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Discussion Starter #1 (Edited)
UPDATE: http://www.355nation.net/forum/i4-i5-engine-drivetrain/121850-2008-3-7l-i5-i-give-up-engine-lights-2.html#post3308218


Hello All,

I have been scanning these forums for almost 2 years now and the topics on here have been so helpful. I have been able to fix many problems thanks to the coloradofans and 355nations community without posting! Although I have come to the point where I cannot fix this particular issue and hopefully you can all help. I have searched, i swear every topic trying to fix this issue.

The Problems:
Engine light codes: P0420 - Low emissions, P0302 - Misfire Cylinder #2
MPG: 16-17 MPG Highway and city...no matter what its the same, so at least its consistently bad!
Symptoms: Rough Shaking idle (whole truck shakes), below par MPG, Engine lights, Loss of power

What I have done:
-I seafoam, replace the air filter and clean the maf sensor, and throttle body every 6 months.
-I use injector cleaner every once in a while
-I have changed my spark plugs and even replaced the coil pack on the #2 cylinder.

What I know (or think i know):
-There was a recall for the 2004-2005 (06?) with the head issue. Not sure if this happens in the later years. If so, i am not the original owner so I am not sure if the warranty applies. (I am at 150k km)
-If the cat has failed, causing the P0420 emissions code, then there is a bigger issue which caused the Cat to fail in the first place.
-If it was an 02 sensor, a different code should be coming up? then again I don't think that would cause a misfire. Although they look to be original.
-There could be a short in the coil pack wiring?

My Next Steps:
-Replace original O2 sensors
-Get the fuel injectors cleaned
-Have a compression test done at the dealership (last resort)

I obviously want to avoid going to the dealership, although I am almost at that point. I could be missing something in all of this.

Any help is greatly appreciated! Thank you.
 

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maniac mechanic
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Replace the o2 sensors. Also swap the number 2 coil to another cylinder and see if the misfire follows it, if so replace it. Also check the exhaust manifold for leaks.

Do those and report back for now.

Sent from AutoGuide.com Free App
 

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Discussion Starter #3
Replace the o2 sensors. Also swap the number 2 coil to another cylinder and see if the misfire follows it, if so replace it. Also check the exhaust manifold for leaks.

Do those and report back for now.

Sent from AutoGuide.com Free App
I have already replaced the #2 coil, and it didn't fix anything. I will try the other 2 items.

Thank you
 

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US Army 19yrs and ......
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^^ Great advise from Fixxer

What plugs did you use and the specs. These motors really like the Iridium plugs not the factory PLAT shits
 

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Discussion Starter #7
Check see if the cats are ruined. May have been caused by the missfire.
I was thinking that's why I am getting the P0420 code. Not sure how to check the cat, will do a google search. Thank you!

^^ Great advise from Fixxer

What plugs did you use and the specs. These motors really like the Iridium plugs not the factory PLAT shits
I replaced them with the Bosch Platinums. Do you suggest trying a different plug?


Also, can bad 02 sensors cause a misfire?

Thank you very much everyone :)
 

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You did not post your mileage, but..

For very poor, rough idle and slightly reduced driveability at 130K, members here advised changing not only the O2 sensors but the MAP sensor.
(I had fresh irridium plugs)

That did it for me. She began running more-or-less like new, with smooth idle.
So my point, in addition to what others have posted is replace the MAP.
It's cheap and can be replaced in about three minutes.


I've posted before that a mechanic I very much respect advises that O2 sensors are only really good for about 60K miles. After that they continue to function, but their accuracy is reduced, which affects fuel mileage, power, and overall driveability.

.
 

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For misfire I would be looking for a possible vacuum leak in addition to all else.
 

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Discussion Starter #10
You did not post your mileage, but..

For very poor, rough idle and slightly reduced driveability at 130K, members here advised changing not only the O2 sensors but the MAP sensor.
(I had fresh irridium plugs)

That did it for me. She began running more-or-less like new, with smooth idle.
So my point, in addition to what others have posted is replace the MAP.
It's cheap and can be replaced in about three minutes.


I've posted before that a mechanic I very much respect advises that O2 sensors are only really good for about 60K miles. After that they continue to function, but their accuracy is reduced, which affects fuel mileage, power, and overall driveability.

.
My apologies, I have 150k on the colorado and they look to be stock. I will look into getting these parts.

Thank you very much for the feedback!
 

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Iridium plugs came stock on the 3.7. AC delco 41-103 are good . Drive the truck with the o2 sensors removed for a short distance.
 

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If you have a miss on cylinder #2 and based on what you have done. Run a cylinder leakdown test. If you don't have access to a checker do a compression check. I would also replace the platinum plugs with some iridium plugs. These engines are funny about what plug it likes. Also an easy way to check your cats is with a inferred temp gun. The rear outlet of the converter should be 50 degrees or so more than the front. Like others have said do the O2's first but they wont cause your miss.
 

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If I were you get rid of the Bosch spark plugs. People have had issues with them. I have always ran the stock a/c delco plugs with no issues.

Also not sure what you are using for cleaning the fuel system. But I had a misfire code and tried the chevron fuel cleaner and it did nothing. Went and bought a bottle of redline fuel cleaner and after a half tank the misfire code was gone. That was 2 years ago. Now at every oil change so 6000km I add a bottle of redline cleaner. This may help you.
 

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maniac mechanic
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sorry I read it quick and missed the coil part. Also follow their advice on the plugs the coil system doesn't seem to like anything but iridium plugs.
 

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Discussion Starter #16
Thankyou very much everyone for the suggestions. I will try each suggestion through trial and error and hopefully I will get this solved. If it does work, then there is some valuable info for people with this issue in the future! :)
 

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Discussion Starter #18
I will be replacing my spark plugs with AC Delcos today as well as both o2 sensors. I will post my results. Again, thank you everyone!
 

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are you getting juice to the coil ? is it the only cyl miss firing ? hows your fuel press ?
 

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Discussion Starter #20
UPDATE: Okay, so I replaced both o2 sensors, and all of the spark plugs (with ac Delco's). Let me just say...changing the back o2 sensor was not one of the easier things I have done...you need some sort of micro hands for that job!! I guess the cat and o2 sensors were moved in the later colorado years.

Anywayssss.... everything seems to be running a little smoother. I will cross my fingers and hope that the light wont come back on. As for the rough idle... i am starting to think it's natural. I am currently idling at 550-650 rpms...is this to low? is that what is causing my engine to shake so bad?

Thank you for the help everyone!
 
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