Seeking edgeucation about the in cab EEC fuel tank system.

[71CHEVYSHORTBED402]

Decent write up here: http://what-when-how.com/automobile/...ms-automobile/

Some questions though.

1. The vent pipe assembly for lack of better term hangs from the cab's RH rear pocket. By chance is that a liquid-vapor separator, part of, or? Perhaps it's just a way to connect the fuel tank to the charcoal can.

2. The filler neck takes a hose connected to the top LH side of the fuel tank. Is that for the overfill limiter?

3. The above link reads:

most EEC caps "are incorporated with built in pressure-vacuum relief (Fig. 17.20), so that a vacuum lock may develop due to the fuel expansion or contraction.

and;

"The cap vents excess fuel tank pressure to the atmosphere in case the vapor delivery line is blocked"


3A. I've read the caps don't actually vent or perhaps some do, varying by year I suppose, 70 & 71 Vs 72. Clarification appreciated.

3B. My 1971 C10 NOS cap reads "VACUUM PRESSURE". Does that imply it's filling the tank with air as fuel is used and/or it's a pressure vacuum relief so that vacuum lock develops for fuel expansion or contraction as stated above?

3C. GM installed charcoal cans for the EEC system. Why would the cap need to vent considering the tank vented to the charcoal can? Seems to me this NOS cap doesn't vent.


Thanks

For what worth, fuel return lines weren't installed until 1972, though I suppose some 1971 had them. Some believe A/C systems or something like required a return line, but they weren't installed in 70-71 regardless of AC. In other terms, the fuel pumps and senders on the 71 and earlier had one tube each, the 72 models had two. A return line might make a difference for what cap was used, couldn't tell you. My cap is specifically for a 1970-71 C10 EEC system.

[tutone]

Subscribed.
I do remember when I bought my grommet for the filler neck for my 71, I had to specify if my truck had the system or not. Must be some sort of difference there as well.

[71CHEVYSHORTBED402]

Ttd

[geunther]

My understanding of the system: I am no expert but have studied it some as my 72 truck has it.

With the canister, the caps are usually not vented at least 72s from my understanding. So they are marked with the pressure / vacuum. Meaning they will develop both. No need to be vented here as it is vented through the canister.

The u shape bend (inside the RR cab) is just designed to prevent any liquid from the tank from finding its way up and over the inverted U. If this line is plugged, the tank will develop pressure/vacuum with the non-vented cap. The line should not be filled with liquid. Though vapors will be in this line and that dictates at times it could condense and become liquid.

I believe the little short hose next to the neck is unrelated to the canister system.

[HO455]

I believe the factory caps had a check valve in them. It allows air into the tank as the fuel level drops but it prevents pressure from escaping until it rises above 3 or 4 PSI. Hence the Pressure/Vacuum identifier.
Most later replacement caps don't have the pressure control valve as most later vehicles used a simple unvented cap. The later caps can cause the air whooshing out effect when removing the fuel cap.

[71CHEVYSHORTBED402]

Thanks a bunch. Here's an interesting illustration. I have the tank in the lower illustration. The upper vent tube actually takes three clips. Two as shown in the illustration, and one that clips onto the lower RH edge of the fuel tanks perimeter. Maybe I'll take some pics. when this is done, add them here. Thanks again.

http://67-72chevytrucks.com/vboard/a...1&d=1368198856

[71CHEVYSHORTBED402]

Couple pics of a 1971 EEC tank. Nice to have this done, original parts, I just cleaned them. I only need to run the lines to the canister, install a bench seat, battery and air cleaner and this should be ready to roll. Only took 6-7 years