Cadillac DTS Northstar with Constant Misfire

Vehicle details: 2007 Cadillac DTS 4/Automatic transmission/ Cadillac Northstar V8 engine 124,820 miles

Technician: Todd Hansen

Initial condition:

  • Stabilitrac light on
  • Loss of engine power
  • Rough running

Out of State Cadillac dealership attempted ABS repairs / no luck.  Recommended engine tear-down and partial (or possibly complete) engine rebuild.  Client came in looking for an alternative solution.

Diagnostic Process:

  • Monitored engine data with scanner and verified misfire on #6 cylinder as well #8 cylinder.
  • Removed #6 and #8 spark plugs, performed simple/basic compression test on those 2 cylinders, results:
    •  #6: 0 psi
    •  #8: 175 psi
  • Tested Cylinder #6 further:  leak-down test and wet cylinder compression test – still:  ZERO compression.  So far – dealership recommendations are correct:  The engine clearly has a mechanical problem.
  • Valves, or their ability to seal compression into a cylinder, usually fail for one of three reasons:  They overheat and their sealing surfaces pit – causing poor sealing (this is condition is called a BURNT valve).
  • We ruled this out because burnt valve conditions will cause low compression that will increase during a wet-cylinder test.
  • Knowing the client was looking for the possibility of an alternate solution we got out the borescope camera to get pictures of the inside of cylinder #6.
  • The borescope found one of the intake valves stuck open in cylinder #6 – creating a no-compression condition.
  • We now know:  the engine has a possible bent valve or broken valve spring.
  • We also know:  a BENT valve was not very likely because this problem was only in ONE cylinder.  Bent valves are caused by out-of-synch timing components and thus almost-always involve more than one.
  • Removed valve cover
  • Removed intake camshaft for that cylinder bank and confirmed that the valve was stuck open due to a broken valve spring:
Northstar broken valve spring.

Northstar broken valve spring.

  • Replaced the valve spring, valve seal and keepers – a job that can be done with the engine in-chassis and without engine disassembly if other tricks are used.
  • Re-assembled engine and retested:  Smooth running engine / no more misfire codes!

Conclusion:

Often dealerships will take the less risky road with repairs.  This is common with franchises since they are required to perform their services and their service recommendations within standards agreed upon that allowed them to become a franchised representative of the manufacturer.  Our alternate repair required buy-in from the vehicle owner and more risk since the possibility existed that time taken to diagnose and perform this alternate repair could have proven unsuccessful and thus resources would have been wasted.  The tipping point came from the use of the bore-scope – a tool that is simple in concept but very powerful.  End result:  better information lead to a better solution.  The client’s out-of-pocket expense was less than 25% of the dealership’s proposed repairs.