New to us 2002 Yukon XL Denali, EVAP Code and Hard Shifting.

[Gentleman_Giant]

Hey everyone,


So I just recently purchased a fairly clean 2002 Yukon XL Denali with 200,000 miles on the ODO and, to no one's surprise, we got a check engine light a week later. I'm going to troubleshoot the issues myself but I wanted to see if I could get some pointers from those who are more knowledgeable than I.

I don't have a fancy OBD2 tool, just a cheap RepairSol2 I bought from autozone years ago. It will give me codes and some metering.

When I pull the data from all systems the first thing that pops up is a P0446 Evaporative Emission System Vent Control Circuit code. This is the biggest issue with the vehicle that I'm hoping to gets pointers on.
The few things I know so far:

1. I don't smell fuel anywhere around the vehicle after driving around town (roughly 20min average drive time) so I don't think it's a stuck open purge valve.
2. I watched the feedback from my TPS, MAF, and other intake sensors, but they all look normal to me. No sudden spikes or unexpected readings.
3. I have noticed that the transmission will intermittently shift harder than normal, which kind of has me thinking a sensor is providing bad data to the PCM, but I'm just guessing (and hopeful that the transmission is symptom of the EVAP issue and not a transmission issue itself...).

I also have two faulty wheel speed sensors that I don't think are related, but I'll mention it just incase.


And that's pretty much the gist of it. I'm sure I can get it figured out in time, but if you guys can help point me in a direction and save me from wasting money by throwing parts at it, I would greatly appreciate it. If I need to provide any other details, get readings, or perform other tests just let me know. I monitor my email frequently so I should respond pretty quick.

Thank you.

 

[Scottydoggs]

the vent solenoid is under the truck near the gas tank, its a 30-40 dollar part. take ya 15 minutes to swap tops. loads of vids how to change it on youtube.

evap code has zero to do with hard shifting. thats generally a trans issue. like internal.

the speed sensors, follow the code for what ones are setting the fault and replace em. another $30-40 a pop.

 

[Marky Dissod]

... transmission will intermittently shift harder than normal, which kind of has me thinking a sensor is providing bad data to the PCM ...

Carefully & gently clean the MAF.
Thoroughly check the TPS sensor under duress. If it does not perform admirably (not adequately), replace it.

 

[Joseph Garcia]

Welcome to the Forum from NH.

Lots of knowledgeable folks here who freely share their knowledge, experiences, and perspectives. Knowledge is power.

I hope that you will become a participating member in the Forum's discussions.

Pics of the truck, please.

You are already receiving sage advice from the knowledgeable folks on this Forum.

The evap code issue is generally a result of folks trying to squeeze that extra pint of gasoline into the truck, after the pump automatically shuts off. DON't do it, and you don't know if the prior owner did. If this turns out to be the source of your evap code issue, you may have charcoal pellets invading the rubber hoses connected to that system, and they will need to be blown out.

 

[Gentleman_Giant]

Welcome to the Forum from NH.

Lots of knowledgeable folks here who freely share their knowledge, experiences, and perspectives. Knowledge is power.

I hope that you will become a participating member in the Forum's discussions.

Pics of the truck, please.

You are already receiving sage advice from the knowledgeable folks on this Forum.

The evap code issue is generally a result of folks trying to squeeze that extra pint of gasoline into the truck, after the pump automatically shuts off. DON't do it, and you don't know if the prior owner did. If this turns out to be the source of your evap code issue, you may have charcoal pellets invading the rubber hoses connected to that system, and they will need to be blown out.

That makes sense, the squeezing for that extra pint. I'll see if I can get underneath it this weekend and give it a look and a blow.

Posted some pics of the rig below. Not too bad of shape for +200k miles.