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2014 Silverado misfire no code?


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I got 2014 Silverado with 5.3. Just changed my spark plugs because of a weird misfire under low rpm. Doesn’t do it if you get on it. Changed plugs and wires and was way over due. 150,000. One plug was a little wet and others were dry and all had carbon build up to the point not much gap anymore. Still does the same thing. Not throwing a code. Does it pretty bad at around 30 mph kinda like its dropping for 8 to 4 cylinders but misfire??  Any ideas?

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Might need to have your intake valves looked at for carbon buildup. DI engines are more subject to valve buildup because there is no fuel being sprayed on them to clean away the PCV byproducts like there is with port injection. 

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What modifications have you done to the vehicle? Was any recent work performed on it? Even if nothing was fixed, has anything been removed, loosened or unplugged recently? What did your old spark plugs look like? Black and greasy? Grey? Pitted?

The fact that it doesn’t occur under heavy load suggests a manifold vacuum leak. Since vacuum is highest at low RPM, if unmetered air is getting in, you may be going lean enough to cause a misfire.

At wide open throttle, manifold vacuum is low and your fuel ratio would richen. Both conditions might decrease the impact of a small manifold leak. Not saying this is the problem, but it’s something to consider.

Another possible cause is your oxygen sensors. If you haven’t replaced your oxygen sensors yet, they are long overdue. I’d purchase and replace all four of them at the same time before investigating the issue any further. They have a big impact on your AFR and if they are providing the ECM faulty data, you’re going to be chasing your tail trying to troubleshoot the issue.

If it were a problem with the injectors or fuel system, such as a clogged or failing injector or pump, it would seem the problem would get worse as the demand for fuel increases like it would under heavy load. (There are others here with more knowledge about the fuel system than I and they may chime in with more useful information).

The same goes for the ignition system. As more air and fuel enter the cylinder the more likely a weak spark would result in a misfire compared to at lower RPM.

Dirty intake valves may be contributing to the problem. If it were my truck, and I was still having problems after I had already addressed the other issues above, I’d pull the intake manifold and look at the intake valves before squirting anything on them. It’s not that difficult, especially if you spend $20 and buy the Haynes manual that takes you through it step by step.

If the valves are so dirty to obstruct the intake air flow, enough to cause a misfire, you may create a much bigger problem if you dislodge that carbon buildup and it’s ingested into the cylinder. The sprays have their purpose and place, but I wouldn’t do it if I suspected large chunks of carbon were casing an obstruction. If you find large amounts of carbon on the valves, you can pull the heads and clean them manually or you can media blast them in place through the intake ports. Other people have had success by soaking them in solvent through the intake ports also. All of these remedies require a meticulous approach to assure nothing enters the combustion chamber that shouldn’t be there.

Not every misfire causes a MIL. It’s recorded somewhere, you just need the right scanner to find the data. Good luck finding the problem. Please post a follow up once the issue is resolved to let us know what it was.

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I just re-read you post and see the info about the plugs. I also overlooked the part about being it’s worst at 30mph and feeling like a failure AFM to properly engage. You could have a lifter problem if it’s occurring specifically when the AFM kicks on or off. Lifter collapse is a known problem on the AFM cylinders. I hope this isn’t the case since it’s not a cheap fix. It might be worth pulling your valve covers and evaluating your valve train to look for a collapsed lifter.

 

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