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Jared D
Jared D, ASE Certified Technician
Category: Chevy
Satisfied Customers: 1333
Experience:  Shop Supervisor at County Fleet
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My 2002 Trailblazer does not have any error codes,but,it

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My 2002 Trailblazer does not have any error codes,but,it doesn't idle that smooth.It is showing a misfire on the #4 cylinder which is probably a fuel injector problem that I am aware of,I did a tune up last March.My diagnostic scanner is showing the spark angle at an idle is bouncing up and down from as low as 13 to as high as 24.It does stay between 17 and 20 before going to the other extremes.What could be causing this?

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:

https://www.amazon.com/Genuine-1052626-Engine-Injector-Cleaner/dp/B000QII698/ref=sr_1_2?ie=UTF8&qid=1475792488&sr=8-2&keywords=GM+upper+cylinder+cleaner

Customer: replied 1 month ago.
Thanks,I did have a bad coil on #4,it was replaced in March,I don't think there is an issue with the throttle body,I sprayed that real good.Upper intake cleaning I have not tried......

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.

Customer: replied 1 month ago.
Thanks again!

Please let me know if you need more info. Don't forget to rate.

Hi, do you still need help with this?

If not, please don't forget to rate.

Jared D and 5 other Chevy Specialists are ready to help you
Customer: replied 4 days ago.
I think I already mentioned I cleaned the intake,I used Seafoam,I have had no more misfires on #4 except for an occasional 1 or 2 you were spot on about that.Still have the ignition and spark angle going up and down at an idle,still idles rough,from the previous conversation it sounds like my only option is to get a reflash done?
Customer: replied 3 days ago.
I think I already mentioned I cleaned the intake,I used Seafoam,I have had no more misfires on #4 except for an occasional 1 or 2 you were spot on about that,now Fuel Bank #1 is showing it is running rich,I did put some Seafoam directly into the intake before it was sprayed into the throttle body,no service engine light is on.Still have the idle, ignition and spark angle going up and down at an idle,still idles rough,from the previous conversation it sounds like my only option is to get a reflash done?