P0300 becomes p0307

>p0300 becomes p0307
>no cel but rough idle
fuck

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Vaccum leak / air sensor causing bad A/F ratio? Cylinder 7 might be particularly succeptable, but if you're getting rough idle and codes that bounce around cylinders then it's probably something that affects the entire engine.

I bought the truck (sierra 1500 w/ 5.3vortec) used yesterday and I think the dealership cleaned the engine bay. The problem is it's cold enough to freeze hell up here. Could ice be an issue? It runs smoothly when it warms up but still has a rough idle at traffic lights.

youtube.com/watch?v=OyvdWeyDHf4

Once it heats up it would be idling based on O2 sensor readings ("closed loop"), so you'd expect it to smooth out unless something is so far out of whack the ECU can't learn a workable fuel trim.

If it's rough when cold it would be running open loop (not paying much attention to the O2 sensor) and on some sort of cold map where it's supposed to be adding extra fuel and probably just making fueling assumptions based mostly on the air sensor and throttle position sensor or something like that. So it's also possible you could have a coolant temp sensor issue that's causing it to not use the fuel-enriched cold map, or that it has learned wacky fuel trims fuel trims. A couple dozen highway miles when warm should get it to relearn good fuel trims unless there's a sensor problem or vacuum leak or whatever that's causing the ECU to become confused.

>dealership can't figure out problem with a vehicle they specialise in even with sealer exclusive diagnostic tools
>Dealership puts up ad for "great deal that just needs a MAF sensor or something lol our mechanics paid to fix our vehicles just didn't have time XD"
>OP snatches that shit up and gets down to business with his $10 ChinaBest scan tool and YouTube comment repair instructions

Top lol son

it was perfectly fine when I test drove it in equally cold weather

Srsly tho if you just bought it from a dealer and it's not running right, that's a prime opportunity to take it back and tell them they need to make it run right.

I'm going to. I just don't understand how it can go from being perfectly fine one day to being defective now

Maybe it's actually intermittent and just didn't show up before?

If it is a sketchy MAF or something it might only happen when the learned fuel trim gets too far out of whack and the rest of the time the ECU has things running close enough that it doesn't misfire. Or maybe you just got really lucky and something managed to pop off or flake out since yesterday.