Evap Gone Wild!

Evaporative Emissions Systems stink, and that's when they're working correctly. There is some complication to them, but they serve an important purpose: give a hoot, don't explode.

When your fuel tank gets hot (like sitting all day in an asphalt parking lot in August) the liquid gasoline is trying very hard to becomes gaseous. Without a way to vent the tank, the tank could bulge or spring a leak. But just drilling a vent into the tank is a poor method as well, and dumping combustible gas on the ground is not only throwing your money away, its a real hazard for anyone nearby. (Flick a butt under your are and watch all the fuel vapors ignite in a flash: FOOP!) "Am I missing an eyebrow?"

The VW evaporative system (which has evolved, but is still in use in today's cars) allows the fuel tank to vent flammable gas out of the tank safely, first to long copper tubes that travel out to the sides and then back to the rear of the bus, then up to condenser chambers in the air inlets at the rear corners. Here it has a chance to re-condense and run back down into the tank. If the vapors are quite thick, they'll travel down again, joining up and entering a holding tank.

That tank is the carbon canister. Depending on your year and model, it may be in a few different locations in your engine bay. Thus, you don't waste fuel by venting it to the air around your car, your car doesn't stink because its sitting in a haze of its own fuel, and you don't deform your fuel tank because you give the fumes someplace to go. And you don't explode.

Of course, there are two assumptions here: one is that all of your evaporative emissions hoses are in place and that they 'air tight' so you're not leaking fuel fumes in to your engine compartment, or worse, into the cab of the vehicle.

The other assumption is that your carbon canister is up to the task of trapping the fumes. If it is plugged up or the carbon is just OLD, it can't do its job. VW recommended replacement of the sealed canister every 48,000 miles. You can imagine how often this maintenance got skipped.

The real tender part is that all of those blue, red, green and purple hoses have to be replaced because they've  been rotting for more than three decades. The purple ones are especially exciting, since they are your fuel fill hoses and if one of them fails, you can take a left turn a little too fast and leave $20 of fuel on the ground.

I went through the whole gymkhana of replacing all of these hose links with Ethanol resistant hose, so I won't need to do that again for many years. Only the blue rear three hose links and the red hose can be replaced with the engine in situ ; all others require the engine to be removed first. Amazingly awkward.

I was especially pleased to find that both of the purple hoses were in very bad condition, so the effort was worth its weight. Once the fuel level sensor (on backorder for the last month, dammit) gets here, I'll get that screwed in and the fuel tank can be remounted and strapped back into place. Finally, the firewall can go back in and we'll be back to just dealing with the engine. As if that isn't enough.