Dead Miss Detective

Diagnose This! is a new regular feature in Truck Parts & Service. Each month we will present a real-life service scenario and provide clues as to what the ultimate diagnosis was. If you think you know the answer, be the first to e-mail with the correct response for free Truck Parts & Service merchandise. The answer will be revealed in a future issue of the magazine.

This month’s Diagnose This! is submitted by ASE master technician and owner of Fleet Guardian, Damon Russell.

Vehicle: 2000 Peterbilt Model 379

Key components: Caterpillar C12 engine; 690,000 miles on odometer

Scenario: The customer complained of a sudden weak engine brake and odd engine sound. The truck clearly had a dead miss.


  • Scan data showed no codes, but a balance test pointed to the No. 5 cylinder.
  • With rocker cover removed, a broken lash adjuster on the No. 5 injector follower was evident.
  • The camshaft was relatively damage free and did not require replacement.
  • Here are the broken parts that were removed.
  • All the loose parts were found inside the rocker box and a new lash adjuster was installed, but the lash was excessive when adjusted and the adjuster was at the end of its adjustment. A closer look revealed a failed camshaft follower, requiring cylinder head removal.
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