I got an updated 87 octane tune from Nelson the other day. He didn't disable the P0300 code so I keep getting a misfire DTC. Shortly after that, my truck started running like I lost a lot of power and it started hesitating a lot.

Now, it will hardly start and once I get it started, it will die if I press the gas.

I talked to Allen and he said that the P0300 DTC wouldn't do that, check the spark plugs and front O2 sensors. I changed all that and disconnected the battery to reset the faults. It's still the same. The spark plugs did need to be changed, there was a lot of buildup on them.

What else can it be? I have 155k miles and 1 stock cat. I think that may be it but how do I know for sure? I'd rather not have my truck towed to a exhaust shop. Could I take the Y pipe off and test it somehow?

Thanks,
J.R.

 

just for a quick check you could unbolt the collector and start it with open headers, that same problem happened to a guy I know, we were amazed at how horrible it ran with a clogged cat.

 

Dustin

 

I was thinking about that but I figured that with the O2 sensors out of the loop it wouldn't run good anyway.

Thanks.

 

Okay, it's not the cats... I dropped the exhaust and it's still the same.

I did get it to run okay for a few minutes by wiggling a black canister that's on top of the intake manifold. It has a couple of wires on 1 plug and a black plastic line that runs along the manifold and down between the engine and firewall. It has a green cap in the middle of the line. Does anyone know what this it? I took it off to see if there was a part number or something and now it's back to running like

Any other ideas?

 

that canister is the evap.

did it run fine before the tune?? Do you still have the other tune??

 

Yes, it ran fine before the tune. I don't have the old tune but Allen is sending me a new one. He doesn't think that it's the problem but I'm not too sure.

What is the evap for? Would it cause this problem?

 

Any other suggestions?

 

I thought that if ur evap was low ur truck would run like ****? and it was supposed to have about a pound in the line? I may be talking out my *** but Im pretty sure I heard that somewhere?

 

I have no clue what the evap does. How do I check?