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This one is more of a lesson learnt the hard way more than anything else.
Last year, my kid’s 2003 Honda Civic was suffering from overheating caused by a bad fan switch. As this was his first car, he did not relate the temp gage at HIGH when idling to anything serious and of course let it ride. Fast forward to a couple of months later to me taking the car for a drive and noticing this problem, and finding oil in the coolant.
After taking off the cylinder head and getting it machined and pressure tested, new head gasket, cleaning out everything there was to clean in the cooling passageways and EGR system, new timing belt, water pump, thermostat, I decided to go the extra mile and get an ultrasonic injector cleaning as a preventative measure. I put everything together. Immediately started getting misfire problems. It was drivable but not what I had expected after doing a head gasket change and full automotive enema. I checked everything twice, intake manifold leaks, coil packs, compression & leak-down tests, spark plugs, all OK. I swapped a couple of injectors and the misfire seemed to follow a particular injector. I replaced that injector and the problem seemed to go away for a while, then came back. Tested everything again, including injector wiring with noid lights, fuel pressure, you name it, but the problem persisted. Finally waved the white flag and took the thing to a mechanic who did all the tests I did and found nothing. Then the next day he said the problem was in the injector wiring and replaced the harness with one from a scrap yard. When I picked up the car, I was charged $600 and told the misfire had ‘mostly’ gone away but not enough for inspection.
Throughout all this, the symptoms were that the random misfires would not be present on a cold engine, but only after starting from warm. It got more serious and the check engine light flashed for the first minute or so after warm starting at every start up (gunning the engine after starting eliminated the problem more quickly), but it needed to be addressed, as inspection loomed.
I took the car to the Honda dealer and they did all the tests I had done and found nothing. Finally after 2 days, they told me they believed it was a bad injector. I told them this was unlikely as I had replaced one and the others had been ultrasonically cleaned by a reputable online service, that had been reviewed by a somewhat well known TV show. They said they would like to try an injector swap and ordered one. This solved the problem. They said even though I had swapped the injectors around and that technically the injectors only affect a single cylinder, a single bad injector (one of those cleaned) can cause multiple random misfires.
I guess the moral of the story is not to trust injector cleaning services, and if you are getting multiple random misfires and it looks like an injector problem, swapping between cylinders may not always be a valid diagnostic test.
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