740 Smog Cannister Valve and ports

[waynegarbo]

I am interested in testing the function of the ports and check valve on the Smog Canister. So far here's what I've got and am looking for confirmation or correction please.

I did a vaccum test on the smaller port. Pumped it to about -15 and the valve held up. This confirms the valve works but is there
another test to confirm the gas port into the canister and the larger port that draws the vapors out is indeed in sync? How would I know if the smaller vac valve is stuck?

What I read was the small port is the valve that controls vaccum and the flow of gas vapors from inside the canister to be purged and drawn through the larger port that gets drawn out and into the intake. When I did a vac test on the larger port that purges the vapors out, there was no neg pressure. I am assuming that is suppose to be an open port when the check valve opens. But what about when the check valve is closed? Should I be getting a neg pressure reading on that larger port out of the canister?

I know the lower port receives gas vapors from the gas tank which pressures vapors into this port and into the canister. Is there a test through this port I'm suppose to check?

Thanks.

[93Regina]

Smog Canister.

Evaporative Emission Controls


Canister Purge Valve

Remove the inlet hose to the valve.

Connect a hand-held vacuum pump to the fitting and pump up vacuum.

If vacuum holds, valve is ok. If vacuum falls off, valve diaphragm is leaking, replace the valve.
=================

Evaporative control system

The 740 is equipped with a gas-evaporative control system, which prevents gasoline fumes from being released into the atmosphere.

The system is comprised of an expansion chamber in the fuel tank, a roll-over valve on the cross member in front of the fuel tank, and a charcoal canister with built-in vacuum valve under the left-front wheel housing.

The components are interconnected by hoses which channel fuel vapor from the gas tank to the charcoal filter, where it is stored until the engine is started and then drawn into the engine's fuel-induction system.

[waynegarbo]

Smog Canister.

Evaporative Emission Controls


Canister Purge Valve

Remove the inlet hose to the valve.

Connect a hand-held vacuum pump to the fitting and pump up vacuum.

If vacuum holds, valve is ok. If vacuum falls off, valve diaphragm is leaking, replace the valve.
=================

Evaporative control system

The 740 is equipped with a gas-evaporative control system, which prevents gasoline fumes from being released into the atmosphere.

The system is comprised of an expansion chamber in the fuel tank, a roll-over valve on the cross member in front of the fuel tank, and a charcoal canister with built-in vacuum valve under the left-front wheel housing.

The components are interconnected by hoses which channel fuel vapor from the gas tank to the charcoal filter, where it is stored until the engine is started and then drawn into the engine's fuel-induction system.

I already know this. No one has clarified which port is the valve. And I am asking if the larger ports are tested to confirm the valve is in sync.

[93Regina]

I already know this

OEM vehicles comes with an emission sticker, showing how it routes hoses.

PIA, you manually check each hose's fitness...

RE: I am asking if the larger ports are tested

Put a vacuum on diaphragm port...then see if air passes thru diaphragm unit, and does not without a vacuum. Use your mouth's air to provide postive air pressure.

[waynegarbo]

I already know this

OEM vehicles comes with an emission sticker, showing how it routes hoses.

PIA, you manually check each hose's fitness...

RE: I am asking if the larger ports are tested

Put a vacuum on diaphragm port...then see if air passes thru diaphragm unit, and does not without a vacuum. Use your mouth's air to provide postive air pressure.

Yeah I put vaccum on the diaphram port and blew through the gas inlet port. I could hear air flow from the pcv port and a little hot air up to my ear but I don't feel a significant flow

[waynegarbo]

I already know this

OEM vehicles comes with an emission sticker, showing how it routes hoses.

PIA, you manually check each hose's fitness...

RE: I am asking if the larger ports are tested

Put a vacuum on diaphragm port...then see if air passes thru diaphragm unit, and does not without a vacuum. Use your mouth's air to provide postive air pressure.

Yeah I put vaccum on the diaphram port and blew through the gas inlet port. I could hear air flow from the pcv port and a little hot air up to my ear but I don't feel a significant flow

Another thing, my gas tank doesn't pressurize. 2yrs ago when my car past smog it was this way. The tech said I had a leak but he passed the EVAP. I have been under my car and all my hoses are connected and I don't see anything buggered up. Even at that
two way valve underneath, it was hooked up. The hose is hooked up to the side of my filler neck. I did change my sending unit seal. And all those hoses are hooked to and from the sending unit. No rust or corrosion.

[93Regina]

Another thing, my gas tank doesn't pressurize.

Try reading this: Evaporative Control System (Charcoal Canister)


"On 1974 and later models, the expansion tank was relocated inside the fuel tank and an equalizing or balance valve is used to regulate pressure buildup in the fuel tank. When the expanding fuel vapors create too much pressure in the tank, the valve opens and allows the vapors to be fed into the charcoal canister. When too much vacuum is present in the fuel tank (due to sudden cooling or low fuel level) a second valve opens and allows air from the venting filter back into the fuel tank to prevent the tank from collapsing. The equalizing valve also prevents fuel from entering the vapor line during hard cornering."

I have no idea where this valve is located....maybe its the Rollover Valve....diagram suggests a vent is within it...see TB thread: Evap Pressure test specs for 1991 740T?

I think TB thread will explain it for you