Well I replaced the collapsed tank with a good used one but didn't take the time to figure out why so much vacuum had developed. I knew that the problem still existed because letting the car run for half an hour or so caused a fairly high vacuum to develop in the tank and a great whooshing noise would result when I removed the fuel filler cap.
A couple of weeks ago I made my final repairs to the car in order to get it back into daily service so I needed to figure out what had happened.
I got the car on ramps and traced all the fuel lines between the tank and the engine compartment. It all came down to the check valve and tubing to the charcoal canister so I disconnected the tank end and started troubleshooting with my air compressor and a MityVac pump.
When I removed the left fender splash shield to follow the vent line the problem became obvious. See the pics.
In the second instance I've seen, the charcoal canister behind the left fender rusted, retained water, and rusted some more. Whatever contains the charcoal bits inside disintegrated allowing the charcoal to get sucked into the fuel tank vent tubing.
I had a spare charcoal canister on hand from the 89 300E parts car but I'm *so* glad that I didn't just install it and call it a day. The problem would have continued...
Using compressed air from the tank end I tested the vent line and found that it was full of charcoal bits and completely plugged. It took quite a bit of carb cleaner and repeated high pressure blasts to clear all the junk out of the vent line.
With that done and everything reassembled the tank now operates at only a slight vacuum level as designed.
Watch out for this, especially if you live in wet/rainy climates. A disintegrating charcoal canister can really cause some serious problems. It's easy to partially remove the left inner fender splash shield and see if the canister is developing rust on the top surface. The canister has a lip that holds water very nicely and that can cause a lot of grief.
