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

Having completed her work on the rear wheel differential, auto mechanics student Sarah Powell now focuses on engine diagnosis and repair. As most people know, the engine is where power is generated. The core of the engine is the cylinder, and inside each cylinder, a piston moves up and down, compressing air and fuel inside the combustion chamber.

A spark plug ignites the air / fuel mixture causing a controlled explosion, which forces the piston back powering the vehicle. Most vehicles have four, six or eight cylinders working simultaneously. The more cylinders, the more power generated. For a cylinder to work properly, it requires several things: fuel, air, compression, a spark and adequate exhaust removal. Remove any of one these things and the cylinder will misfire, causing a lack of overall power ....

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Figure A

Here we follow Sarah to the auto shop where she apprentices under master tech Jeff McCoy. Jeff "bugs" a six-cylinder engine to test Sarah’s progress in diagnosing and correcting common problems (figure A). The results of the bugs cause the engine light to blink on and off and noticeably affect the vehicle’s performance. Sarah must correctly run a battery of tests to isolate and repair the problems.

Basic Tools:

Diagnostic Tester
Service Manual
Screwdriver
Wrenches
Electrical diagnostic meter

• After reviewing the customer work order, which contains descriptions about the car’s performance, Sarah starts the car and listens carefully for telltale symptoms. She notices that the engine makes a slight pinging noise and the engine block seems to be shaking more than usual. Then she takes the car out for a test drive to verify problems listed by the owner (figure B). While driving, she notes that the steering wheel shakes and there seems to be a significant loss of acceleration power. She hypothesizes that the problem stems from a cylinder misfire.

• Sarah stations the car in a work bay and hooks up a vacuum hose to vent the exhaust (figure C).

  Figure B

  Figure C

  
  

• Sarah communicates with the vehicle’s computer using a hand-held diagnostic tester called Consult II. She connects the tester to the car's computer located under the dashboard (figure D).Because today's vehicles feature an internal computer, the diagnostic tester has fast become one of the most used implements for the automotive technician (figure E). The tester reads that there are multiple cylinder misfires, confirming Sarah’s initial hypothesis.

  Figure D

  Figure E

  
  

• Hoping to further isolate the problem, Sarah cross-references the DTCs or diagnostic trouble codes issued from the tester to one of the many service manuals stored in the shop computer (figure F).The codes still indicate several possibilities. Sarah prints out a specific workflow checklist to reference as she continues to identify the problems (figure G).

  Figure F

  Figure G

  
  

• Sarah performs a power balance test to find the malfunctioning cylinder (figure H). After starting the car, she punches in a command on the diagnostic tester and cuts off the fuel and spark to each cylinder in turn. She watches the RPM readings carefully. When a properly working cylinder is shut down, then the overall RPM will have a noticeable drop. If the RPM is not affected, then that cylinder is likely misfiring. According to her test, she determines which two of the six cylinders are misfiring.

• After removing the engine cover, Sarah checks the fuel injectors on the misfiring cylinders. She carefully listens to the fuel injectors using a long screwdriver as a makeshift stethoscope (figure I). A clicking noise indicates that the injector is working. She determines that one injector is not clicking at all which would cause a misfire due to lack of fuel.

  Figure H

  Figure I

  
  

• To access the fuel injectors, Sarah removes the cover and upper section of the plenum, which houses the cylinders and provides air distribution to the engine (figure J). Sarah makes sure to protect the exposed air intake valves from foreign objects by stuffing rags into the holes (figure K). Sarah soon identifies Jeff's first bug: the fuel injector power connector is disconnected (figure L).

  Figure J

  Figure K

  Figure L

  
  

• After properly reseating the injector connecter, Sarah performs a continuity check to make sure the circuit works properly. Using an electrical diagnostic meter, she taps into the connector on the injector and measures the current, which is now within specs (figure M). With the injector reseated, the engine light no longer blinks. (The engine light blinks to warn that the catalytic converter is at risk of damage). With the plenum removed, she makes sure to check the continuity of remaining five injectors.

• Sarah replaces the plenum and cover, following a specific installation and torque pattern found in the service manual (figure N).
  

In the next segment, Sarah runs another diagnostic test with the Consult II, which tells her there is still a cylinder misfire. She'll have to continue her detective work to find Jeff's remaining bugs ....

Figure M

Figure N