gas cap, emmissions, etc...

[sactoC10]

I'm trying to get some answers and according to this bit of information, the non-vented gas cap on my 1971 truck should not be a problem...in fact according to this I should have a non-vented cap. Can anyone explain further why my non-vented cap is creating suction in my tank?

EVAPORATIVE CONTROLS
Gasoline evaporates quite easily. In the past these evaporative emissions were vented into the atmosphere. 20% of all HC emissions from the automobile are from the gas tank. In 1970 legislation was passed, prohibiting venting of gas tank fumes into the atmosphere. An evaporative control system was developed to eliminate this source of pollution. The function of the fuel evaporative control system is to trap and store evaporative emissions from the gas tank and carburetor. A charcoal canister is used to trap the fuel vapors. The fuel vapors adhere to the charcoal, until the engine is started, and engine vacuum can be used to draw the vapors into the engine, so that they can be burned along with the fuel/air mixture. This system requires the use of a sealed gas tank filler cap. This cap is so important to the operation of the system, that a test of the cap is now being integrated into many state emission inspection programs. Pre-1970 cars released fuel vapors into the atmosphere through the use of a vented gas cap. Today with the use of sealed caps, redesigned gas tanks are used. The tank has to have the space for the vapors to collect so that they can then be vented to the charcoal canister. A purge valve is used to control the vapor flow into the engine. The purge valve is operated by engine vacuum. One common problem with this system is that the purge valve goes bad and engine vacuum draws fuel directly into the intake system. This enriches the fuel mixture and will foul the spark plugs. Most charcoal canisters have a filter that should be replaced periodically. This system should be checked when fuel mileage drops.

[Harold Shepard]

Look at the post I left at the end of your previous post. The canisters are still available from GM for $40 or less. I put a new one on my restoration. Hope some of this helps. Harold

[Tx Firefighter]

Didn't you disable your canister and the rest of the Evap system? That throws everything out the window.

If so, just use a 70 model gas cap wich should be vented. That should stop your vacuum problem.

[sactoC10]

Tx,

Thanks, maybe this is the simplest answer. Since most of the emissions system has been scrapped, then what remains should simply be treated as a non-emissions set-up, thus...need a vented cap.

But what does the set-up on pre 70 non emission vehicles look like?

Hmmm.

[Tx Firefighter]

Originally Posted by sactoC10

Tx,


But what does the set-up on pre 70 non emission vehicles look like?

Hmmm.

Nothing. There is a vented gas cap on the tank. One fuel line runs from the tank to the fuel pump. Then another line from the pump to the carburetor.

There is only 1 fuel line running down the frame rail. No charcoal canister, or the vent loop deal that goes up into the back of the cab. No secondary fuel line on the frame.

Disconnect all that if you want, and put an earlier vented cap on.

[crustysarge]

Nybody got a P/N for the charcoal canister?