The canister can leak, so can the filler cap, filler neck, fuel tank, fuel pump module O ring, vapor lines, (2) from the tank to the canister, the canister to the vent valve line, the vent solenoid, vapor line from the canister to the purge solenoid, the purge solenoid itself, and the seal for the fuel tank pressure sensor. The vent solenoid is a likely spot. It's located under the vehicle near the fuel tank and it's exposed to dirt and dust that can keep the valve from sealing. If the tank is over filled, (keep clicking the pump after it shuts off when filling) can cause liquid fuel to enter the canister. When that happens, pieces of the charcoal can make their way to the purge solenoid and keep the solenoid valve from sealing. If the canister is subjected to liquid fuel, it must be replaced. Keep in mind that you're looking for a leak about the size of a pin, or smaller.

 

This truck has plenty of dirt and grime underneath. It came from Alaska originally and somehow wound up in the lower 48 where I purchased it.

Is the vent solenoid the one above the spare tire on the frame cross member? If so, is there any way to test it apart from the entire system? I don't over fill the tank but there's no telling what the previous owners may have done.

I've got a year to fix this particular problem. I don't need an emissions test until April 2014. I'm hoping to fix it this week, though.

Thanks for all of your help!

 

If there's as much crud underneath as you say there is, it's a good possibility the vent solenoid is stuck open. Yes, the vent solenoid is on the crossmember just forward of the spare tire on your 4 door, and looks like this:



Without special test equipment, you'll need to remove it to test it.

 

Great! I already removed the solenoid bracket from the cross member so it should be easy to remove it and test it.

How do I test it?

 

On the solenoid, there are 2 pins in the connector: if you apply battery voltage to one pin, and ground the other, you should hear the solenoid click. The fitting should be completely sealed with no voltage applied, and should flow freely when voltage is applied.

 

I checked the vent solenoid out as instructed. It works perfectly. When I applied battery voltage the solenoid closed and it held both vacuum and pressure. It was normally open and closed with battery voltage, which is the opposite of what you outlined above. Since I'm pretty sure it's the stock solenoid I'm confident that's the normal operation.

So with that eliminated what's next? Does anyone have a vacuum diagram for the EVAP system? The EVAP diagram in the factory service manual only shows the vacuum lines under the hood. It doesn't show the lines going back to the tank and canister. If I knew which lines went where I could test the lines individually and hopefully find the leak.

 

I found a diagram of the EVAP system hoses. I'm going to check each of those for leaks and see where I can get from there. It would have been nice for the factory service manual to include something useful like that.

 

The solenoid is fine, and yes, that is correct operation When you say "factory service manual" are you referring to the GM manual or Chilton/Haynes/Mitchell/Alldata? Might want to try the same test procedure with the purge solenoid on the engine, (passenger side of the intake plenum, near the coil). That solenoid is normally closed and opens when you apply voltage and ground.

 

I mean the actual GM manual. I buy factory manuals for all the vehicles I own. The Chilton/Haynes is useful for holding up a short table leg or looking up torque specs for an oil filter.

Since I was able to find a diagram on the Internet I was able to check the purge line from the solenoid on the intake to the canister. I put the hand vacuum pump on the engine end and a vacuum gauge on the canister end. It didn't hold vacuum. In fact, the vacuum slowly bled off. I checked it eight times and got the same results each time. That tells me the purge line is suspicious.

I'll still check the purge solenoid on the engine just to make sure it's functioning properly. I'm also going to check the vent lines from the tank over to the canister. If I've read the diagram correctly I should be able to apply vacuum to the vent line from the tank to the canister and it should hold as long as nothing is leaking.

When this is all done I may write up a diagnostic process to solve this particular code when one does not have a proper scanner and smoke machine.

 

I agree on the Chilton/Haynes, also works well for a bird cage liner.

Applying vacuum to the canister takes quite a bit of pumping using a hand vacuum pump, (large area). You can isolate each line and check each one individually. Very time consuming, but you're on the right track. Gotta be careful when applying vacuum: When the PCM commands the EVAP monitor to run, it closes the vent solenoid and opens the purge solenoid for a given amount of time, (applies a negative pressure to the EVAP system). Then the purge solenoid closes and seals the system. The fuel tank pressure sensor measures the negative pressure in inches of water, (far more accurate than inches of mercury). There's a green tag on the EVAP test port that tells the pressure limits when testing the system for leaks.