Something’s wrong with your car. You do a little deductive reasoning and figure out the general area of the problem and narrow it down to a few things, then throw a new part on. Problem fixed? Great! Move on with your life. But what if that doesn’t fix it? Then you have to move on to the next most likely part, and the next, and the next until the problem goes away or you end up at Sleazy Ron’s Used Car Emporium looking for a car with the pittance you have left. So what happens when the problem stays around after you’ve replaced every conceivable part that could cause the problem you had to begin with? That’s when you have to start back tracking. I’ll give you a case from near the beginning of my automotive career.

My parents had a 1986 F150 with a 302 V8 that served us well for many years, but one day I was driving it and it just died (I was about 17 if I recall). It shut off immediately, just like I turned off the ignition switch. I tried to restart it with no success, so I called my parents and we got the truck towed back to the shop at the house. I did a little preliminary troubleshooting looking for the “Big Three” that an engine needs to run: fuel, compression, spark. Fuel pressure? Check. Spark? Not so much. I did a compression test just to be sure, and all cylinders were in spec. I had an ignition problem, and now it was time to track it down. This was an old school, early fuel injected engine that still used a distributor and it’d been a while since it had a good tune up, so I installed new plugs, cap, rotor, and wires. No dice.

I knew that ignition control modules were known for failing under heat because they’re mounted on the side of the distributor on these engines, so I figured that was a good next step. Cool thing is that you don’t have to just stab a new part on; most of the time you can have them tested at a local parts store. I took mine down and had it tested and it checked out good. Rats! I really thought that was the problem. Oh well…moving on.

I pulled the distributor and replaced it hoping for a bad pickup coil (basically an early cam position sensor) or a worn distributor shaft bushing. Still won’t start! Next, I pulled the upper intake manifold and checked all the injectors for a pulse thinking that the ECM could be bad, sending neither a signal to the fuel injectors nor to the ignition module. They all passed a noid light test, so I had to move on.

This all happened over several weeks and I was at my wit’s end and my parents gave me the go-ahead to tow it to a local shop and have them fix it. They called the next morning and said it was fixed! It was the ignition control module the whole time. “But I had it tested!” I told the mechanic. They were very understanding and charged a couple hundred bucks for the module, labor, and a timing adjustment which was very fair.

One more case study: the customer brought a Honda Civic to the shop with a bad misfire, mostly under load. We were working on diagnosing the car when the tech pulled off a spark plug wire and noticed something pretty interesting. The fire was arcing to the spark plug tube! The customer had replaced the plugs, wires, cap, and rotor with “high performance” parts from eBay, and this was the result.

I’m not knocking parts from any particular vendor or source, just be careful. If the name brand spark plug wires at your local parts house cost $80, you should be wary of spark plug wires on eBay (or anywhere else) that go for $8.32.

You can see the failure point in this photo. The spark had been jumping through the insulation to the spark plug well causing the misfire. If you see a place like this on an ignition component there’s a good chance the spark is “leaking” causing some or all of the ignition energy (the spark) to ground somewhere beside the spark plug tip.

My point after all that rambling is that you can’t always trust new parts, and good diagnostic skills can really save you a lot of cash. I probably should have just replaced the module anyway, but I thought I was doing the right thing. You should really be able to prove that a part is bad before replacing it. Using a logical diagnostic procedure and understanding the function of a part will save you MANY headaches. Anyone can throw parts at a car, but what if you get a bad part? I’ve gotten many sensors, pumps, etc. that were bad right off the shelf. And it’s not just cheap bargain basement parts, either. Quality, trusted name brands aren’t failure-free. Don’t just replace parts; know WHY you’re replacing the part. If you don’t have those skills, it might be time to seek help from a more knowledgeable friend or take it to a professional.

Happy wrenching!

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