1. Valve Train Components

1.4. Valve Guides

LO 9-05: Identify and describe the purpose, construction, and operation of components used in the valve train.

 

Valve Guides

Valves are located in guides that are either integral, meaning they are cast into the cylinder head, or removable, meaning they can be replaced Figure 29. Premium engines have replaceable valve guides that can be easily serviced during overhaul. The condition of a valve guide is critical to oil consumption control. A limited amount of lubrication oil must pass between the guide and the valve. In some engines, a single spiral groove may be machined to allow some oil into the guide. Guide material, such as sintered iron made of compacted iron particles, can better retain oil in pores for lubrication while maintaining narrow valve stem-to-guide clearances. However, oil can coke, or carbonize, in guides, causing abrasive wear of the guide. This means valve stem-to-guide clearances must be kept very narrow to prevent oil from accumulating in the guide Figure 30.

  Fig09_29

FIGURE 09-29: Valve guides can be either cast into the cylinder head or made replaceable from an iron alloy insert pressed into the cylinder head.

   Fig09_30

FIGURE 09-30: Coking oil in a valve guide can cause premature valve stem and guide wear.

  

 

Similarly, worn guides can allow oil to be drawn down the guide and leave thick carbon deposits on the back of the valve head, restricting gas flow. Oil can also enter the combustion chamber or exhaust stream because of the Venturi effect. This happens as a low-pressure area is created at the base of the valve guide by high-velocity gases that draw oil into the exhaust or intake stream, causing high oil consumption that may be accompanied by blue smoke Figure 31. Older diesel engines, particularly turbocharged engines, did not use seals on the intake guides because the intake manifold was pressurized and had little or no vacuum. However, positive umbrella seals are now used on all valve guides and stems to minimize the entrance of oil into the guide Figure 32. The high temperatures of the guide can cause oil to coke and thus accelerate abrasive wear. Oil entering the exhaust stream can contribute to engine emissions.

  Fig09_31

FIGURE 09-31: Valve stem seals are used to minimize the entry of oil into the valve guide. Worn guides can allow oil to enter the exhaust or intake stream.

   Fig09_32

FIGURE 09-32: A positive valve stem seal prevents oil from entering the valve guide.

  

 

  Technicians Tip  
  Exhaust smoke color is an important diagnostic observation. Color depends on the type and size of particles that the light passes through when visible smoke appears in exhaust haze. Large-size particles make smoke appear either as gray or black, depending on the smoke density. However, blue haze is created by vaporized oil and fuel particles, which are some of the smallest particles in the exhaust. The color is called Tyndall blue, named after the scientist who also discovered the greenhouse effect. The blue color of microscopic-size tobacco smoke is another common example of Tyndall blue. As particles increase in size, the light wavelength making up the blue color of light disappears and smoke begins to appear white. To determine whether a white cloud or haze appearing from the exhaust is smoke or steam, carefully observe how it dissipates. Steam or water vapor will rise and dissipate quickly, especially in warm temperatures. Smoke tends to stay closer to the ground and requires more time to dilute with a light wind.  

 

  Technicians Tip  
  Worn guides and missing valve stem seals lead to high oil consumption and blue smoke during deceleration. Oil in the guides can carbonize, accelerating stem wear. Also, any oil consumption in an engine leads to formation of particulate emissions. Long exhaust systems, such as those found on school buses, create a longer and more intense period of low pressure at the valve guide in the exhaust port, especially during deceleration. More oil can be dragged down the stem during deceleration and increase oil consumption, which is almost always higher in school buses when compared to other types of chassis.  

 

  Technicians Tip  
  Combustion quality in diesels at idle is often poor due to low combustion pressures and temperatures. Engine slobber, a residue consisting of unburned fuel, soot, and engine oil, can coat the back face of valves during prolonged idle. When left to cool, slobber will solidify on the back face of an open valve, preventing the valve from closing during engine start-up. This phenomenon is very common in cold-climate conditions when subzero temperatures do not rise above double digits. The result of a valve becoming heavily covered in engine slobber after shutdown is catastrophic engine failure during cold start-up. Solidified slobber prevents the valve from closing and results in piston-to-valve contact during cold start-up conditions.  

 

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