Well, I don't think I made any difference this morning. Got the vent and purge valves swapped, the lines blown out. And won't really know if I did any good for at least a week. Tahoe's tank is 3/4 full, Missus won't be back until after the 4th.
Jacked and braced the vehicle, but just enough room for me to lay on the ground and hug the rear differential as I worked on and around the cannister. Most of the cannister connections face the fuel tank, so the back of my hand was working against the sharp edge of the gas tank shield. The evap line connectors are similar to the firewall heater hose connections, meant to be quickly stabbed on during assembly and a PITA in all other respects. They work like a gill net, two levers that angle in under a rim on the pipe connector. You have to squeeze and angle them open to wiggle the connector off.
Here's an overview shot. Gas tank on the left. Vent valve top of the image, rear diff vent hose in center foreground
The action on the couplers. I wound up using some large needlenose pliers and a long skinny screwdriver to manipulate the cannister couplers facing the gas tank
Vent valve removed, its coupler is at top left. My index finger points to the cannister nipple, after removing the coupler that runs to teh purge valve on the intake manifold.
This is blasting air down the purge pipe
Old and new. blew and knocked lots of talcum-fine dust out of the old vent valve, but it readily passed air out on the very first attempt.
I blew air in the cannister nipple I was pointing at, air came out the other nipple. Couldn't line up my air hose to do the reverse, so remounted the vent pipe and blew air in its end, nothing seemed to come out the purge nipple. This clashes with my understanding of how things should flow. No sign of any debris or charcoal grit, no material expelled by any of the air blowing. There's either a one-way valve in the cannister or some other blockage I'm unaware of.
I stopped there and put it back together. I wasn't going to go thru the trouble of dismounting the cannister without a new one on hand to install. Probably going to wait until the next attempted fill up before I even order the replacement. At 180k+ mi it probably ought to be replaced anyway. But I'm not in a hurry to spend a $100 on it.
I am bugged that air blown into the vent pipe did not come out the purge fitting on the cannister. That's a direction of flow that SHOULD be happening, as I understand the functioning of the purge system.
But air is flowing the other way, from the cannister out the vent nipple. So the tank ought to be breathing right when being filled.
balls. As I type this out I realize I never checked the short pipe section that connects the cannister to the vent valve. If it is blocked, that would explain why air blown into it didn't come out the purge nipple. AND it would correspond to the filling failures.
But I don't see how it could be. It sits at about a 30deg angle. Anything in it would run back into the cannister. Any crud in it would have resulted in a shower of junk when I ran air in the purge nipple. I did that while placing my finger right on the vent nipple and air was coming out and there was no debris. The pipe is about 14" long, 1/2" dia, thin wall plastic. I really don't want to go back out there and work on it again. And the exhaust pipe is hot now.
And ALL of it could be a blockage of the small rubber vapor vent line from the tank to the cannister, too. And I'm not getting to that without dropping the tank. And if I'm going through that evolution I'm changing the fuel pump too.
I'll probably pull the vent valve pipe this evening and run something thru it to make sure it is clear.
eta at this point I'm HOPING that the vent pipe is blocked. Even if it means the new valves were a waste of money. Because that's any easy fix. If the pipe is wide open it means a bunch more money and effort.
