P0300 Code On For Years and Years...

[Bxcliff]

So my 2003 GMC Yukon Denali that my family and I had since 2004 w/ 181k miles has had an intermittent CEL for years, actually for as long as I can remember. The thing is, the truck dives totally fine, the engine light comes on only when I take it on a highway (about 60 mph) and it would start blinking. I've driven this truck out of town on 5-hour road trips like this and there is no problem, so for years, I left it. I should mention it is straight piped (Muffler delete only) but the light was on before we went straight pipe.

Now that I have time and money I've been fixing all the little things wrong with my truck and a while back I tried to fix this CEL and it scanned P0300 multiple misfires and nothing else. I changed the plugs and wires, along with a new throttle body from a previous issue I had (Engine was going in limp mode due to throttle position sensor) and fixed it but still a blinking CEL and P0300 code.

I don't want to start throwing new parts at it, but I wanted to do some testing and see if I can find what exactly is bad or needs to be replaced. Anyone who knows or has been through this before, do you have any advice or things I should look into? Any help is appreciated

 

[gooffeyguy]

slight vacuum leak somewhere

 

[Bxcliff]

slight vacuum leak somewhere

Could that make a sound when I turn the engine off? I've noticed it sounds like a vacuum or air being released whenever I turn the truck off, only if I'm outside the truck using my remote starter or if someone else does it.

 

[mdmcinti]

Could that make a sound when I turn the engine off? I've noticed it sounds like a vacuum or air being released whenever I turn the truck off, only if I'm outside the truck using my remote starter or if someone else does it.

My wife back in the day used to have an 05 Silverado with the 4.3 v6. It had a random misfire cel flash once getting onto the freeway. Every time after we were done driving the truck, we would notice an air leaking sound like if air was leaking out of an inner tube or something like that. I pin pointed it to the brake booster vacuum line where it goes into the booster. I could never get it fixed, but I wonder if you have that same issue.

 

[S33k3r]

I have been made aware of a TSB for our intake manifolds. There's a plastic section that can crack and cause this. Might try there. I am investigating something similar on my Silverado (this is for both 5.3s and 6.0s, though mine is a 6.0), but haven't had time to thoroughly dig into it yet. I was turned onto this by someone in another group to which I am also a member. Good luck, and I'm interested in hearing if you figure it out.

 

[Tonyrodz]

I have been made aware of a TSB for our intake manifolds. There's a plastic section that can crack and cause this. Might try there. I am investigating something similar on my Silverado (this is for both 5.3s and 6.0s, though mine is a 6.0), but haven't had time to thoroughly dig into it yet. I was turned onto this by someone in another group to which I am also a member. Good luck, and I'm interested in hearing if you figure it out.

I've heard the same thing about the plastic LS intakes in general, that they crack. I've had an 05 Express van a little over 2 yrs with a P0300 from day one.

 

[Bxcliff]

I have been made aware of a TSB for our intake manifolds. There's a plastic section that can crack and cause this. Might try there. I am investigating something similar on my Silverado (this is for both 5.3s and 6.0s, though mine is a 6.0), but haven't had time to thoroughly dig into it yet. I was turned onto this by someone in another group to which I am also a member. Good luck, and I'm interested in hearing if you figure it out.

So what I'm thinking now would be to do a smoke test and do some digging around to find a possible leak. If the intake manifold does happen to be cracked I know ill have to replace the whole thing but would it be wise to change out the knock sensors/harness too?

 

[Tonyrodz]

So what I'm thinking now would be to do a smoke test and do some digging around to find a possible leak. If the intake manifold does happen to be cracked I know ill have to replace the whole thing but would it be wise to change out the knock sensors/harness too?

I def would--you know, "while you're in there".