© 2012 Delmar, Cengage Learning Diagnosing Engine Performance Problems Chapter 48

© 2012 Delmar, Cengage Learning Objectives Perform compression tests in a correct manner Perform cylinder leakage tests in a correct manner Describe the procedures for doing various vacuum tests Explain the differences in types of carbon deposits

© 2012 Delmar, Cengage Learning Introduction Engine and ignition system problem results –Poor fuel economy –High emissions –Poor driveability Requirements for engine to run and start properly –Sufficient compression –Timed spark –Fuel and air mixed in the correct ratio

© 2012 Delmar, Cengage Learning Visual Checks Perform visual check when an engine does not start or is running poorly –Vacuum line –Electrical wires –Fuel gauge Believe it or not, sometimes the vehicle is out of gas

© 2012 Delmar, Cengage Learning Ignition System Checks Ignition system must produce a strong spark at the correct moment –Checking for spark is quickest thing to do Fuel injection –Easier to test compression than check for fuel on port injection engines Sometimes engine starts and runs but one cylinder has a dead misfire –Ignition or mechanical problems

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Engine Performance Testing Fuel problems –Dry black soot at exhaust pipe –Fuel in the crankcase Air-fuel mixture –Can be affected by air leaks Other problems and causes –Poor quality fuel blowby and EGR gases cause gummy deposit behind throttle plate –Dirty fuel injectors cause driveability problems –Plugged fuel filter causes starvation at high rpm

© 2012 Delmar, Cengage Learning Compression Loss Causes of low compression –Engine breathing problems –Compression leaks Breathing problem causes –Carbon buildup around neck of valve –Intake restrictions –Blocked exhaust Catalytic converters operating for a long period when there is a misfire –Become plugged

© 2012 Delmar, Cengage Learning Vacuum Testing Intake manifold vacuum readings –Useful in determining engine problems Leaking intake manifold –Causes rough idle Low, steady vacuum –Check ignition timing Leaking valve –Indicated when needle drops at regular intervals

© 2012 Delmar, Cengage Learning Vacuum Testing (cont'd.) Restricted exhaust systems –Cause hissing sound Cranking vacuum test –Performed to check for internal leaks or incorrect valve timing

© 2012 Delmar, Cengage Learning Other Vacuum Tests V-type engines –Can have a vacuum leak Test: pinch off PCV valve hose to the manifold and breather hose to the air cleaner Vacuum at oil filler opening: internal vacuum leak Vacuum signals –Can be used with a digital storage oscilloscope Smoke tester –Finds vacuum, exhaust, cooling, and oil leaks

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Compression Problems Compression leak causes –Blown head gasket –Burned valves –Worn or broken piston rings Fast way of locating a weak cylinder –Cylinder power balance test Compare performance of cylinders Compression test –Pinpoints compression problems

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Compression Problems (cont'd.) Causes of variations in cylinder –Ignition or fuel system Engine vacuum leaks Compression problems Hand-held scan tools –Have power balance test capability Compression tester –Common piece of equipment

© 2012 Delmar, Cengage Learning Compression Problems (cont'd.) Compression test procedure –Twist rubber boots on spark plug cables –Before removing spark plugs, blow dirt away from their base –Block the throttle in wide-open position –Connect remote starter switch –Insert compression gauge into spark plug hole and crank engine –When cranking, disable ignition system

© 2012 Delmar, Cengage Learning Compression Problems (cont'd.) Interpreting compression test results –Check manufacturers’ tolerance specifications –Two cylinders next to each other have low compression: indicates blown head gasket –Burned exhaust valves: can cause of one or more cylinders with low compression –All cylinders have low compression: incorrect valve timing could be the cause –Cylinders with compression higher than specified: can result in carbon buildup

© 2012 Delmar, Cengage Learning Compression Problems (cont'd.) Wet compression test –Used if all cylinders show poor results –Oil is squirted on each cylinder Running compression test –Measures how well the cylinders can draw in air and fuel –Running compression should be about 80% of cranking compression

© 2012 Delmar, Cengage Learning Cylinder Leakage Test Used to pinpoint causes of leakage in a combustion chamber Leaks can be pinpointed by listening in different places –Oil filter: leaking rings or pistons –Air cleaner: leaking intake valve –Exhaust: leaking exhaust valve –Bubbles in radiator: blown head gasket or crack in head or block

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Cylinder Leakage Test (cont'd.) Advantages of leakage test –Performed on an engine removed from a vehicle –Performance of camshaft will not affect the results –Source of leakage pinpointed before engine disassembly

© 2012 Delmar, Cengage Learning Carbon-Related Problems Carbon causes performance problems –Increased compression ratio –Fuel absorption –Blocked air and fuel flow Kinds of deposits –Oil based and carbonaceous Carbonaceous deposits –Result from fuel –Are difficult to remove and cause driveability problems