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Installation of new ACDelco 41-950 Spark Plugs

Cadillac's Northstar engine comes equipped from the factory with dual platinum spark plugs, ACDelco part number 41-950.  The center electrode is platinum and a small platinum "pad" is attached to the ground electrode.  This provides for an extremely long life with minimal gap opening.  The advertised service life for these spark plugs is 100,000 miles, although mine had a few more than that.

With 149,100 miles, the car still had great power throughout the RPM range, and started with ease.  There was some roughness when at cold idle, and the wires had already been replaced, so I decided it was finally time to change the spark plugs.  I used the procedure below, and it worked well for me.

All eight of my spark plugs required the electric impact gun for removal.  The front four (2/4/6/8) were relatively easy to remove.  But the back four (1/3/5/7) were not, and required a universal joint.  At the time, the only universal joint I had was designed for hand ratchets (as opposed to an impact gun), and I could not remove the rear plugs using that universal joint.  The small (and wobbly) u-joint allowed too much "play" in the ratcheting action, and in fact broke the porcelain insulator on one of the plugs.  After acquiring a universal joint designed for impact guns, the four back plugs zipped right out.  The proper tools were indispensable on this job.

Tools required:

  • Ratchet(s)

  • 1/2" socket

  • 10mm socket

  • 5/8" spark plug socket with rubber insert

  • 3/6" extension(s)

  • 10mm wrench

  • Wire spark plug gapping tool

Optional tools:

  • Air/electric impact gun

  • Impact extension

  • Impact universal joint

Step 1: Remove the four 1/2" nuts retaining the plastic beauty cover.  Remove the beauty cover and set aside.

Step 2: Remove the four 1/2" nuts retaining the strut tower brace, if equipped.  Remove the strut tower brace and set aside.

Step 3: Remove the four 10mm bolts retaining the Ignition Control Module (ICM).  The two middle arrows indicate the approximate location of the two rear 10mm bolts.  Once the bolts are removed, unplug the four harness connectors (two on each side) and pull the ICM forward and let it sit on top of the intake manifold, for more working room at the rear plugs.

Step 4: Gently pull one of the spark plug wires from the spark plug.  Use the wire boot to remove the wire; do not pull on the wire itself (it can damage the plug wire).

Step 5: Using compressed air or a vacuum, blow/vacuum any corrosion and dirt from the spark plug well ("hole").

Step 6: Using the spark plug socket and an extension, remove the spark plug.  Depending on age/mileage, the plug may be difficult to remove.  This is where an electric impact wrench may be helpful.  It removed all eight of my spark plugs with ease, and left the threads in the head clean as can be.

Step 7: Gap the new spark plug at the measurement indicated on your underhood emissions decal.  My '97 indicated .060", but some other years indicate .050".  Use the gap indicated for your model year.

Step 8: Using the torque wrench, set to 15 N*m or 11 lb*ft, install the new spark plug.  Use a torque wrench to achieve the proper torque.  Attach the spark plug wire to the new spark plug.


Step 9: Reattach the ICM to the rear cam cover using the four 10mm bolts.  Tighten until snug.  Remember to connect any ground straps and the four harness connections.

Step 10: Start the engine and verify proper operation.  It should idle smooth with no vibrations or misfires.  If the engine shakes or vibrates, turn it off and confirm that all spark plug wires are firmly connected to both the spark plugs and the ICM.

Step 11: Reattach the strut tower brace (if equipped) using the four 1/2" nuts.  Reattach the beauty cover with the four 1/2" nuts.

Pictured below are my eight original spark plugs.  The gaps ranged from an estimated .055" to about .065" (I only have .050/.060/.080" wires on my gap tool).  The #8 plug still contains its platinum pad on the ground electrode.  The cam cover seal on the #1 spark plug well has a slight seepage, and left about a quarter teaspoon of oil in that spark plug well.

Many thanks to all the fine folks at the CaddyInfo.com message forum.  Without their help, this job would have taken me a lot longer and gotten me more frustrated.  With their guidance, it was super easy.


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