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Engine Diagnosis and Repair, Part Two Engine Diagnosis and Repair, Part Two From "Trade School" episode DTRS-104

To complete her engine repair project, auto mechanics student Sarah Powell continues diagnosing the causes of the misfiring cylinders ....

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Figure A

Basic Tools:

Diagnostic Tester
Service Manual
Screwdriver
Wrenches
Electrical diagnostic meter

• After replacing the plenum, Sarah starts the car, which still seems to be idling rough. She refers to the Consult II diagnostic tester and finds that a cylinder is still misfiring (figure A).
  

• Responding to a hint from Jeff, Sarah visually inspects the coils that amplify the power to the spark plugs and finds a loose connection (figure B). She quickly reconnects the coil to the spark plug and replaces it (figure C).

  Figure B

  Figure C

  
  

• The engine still seems to have a cylinder misfire because it's still running a little rough. Sarah refers to the diagnostic tester, but finds no trouble codes (figure D). She runs another power balance test, but still finds no answers.

• Then Sarah uses the diagnostic tester to determine the ratio of oxygen to fuel in each cylinder (figure E). She revs the engine to 2,000 rpm and gets a read-out. The mixture is well within specs.

  Figure D

  Figure E

  
  

• Sarah refers back to her workflow checklist and decides that the remaining misfire could be caused by a bad spark plug. She removes each one and compares them to a new spark plug (figure F).

• Sarah finds Jeff's third and final bug. The electrode on one of the spark plugs is pushed in too far. The gap between the electrode and the ground needs to be big enough to allow the spark to "jump the gap" which ignites the compressed air/fuel mixture (figure G). If no gap is evident, the electrode simply creates a closed circuit and will not create a spark. She replaces and hand-tightens a new spark plug.

  Figure F

  Figure G

  
  

  Figure H

• After replacing the plastic engine cover, Sarah and Jeff take the car out for another test drive (figure H). The car no longer idles rough, the power is regained and the car is much more responsive.

School Contact Information:

Arapahoe Community College
5900 S. Santa Fe Drive
Littleton, CO 80160
Phone: 303-797-4222
Website: www.arapahoe.edu