MY 85-86 evap/tank vent system
 

The gas tank on my 86 collapsed recently and I thought it would be a good idea to figure out why. I have taken everything apart and blown out all the lines and did not find anything clogged or restricted. It appears that the collapse occurred because the EVAP system was pulling air out of the tank faster than it could be replaced and/or the rollover valve on the vent line to the outside world (located in the right rear wheel well) also acted as a check valve. There are two vacuum operated valves in the MY 85-86 system that that act in tandem to pull fumes from the canister. Valve 1 is located near the coolant reservoir and valve 2 is on the strut brace. Valve 1 appears to open under slight vacuum. I don't have a clue how valve 2 works, but it does have a lot of hoses connected to it.

The top of valve 1 - where the vac line fits - had popped off the base. Remarkably, the diaphragm was still in place and the spring that fits between the top and diaphragm was nearby. Valve 2 appeared to be intact. Since valve 1 was apart, the mystery appears to be solved. Nonetheless, after going through my service manual and the MY 86 technical supplement, the only information I can find on how the system is supposed to work, or how to diagnose it, is a diagram for the MY 87 and up cars where valve 2 is replaced by a solenoid valve driven by the engine control unit.

A new valve 1 is on its way and will be installed. My question is whether anyone has any info on how the system is supposed to work or has any info on any tests that can be done to make sure it is working as it should? I would prefer something more refined than watching the tank to see if it collapses again.

Thanks for your help.

Hi Otto, this is how I believe it works

Valve 1 (near the coolant tank) is the main one connected to the Fuel Tank via the charcoal filter in the Right fender well, it only gets vacuum to open when the coolant temp reaches operating temp by that supply coming from the rear of the Throttle body. and being switched by the temp valve on the back of the 1-4 head.
Once the temp valve is open the tank can be vented but only when Valve 2 (the one on the support brace) tells it so. This is determined by vacuum from the 6-way variable vacuum feed that also feeds the FPR/dampeners, EZF and AutoTrans (if equipped).
When it does open the fuel vapes are sucked through the Intake for re burning, IIRC this only happens at "cruse" vacuum conditions, not WOT or idle.

Most of it can be removed and plugged up as it is all a source of leaks and just rerouted back to the tank vent in the fender well (or by just plugging the hose that supplies Valve 1 (it goes into the fender).
I have it by-passed and plugged on two of my cars, the Twin Screw and a moded manual.

Edit: I thought I would add, on the S4 and later MY's the ECU takes over in-place of a vacuum Valve 2 (on cross brace) and an electric (valve 2) is used (sits above water pump on the block. ;)

Dave K

Taking your lead, I have plugged up the hose to valve 1 and capped off the hose from valve 1 to the mysterious control valve on the strut brace. Just for jollies, I tested the mysterious valve #2 on the strut brace with my mityvac and it held vacuum, so that would appear not to be the culprit.

thanks for the info.