A while back my 2006 (out of warranty) Hummer H3 started to idle roughly and I was presented with an amber “check engine” light on the dash. I went to the local auto parts store and borrowed their code reader, and plugged it into the connector beneath the dash. The system was reporting that cylinder five had been misfiring.
I had had rough idle issues previously and I was able to correct the issue by cleaning the throttle body, but this time there was no improvement. I also ran a couple tanks of fuel with fuel injector cleaner through it with no improvement.
Having received a recall regarding a bad aesthetic trim clip, I brought the vehicle into the dealership. Since I was there anyway, I had them check into the rough idle issue. I was told that the inline 5cyl motors occasionally had issue with the last cylinder (number five) in that it doesn’t cool as well as the others and as a result the valves have a tendency to hang up and cause rough idling. I was told that I should replace the throttle body, and the cylinder head which would cost approximately $4,500 plus labor. Just as shock was starting to set in, the service manager told me the work would probably resolve the issue. I told him that I was not about to spend that much money for a “probably.”
When I got home I thought about it a bit and went back to basic troubleshooting. In order for an engine to run, an engine needs air, fuel, and an ignition source. Since I had cleaned the throttle body, and checked the air filter, I figured that it was a safe bet that it was getting air, and having run a couple tanks of fuel with a fuel injector treatment, I was most likely getting fuel, so the only thing left was spark.
Interestingly enough, the engine has five identical ignition coils, one for each cylinder. So I swapped the ignition coils of cylinder five, where the problem was reported by the code reader, and cylinder one where I had never had any issues. A couple days later, the rough idle returned and was accompanied by the check engine light. Again, I went to the auto parts store, borrowed their code reader and … the system was now reporting misfires on cylinder one! Therefore, problem had to be the ignition coil since the problem followed it to cylinder one.
I brought the code reader back into the store, purchased a new ignition coil for $60, and installed it right there in the parking lot in a couple minutes. The engine ran wonderfully smooth and I have not had any problems since. I was very pleased that just some basic troubleshooting saved me from a $4,500 + repair bill, which incidentally would not have solved the problem!
Scott Moore is the chief engineer for Alliant Techsystems’ Sporting Group. He has been with the company for 11 years in the manufacturing engineering department where he has served as engineer, engineering manager, and now chief engineer.
Tell us your experience in solving a knotty engineering problem. Send stories to Jennifer Campbell for Sherlock Ohms.
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