Tech Tip Tuesday: Spark Plug Wires

In this installment of Tech Tip Tuesday (which has been carelessly forgotten a few Tuesdays recently) we’ll examine the importance of dielectric grease when installing spark plug wires. This particular example is on a 2005 Mercury Mountaineer 4.0 V6. The customer said he had a misfire, usually under load, but not all the time and said the spark plugs and wires had been replaced about a year ago or so. I immediately ruled them out as the cause because, hey, they’re new, right? I pulled the codes with my Modis and found a P0302, Misfire Cyl. #2. After driving around for 30 minutes or so with the scan tool showing fuel trims, O2 sensor data, etc. I was still at a bit of a loss for the intermittent misfire. I pulled the vehicle into the shop and decided to pull a plug to see if the porcelain was cracked or something. When I pulled off the #2 plug wire, I was greeted with a nice crusty plug and wire.


old vs new

In the above photo, you can see the junk in the left spark plug wire caused by corrosion because the connection was not weatherproof. The plug wire on the right has a small amount of dielectric grease on the tip to seal the new connection from the elements.

I pulled off a few other wires and some of them were ok, some of them were not, but all the other plug terminals could be cleaned and reused. I informed the customer that he needed one spark plug and a set of wires with which he was fine.

If the previous tech (from another shop in town) had simply applied a little dielectric grease on the tip of the spark plug wire, the customer would not have had to bring his vehicle to me to fix the problem again. As shown in the last In the Shop post featuring a front brake job, it’s the little things that separate a technician from the average parts changer. We all have to make our own mistakes sometimes, but we should do all we can to learn from other’s mistakes instead of making the same blunders over and over again!

Check out the following pictures showing just what a poor spark plug wire connection can do in just a few short months.

corrosion split boot old vs new

*All pictures were from my iPhone 5C.

I tried a new photo size in this post, so let me know if you prefer the new larger size or the smaller ones in all my previous posts.

Happy wrenching!

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