A few times now I thought I'd figured out my P0456 code, but it eventually returns. I may have to get a smoke test to find the leak. Of more concern is that I have this intermittent loping idle on cold start. I can't reliably replicate it, but I can say that it happens on old start, and after I move the truck a short distance AND put it in park or neutral. A short distance is probably only about 30 feet. I just back up, and pull it up next to the garage to load something in it. What happens is that the idle will fluctuate to the point of it almost dieing, then revving up... the length of time it does it varies as does the severity. The first few times it ever did this, I just went back, gave a nudge of the gas pedal, and that corrected it. After it did it a few times, I started to wait it out to see if I could get a check engine light to come on, but it never does. At least one time I held the gas down a bit and it felt like it was still loping, but at a higher RPM.

So firstly, I want to know if the evap leak is maybe functioning like a vacuum leak, or if it can even be related.
If it's not related, what should I be checking? The throttle body looks clean. The plugs (Champion Coppers) only have around 9K miles on them, and I pulled one at random and it was fine. I replaced one coil pack because of a misfire code, but that wasn't really necessary as it was just water logged from a car wash.
I did notice an engine ground strap on the right side was laying on the exhaust manifold, and almost burned/corroded through. I could not see where it attached on either side, so I hadn't replaced it yet. Alternatively, I was just going to make a new ground strap and put it somewhere easy to access.

I would really appreciate some direction of what to look for. I have a bluetooth code reader with the free version of Torque, but I've never been able to go get it and get connected in time before the idle issue corrects itself.

 

Check and make sure you don't have any broken or loose exhaust manifold bolts. With an 03, you're overdue for new O2s.

 

Well, after looking at what I can find of the EVAP system routing, it's at least possible it's dumping fuel or air into the system. So I guess I have to look at that more, and maybe get a smoke test.

I did look or feel the exhaust studs and it all felt tight. Maybe I'll throw on some O2 sensors, but I might also just wait more for it to finally set a code. I hate just throwing parts at it, but the issue is so hard to reproduce and refuses to set a code other than P0456.

 

Download a copy of the Srvc Man from the links in my signature and use the diagnostic and troubleshooting steps as outlined in the section for DTC codes

 

Check the hoses coming out of the charcoal canister under the truck.

SJ650