Engine Mechanical Diagnostics 1

2 Introduction To Diagnosis

 Engine vacuum should regularly be between 17 and 22 inches of Mercury (in. Hg) at idle. Many factors can affect this including the number of cylinders and even barometric pressure, so be sure to reference the OEM specifications for the vehicle you are repairing. 3

 Open and close throttle ◦ Needle should drop below 5”Hg and bounce up to around 25”Hg 4

 Open and close throttle suddenly - needle drops to 0”Hg then back to around 20”Hg ◦ Possible leaking rings 5

 Vacuum readings lower than normal ◦ Possible late valve timing or ignition timing 6

 Extremely low but steady vacuum reading ◦ Possible air leak in carburetor, intake manifold or gaskets 7

 Vacuum that goes low on hard acceleration and stays low with low engine power indicates a plugged exhaust 8

 Purpose ◦ Determine mechanical condition of cylinder ◦ Exact cause of pressure loss in upper part of cylinder 9

 Evaluation of test ◦ Conditions in the cylinder may be determined by utilizing a compression test and a CLT test. When combining information gathered by performing both tests, a large amount of information can be gathered about cylinder and engine conditions. ◦ Defects must be corrected before an effective tune up may be performed and engine performance restored. 10

 What is compression? ◦ Compression is the tight squeezing of the air fuel mixture that promotes a rapid, complete and powerful burn.  Decreases in compression typically decrease power, and very low compression may lead to a misfire.  Increases in compression typically increase power, but also contribute to higher cylinder temperatures which may lead to detonation or preignition if compression becomes excessive. 11

 When checking compression: ◦ Some manufacturers suggest that a compression test only be done when the vehicle is warm, so refer to the manual for specific testing procedures. ◦ Disable the engine by removing fuel rather than spark to prevent a fire hazard. ◦ Ease the load on the starter by removing all spark plugs from the engine before testing ◦ Make sure the proper adapter is used, and that it is threaded completely in the spark plug hole. 12

◦ Ensure the engine has an adequate air supply during testing by holding the throttle wide open ◦ Make sure the engine is turned over at least ten times before recording the reading. ◦ If pressures vary cylinder to cylinder by more than 10%, the need for further testing is required. ◦ Squirting a small amount of motor oil into the cylinders and performing the test again will help you determine if the rings are not sealing properly if all cylinders were even but low after the initial test. 13

 Manufacturers have several different cylinder leak down testers, make sure you are familiar with the model you are using. ◦ Make sure the proper adapter is being used and that the cylinder being tested is at TDC compression.  It is important that the valves are closed completely for this test. 14

◦ Leakage amounts and locations can give you a general condition of the cylinders and where they are leaking.  Great condition – 5% to 10% leakage  Good condition 10% to 20% leakage  Possible trouble – Anything over 30% leakage ◦ Air blowing from the tailpipe = leaky exhaust valve ◦ Air blowing from throttle body = leaky intake valve ◦ Air blowing out of breather vent = worn rings, worn cylinders or a hole in the piston ◦ Air blowing out of radiator neck = head gasket 15

 Compression is poor, but leakage is good ◦ Possible valve train problem including worn cam lobe, collapsed lifter or bent push rod.  Compression is low, but leakage is good ◦ Possible valve timing problem, jumped timing belt or chain  Compression good, leakage good, but cylinder misfires ◦ Possible fuel delivery, ignition, or vacuum leak problem 16

 Components of an engine management system are utilized to provide better power, mileage and longevity.  Identifying and testing these components is the first step in becoming a successful diagnostician. 17