The Story:

The CTS is back! I really do love this car, but it seems that everything that was going to go wrong with it went wrong within a 6-month period. This time she limped in with a misfire. We first noticed something was up one night when we went to aggressively change lanes. I preface that statement with the word “aggressively” because the symptom would only manifest itself above 5k rpm’s under ¾ or full throttle. Otherwise when driving around town she felt perfectly fine. In hindsight she probably had a slight stumble, but nothing that you could notice.

When we punched the gas though, we knew something was instantly wrong. Physically the car had a difficult time maintaining the rpm range above 5k. It would noticeably misfire and you could audibly hear the engine making a “pop, pop, pop” sound. Luckily it wasn’t a mystery symptom, as the SES light instantly turned on. Getting home we popped the code reader on and P0300 & P0301 came up. P0300 is a general misfire code and P0301 is a specific misfire code on cylinder 1. 

Parts for this DIY:

  • New 3.6L Direct Injection V6 Ignition Coil
  • Pack of 6 new Spark-plugs

 Tools You’ll Need:

  • A Ratchet Set
  • A Screwdriver Set
  • A OBD2 Code Reader
  • Dielectric Grease

The Fix:

After doing a bit of research on the various CTS forums it seemed like there was likely two potential culprits. The first, and most easily replaced would be a bad spark plug. The second would be to replace the actual coil module. Given that the car was rolling up on 50k miles, we went ahead and changed all 6 spark plugs. Luckily changing the spark plugs on these 3.6 DI engines isn’t terribly difficult. They are easily accessible from the top of each cylinder head bank. You’ll probably spend more time removing and installing plastic beauty panels than doing the work.

To replace the plugs you’ll need to remove each coil module assembly, giving you access to the spark plug buried within the cylinder head. When removing the spark plugs make sure you use a proper spark plug socket with a rubber insert. This will let you pull the unit out. When installing the new plugs (New Spark Plug (Set of 6) BOSCH 4515 Fusion Platinum Plus Iridium) there are a couple things to keep in mind. First, the plug should go in very easily. Take your time and thread it by hand. It should go in quite a few turns with little or no resistance. If you feel too much resistance too quickly, unscrew it and start again. You must make sure that you do not damage the spark plug threads on the aluminum cylinder heads as stripping them would prove disastrous. Secondly, once you snug up the new spark plug do not overtighten them. These plugs only take 15 ft. lbs. of force.

With the plugs in you simply need to reinstall the coil module assemblies. Before doing so though you smear some dielectric grease to the inside of the coils. The purpose of the grease is twofold. First, it completely bridges the contact area between the coil and the spark plug, allowing for maximum electrical transfer. Secondly, it maintains a nice greasy joint between the plug and coil. This is important for the next guy who has to replace the plugs as they tend to heat up and almost weld themselves together with the coil. When that happens you have to yank and pull at it until it finally rips apart. A really crappy process that’s easily avoided with just a dollop of dielectric grease. The bolts on the coils should be torqued down to only 7.5 ft. lbs., so go easy on them.

With everything back together we went ahead and cleared the codes and started her up. The CTS was running great. We took her for a spin, gunned her and she performed as expected. Perfect!

Three weeks later she was back. Same symptoms, same trouble codes. Next step then was to replace the ignition coil module on cylinder #1 (based on the P0301 code). Usually I’d wait and order the part online as it’s much cheaper, but the car needed to be fixed on the spot so I made my way down to the local auto part and ordered a new one. When it’s time for you to get a new coil make sure you are getting the correct version, as the coil for the Direct Injection engines is different from the regular Fuel Injected engines. I unfortunately did not follow this advice and trusted the employee at Autozone. In fact, I didn’t realize it was the wrong coil until it was already screwed on. (The one you need for the Direct Injected engine is ACDelco D515C GM Original Equipment Ignition Coil). They are very similar to each other so you can almost get it kinda on, but the electrical plug will be different. To add to the issue, as I went to remove the incorrect unit, the screw that holds it in place started to spin. Ultimately this can happen to you regardless of using the correct part or not so I wouldn’t blame it solely on installing the wrong coil.

If you have a coil module retaining bolt start to spin on you, don’t fret. The metal sleeve that the bolt screws into is glued to the cylinder head cover. As such, if the bolt starts to spin that means the adhesive has been compromised. If the glue is shot, then you can just yank it out. I was able to remove mine using a small breaker bar and prying up on the electrical housing of the coil module. Once you pop it loose remove the sleeve using a pair of pliers to hold it in place. With the sleeve now loose you’ll want to pry the two legs slightly apart, as this will cause the much needed resistance when installing it onto the cylinder head cover. Just to be certain I went ahead and put some new adhesive before plunging the sleeve back on.

While I was waiting for the sleeve and adhesive to dry I went back to the Autozone, returned the incorrect unit and picked up the correct one. This time I took the old unit with me to make sure. After that it was just a matter of installing everything back together and clearing the trouble codes. 

The Result:

Just like before the CTS fired right up. A thorough trashing of it proved that the engine was humming as it should. Now that it’s been a number a number of months and the issue has not resurfaced I feel confident that the issue is completely resolved. Ultimately there is nothing preventing anyone from performing this fix. The actual repair is very simple. You’ll just need to keep a level head and find someone or someplace that will read the SES code. After that it’s smooth sailing.

Useful Parts & Tools to Help you with this DIY Project: