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Diagnosing Engine Performance Problems

Diagnosing Engine Performance Problems. Chapter 48. 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. Introduction.

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Diagnosing Engine Performance Problems

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  1. Diagnosing Engine Performance Problems Chapter 48

  2. 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

  3. 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

  4. 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

  5. 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

  6. 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

  7. 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

  8. 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

  9. Vacuum Testing (cont'd.) • Restricted exhaust systems • Cause hissing sound • Cranking vacuum test • Performed to check for internal leaks or incorrect valve timing

  10. 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

  11. 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

  12. 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

  13. 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

  14. 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

  15. 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

  16. 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

  17. 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

  18. 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

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