide.


Detonation occurs when combustion pressures develop so fast that the heat and pressure will “explode” the unburned fuel in the rest of the combustion chamber. Before the primary flame front (ignited by the spark plug) can sweep across the cylinder, the end gases ignite in an uncontrolled burst (Figure 5-14). The dangerous knocking results from the violent explosion that rapidly increases the pressure and temperature in a cylinder. This can be caused by engine overheating, over-advanced timing, low EGR system flow, low octane fuel, and other factors affecting the combustion chamber. In a short period of time detonation can cause serious engine damage, such as bent connecting rods and badly damaged pistons and ring lands (Figure 5-15).

Figure 5-14

Detonation results from an uncontrolled flame front that is started after the spark plug is fired. (A) Combustion begins. (B) Detonation or postspark begins a second flame front. (C) The two flame fronts collide to create a knocking sound.

Figure 5-15

Results of the extra stress placed on the piston due to detonation or preignition.

5-3gKnock Sensor

Most modern engines are equipped with a knock sensor (KS). The knock sensor creates an electrical signal when it senses a particular frequency of knocking or detonation. This signal serves as an input to the engine computer, the powertrain control module (PCM). When knocking is detected, the PCM modifies the spark timing to reduce the potentially dangerous knocking (Figure 5-16).

Figure 5-16

The knock sensor screws into the engine to detect abnormal engine knocking.


Misfire is another type of abnormal combustion. When an engine misfires, it means that one or more of the cylinders are not producing their normal amount of power. The cylinder(s) is (are) unable to burn the air-fuel mixture properly and extract adequate energy from the fuel. The misfire may be total, meaning that a flame never develops and the air and fuel are exhausted out of the cylinder unburned. Hydrocarbon emissions increase dramatically. Misfire may also be partial when a flame starts but sputters out before producing adequate power (due to a lack of fuel, compression, or good spark). When an engine is misfiring, the engine bucks and hesitates; it is often more pronounced under acceleration. Technicians normally call this a miss or a skip. On modern vehicles (1996 and newer) equipped with the OBD II system, a misfire DTC will be set when the PCM detects a single- or multiple-cylinder misfire. This can help guide your diagnosis.

A P0300 DTC means that the PCM has detected a multiple-cylinder misfire. A DTC P0301 means the PCM has detected a misfire on cylinder number 1, and right up to DTC P0312, a misfire on cylinder number 12.

5-3iSupport Systems’ Contribution to Abnormal Combustion

When the cooling, lubrication, intake, exhaust, or fuel system is not functioning properly, it can cause or contribute to improper combustion. If the engine is running too hot due to a malfunctioning cooling system, detonation and preignition are much more likely to occur. A buildup of deposits in a corner of a cooling passage near the combustion chamber can have devastating effects on the engine due to detonation and preignition. The lubrication system must also reduce engine friction and heat to keep the cylinders cool enough to allow normal combustion.

A restriction in the intake or exhaust system can cause the engine to misfire from a lack of air in the cylinders. If an engine cannot breathe, it cannot produce normal combustion. When the fuel system is failing to deliver adequate fuel, the engine can suffer misfire and preignition from running too hot. A lean air-fuel mixture causes hot engine temperatures; the fuel actually helps cool the mixture. If the compressed air-fuel mixture is too hot, a portion of it can autoignite either before or after the spark begins.

5-4Engine Noises

The engine can create a wide variety of noises due to worn or damaged parts. It will be your job to diagnose the causes of these noises and to correct them. Most noises occur because of worn components causing excessive clearances. A typical wear item is crankshaft bearings, which cause expensive damage heard as a deep lower-end knock (Figure 5-17). Valve lifters also commonly wear with higher mileage and cause a higher pitch clatter. These are just a couple of examples of possible noises; in the Shop Manual, you will learn to isolate noises and their causes.

Figure 5-17

The bearings on the left are worn down to the copper underlayer; these created engine knocking from the bottom end.

Chapter Review


  • Proper engine performance requires that the engine is mechanically sound and that its support systems are functioning as designed.
  • The combustion chamber must be properly sealed to provide good engine performance.
  • The valves, spark plug, rings, and head gasket seal the combustion chamber.
  • The octane rating of gasoline describes its ability to resist knocking; the higher the number, the greater the resistance to knocking.
  • Higher volatility fuel should be used in the winter to assist cold starts; lower volatility fuel should be used in the summer to prevent excessive HC emissions and vapor lock.
  • Misfire, preignition, and detonation are three types of abnormal combustion that can cause serious engine damage.
  • Failures in the cooling or lubrication systems can cause abnormal combustion or serious engine defects.
  • Normal engine wear will eventually lead to abnormal noises and reduced performance.