Chevrolet Repair Questions? Ask a Mechanic for Answers ASAP
The old trailblazer with a cylinder 4 misfire.
I've seen a ton of these recently.
The first thing to try is cleaning the upper intake manifold and throttle body. They get really gummed up and mess with the idle which tricks the engine into thinking there is a cylinder 4 misfire.
Next, if it doesn't change anything, swap the cylinder #4 coil to another cylinder and see if the misfire code changes- you might actually have a misfire, but the more common fix is cleaning the throttlebody.
If the coil seems ok and the cleaning doesn't work, then you most likely need a PCM reprogram. GM has released a re-flash that is supposed to fix the false cylinder 4 activation.
This is the best product for cleaning the upper intake on this vehicle. Follow the instructions on the can:
If the coil has been replaced and the throttle body is nice and clean I would probably try the upper intake cleaning first and then if that doesn't work maybe go for the re-flash at the dealer. They usually only charge 1/2-1 hour of labor to do it.
You could also pull the injectors and swap them around to see if it is an injector issue, but that is complicated enough that it would probably be a better idea to just replace the #4 injector while you are in there.
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