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Discussion Starter #1 (Edited)
2010 Traverse 175K miles. Engine has ran well its whole life. Lately it sounded like a tractor on cold start (put-put kind of sound). Tonight it started rough for the first time with mild shaking that smoothed out quickly. Check engine light on with codes PO300, PO304, PO305, PO306.

Would it be unlikely to be coil or plugs with multiple cylinders misfiring? I can do some repairs like change spark plugs and coils (spark plugs changed at 120k miles, but original coils). I could start there (coils) if there was a reasonable chance of success...can bad coil cause multiple misfiring cylinders or do I need a mechanic do you think? thanks
 

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I'm no expert but it could be a number of things causing those misfire codes & diagnosis isn't always easy, but the plugs & coils is a good starting place. Could also be fuel quality related, or maybe even the fuel pump. Could be there is a vacuum leak somewhere. Might also check the electrical connections & wiring harnesses. Just some thoughts on where to start, hope you get it figured out!
 

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carefully take a look for signs rodents were in the engine bay (little paw prints on dusty areas). Check her damaged wiring
 

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Look under the top engine cover. This is the place where they built up their nesting on mine last year.
 

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Discussion Starter #6
I looked around- I don't think it is mice. Last year I put in some new spark plugs that were not OEM. After reading around the net I am going to try going back to AC Delco. Some people report other brands can cause electrical probs that result in misfires.
 

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Jk,
Since you are going to remove your intake manifold,
And you would probably replace the $20 intake manifold gasket when you reassemble your traverse
All to replace spark plugs.... ^^^^

You might consider walnut blasting to remove the carbon on the intake valves. In theory each intake valve should take a half hour, so that would be three hours.....


I believe carbon from intake valves on gdi engines,
Falls off intake valves, and could cause misfire via the process known as Low Speed Pre Ignition. (Misfires)p
The carbon, which is as hard as lava rock,
Could scratch the combustion chamber walls which might result in oil consumption.....
Who knows?
https://www.enginebuildermag.com/2017/10/solving-gasoline-direct-injection-issues-facts-fictions-gdi/


To be clear, I am not saying your misfires are caused by carbon on intake valves, I agree with you about using a different brand of spark plugs.....

Do not use a running engine treatment like crc or BG to remove carbon from intake valves, that will damage your cylinder walls, or could damage your catalytic converter.

No matter what, while you are changing your spark plugs, at least look at the carbon build up on intake valves ......
Take a few pictures , and post them here.....
 

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Yes, please take a few pictures and post, I haven't gotten to do mine yet but hope to have a look-see soon. I bought one of those Bluetooth cameras so I have no excuse. One solution to the carbon buildup is installing a catch can in the Air intake/PCV system. Filtering out the contaminants before they get to the valves would help solve the buildup.
 

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Discussion Starter #9
Sorry no pictures available- I am up here in MN and do my work in the driveway. Snow was forecast so I took the vehicle to a shop. They replaced spark plugs back to OEM, cleaned MAF and throttle body and threw some injector cleaner into gas tank. So far it is running ok with no further misfires. For the past 10,000 miles or so it seems to idle rougher and noisier than it used to and then finally went on to so rough it misfired. I think the carbon build up thing is probably going on here. It is not misfiring since I had this work done, but it still sounds rougher than it did when newer and I don't have total confidence we will get a lot more miles out of this vehicle.
 

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I would strongly suggest the OEM spark plugs and replacement coils. Also, when you have the air intake apart, be sure to clean out the oil sludge that accumulates from the PCV. The snorkel tube will collect all the vaporized oil from the crank case. A thorough cleaning while you have full access to the air intake is a good preventive measure. Also, be sure to stuff rags in the intake manifold to ensure NOTHING falls into the cylinders when you service the air intake. When I was having issues with vehicle hard start, rough idle, and would buck at ~35 mph unless accelerator was pushed all the way, I thought I had major issues. It turned out to be plugs / coils. The engine design does not allow for trouble shooting of a coil / plug which would be done while engine is running. So it is better to simply change them all while you have the upper portion of the engine exposed. If the coils could be accessed when the engine were running you could unplug a coil and diagnose a faulty plug / coil with ease. However, you can not do that on this 3.6L engine, therefore, when in doubt, change it out! Coils and plugs are not the items you should be "going cheap". It is a full afternoon just to access them and the engine controller is too good at masking this problem by playing with timing, fuel map, shift points, etc. You need to rule out the coils and plugs as an issue before moving on to anything else engine related. Your engine will thank you.
 

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Valvoline has a Synthetic Oil for Modern Engines. It claims to help keep the carbon from building up valves on Direct Injection engines. It doesn't say that it cleans the deposits already built up but helps prevent it from happening. I wonder what additive they are putting in, I am sure that there is probably something on the market to add to your oil which claims to so the same thing. Walnut blasting is probably the best process to clean the valves with buildup and PCV, and a Catch can would help to filter out the nasty stuff.
 
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