start Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 OBJECTIVES: After studying Chapter 27, the reader should be able to: • • • • • Prepare for ASE Engine Repair (A1) certification test content area “C” (Engine Block Diagnosis and Repair) Describe how the camshaft and valve train function Discuss valve train noise and its causes Explain how to degree a camshaft Explain how a hydraulic lifter works Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 KEY TERMS: bucket • cam follower • camshaft bearings • contour duration of the camshaft finger follower • flat-link type • freewheeling hydraulic valve lifter • intake centerline • intake lobe centerline method minutes • Morse • roller chain type silent chain type • solid valve lifter • straight up tappets • valve lash Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 The cam is driven by timing gears, chains, or belts located at the front of the engine The gear or sprocket on the camshaft has twice as many teeth, or notches, as the one on the crankshaft This results in two crankshaft turns for each turn of the camshaft The camshaft turns at one-half the crankshaft speed in all four-stroke-cycle engines Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 CAMSHAFT FUNCTION The camshaft’s function is to operate the valve train. Cam shape  or contour is the major factor in determining the operating  characteristics of the engine.  The lobes on the camshaft open the valves against the force of the  valve springs. The camshaft lobe changes rotary motion  (camshaft) to linear motion (valves) The camshaft may also operate the following: Mechanical fuel pump Oil pump Distributor Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Cam lobe shape has more control over engine performance  characteristics than does any other single engine part Engines identical in every way except cam lobe shape may have  completely different operating characteristics and performance.  Figure 27–1 This high-performance camshaft has a lobe that opens the valve quickly and keeps it open for a long time Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Figure 27–2 In many engines, the camshaft drives the distributor and the oil pump through a shaft from the end of the distributor Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Figure 27–3 The fuel pump plunger rides on the camshaft eccentric Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 CAMSHAFT LOCATION Pushrod engines have the cam located in the block. They are  smaller and lighter than overhead cam engines. The camshaft is  supported in the block by camshaft bearings and driven by the  crankshaft with a gear or sprocket and chain drive.  Figure 27–4 Cutaway of a Chevrolet V-8 showing the valve train components Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 CAMSHAFT PROBLEM DIAGNOSIS A partially worn lobe on the camshaft is often difficult to  diagnose. Sometimes a valve “tick tick tick” noise is heard if the  cam lobe is worn.  The noise can be intermittent, which makes it harder to determine  cause. If the engine has an overhead camshaft (OHC), it is usually   easy to remove the cam cover and make an inspection cam lobes  and valve train In an over­head valve (OHV) engine, the camshaft is in the block,  where easy visual inspection is not possible.  Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman 10 ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Here Figure 27–27 indicates a reading of 30° ATDC & 26° BTDC.  Add the two readings together and divide by two (30°+ 26° = 56°/2 = 28°) Move the degree wheel until it is 28° and the engine has stopped  rotating in either direction. Now TDC on the degree wheel is exactly  at top dead center Figure 27–27 Degree wheel indicating where the piston stopped near top dead center By splitting the difference between the two readings, the true TDC (28°) can be located on the degree wheel 42 Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Step #2  Remove the piston stop and place a dial indicator on an in­ take valve lifter. To accurately locate the point of maximum lift  (intake lobe centerline), rotate the engine until the lifter drops 0.050  inch on each side of the maximum lift point Mark the degree wheel at these points on either side of the  maximum lift point. Now count the degrees between these two  points and mark the halfway point This halfway point represents the intake centerline. This point is  often located between 100° and 110° See Figure 27–28 43 Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Figure 27–28 (a) The setup required to degree a camshaft (b) Close-up of the pointer and the degree wheel (a) Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman (b) 44 ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Step #3  Now that both TDC and the intake centerline have been  marked, compare the actual intake centerline with the specs If the actual intake centerline is 106° and the camshaft specification  indicates 106°, then the camshaft is installed straight up If the actual reading is 104°, the cam is advanced by 2° If the actual reading is 108°,  camshaft is retarded by 2° Figure 27–29 Typical valve timing diagram showing the intake lobe centerline at 106° ATDC Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman 45 ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Varying the Valve Timing to Vary Engine Performance If the camshaft is slightly ahead of the crankshaft, the camshaft is called advanced An advanced camshaft (maximum of 4°) results in more low speed torque with a slight decrease in high-speed power Some aftermarket camshaft manufacturers design about a 4° advance into their timing gears or camshaft This permits the use of a camshaft with more lift and duration, yet still provides the smooth idle and low-speed responses of a milder camshaft If the camshaft is slightly behind the crankshaft, the camshaft is called retarded A retarded camshaft (maximum of 4°) results in more high-speed power at the expense of low-speed torque If the measured values are different from specifications, special offset pins or keys are available to relocate the cam gear by the proper amount Some manufacturers provide adjustable cam timing sprockets for overhead cam engines Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman 46 ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 LIFTERS Valve lifters (called tappets) follow the contour or shape of the  camshaft lobe. This arrangement changes the cam motion to a  reciprocating motion in the valve train.  Most older­style lifters have a slightly convex surface that  slides on the cam.  Some are designed with rollers  to follow cam contour. Roller  lifters reduce valve train  friction & increase fuel  economy offset the greater  manufacturing cost.  Figure 27–30 Lifters or tappets are made in two styles: flat bottom and roller Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman 47 ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Valve train clearance is called valve lash. Clearance must not be  excessive, or it will cause noise or premature failure Two methods are used to make valve clearance adjustments. One  involves a solid valve lifter with a mechanical adjustment, and the  other a lifter with an automatic hydraulic adjustment built into the  lifter body, called a hydraulic valve lifter A hydraulic lifter consists primarily of a hollow cylinder body  enclosing a fitted hollow plunger, a check valve, and a pushrod cup.  Lifters that feed oil through the pushrod have a metering disk or  restrictor valve located under the pushrod cup. Engine oil under  pressure is fed through an engine passage to the exterior lifter body.  48 Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 An undercut portion allows the oil under pressure to surround the  lifter body. Oil under pressure goes through holes in the undercut  section into the center of the plunger From there, it goes down through the check valve to a clearance  space between the bottom of the plunger and the interior bottom of  the lifter body. It fills this space with oil at engine pressure Slight leakage allowance is designed into the lifter so that the air can  bleed out and the lifter can leak down if it should become overfilled See Figures 27–31 through 27–34 49 Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Figure 27–31 A cross-sectional view of a typical flat-bottomed hydraulic lifter Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman Figure 27–32 Hydraulic lash adjusters (HLA) are built into the rocker arm on some OHC engines 50 ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Figure 27–33 Hydraulic lifters are also built into bucket-type lifters on many OHC engines Figure 27–34 To correctly adjust the valve clearance (lash), position the camshaft on the base circle of the camshaft lobe for the valve being adjusted Remove all clearance by spinning the pushrod and tightening the nut until all clearance is removed The adjusting nut is then tightened one complete revolution This is what is meant by the term “zero lash plus turn.” Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman 51 ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 CAUTION: Using too thick (high viscosity) an engine oil can cause the  hydraulic lifters or hydraulic lash adjusters to not bleed down as fast as  they should. This slow bleed down can cause a valve(s) to remain open,  which results in an engine miss. Using an SAE 10W­30 instead of the  specified SAE 5W­30 could cause the lifters to bleed down slower than  normal and cause a driveability problem, especially if the oil is not  changed at specified intervals Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman 52 ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 SUMMARY The camshaft rotates at one­half the crankshaft speed The pushrods should be rotating while the engine is running if  the camshaft and lifters are okay On overhead valve engines, the camshaft is usually placed in  the block above the crankshaft. The lobes of the camshaft are  usually lubricated by splash lubrication Silent chains are quieter than roller chains but tend to stretch  with use 53 Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 SUMMARY (cont.) The lift of a cam is usually expressed in decimal inches and  represents the distance that the valve is lifted off the valve seat In many engines, camshaft lift is transferred to the tip of the  valve stem to open the valve by the use of a rocker arm or  follower Pushrods transfer camshaft motion upward from the camshaft  to the rocker arm Camshaft duration is the number of degrees of crankshaft  rotation for which the valve is lifted off the seat 54 Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 SUMMARY (cont.) Valve overlap is the number of crankshaft degrees for which  both valves are open 10 Camshafts should be installed according to the manufacturer’s  recommended procedures. Flat lifter camshafts should be  thoroughly lubricated with extreme pressure lubricant 11 If a new camshaft is installed, new lifters should also be  installed Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman 55 ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 end Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman 56 ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458