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'81 fuel vapor return line

248 Views 4 Replies 2 Participants Last post by  Michal, 
M
I now have the body off the old chassis and can see how the fuel and vapor lines are routed. The canister that is tucked into the driver side fender well is, according to my assembly manual, a vapor canister for fuel vapor collection. When I got the car, this canister's purpose mystified me because it wasn't connected to anything. Most of the emission stuff had been removed, I had assumed this thing was somehow part of that system, and was no longer needed. On close examination of the fuel and vapor lines, I've discovered the fuel vapor line going from the front of the car to the fuel tank, on the driver side, was also not connected to anything at the engine end of it and had been smashed flat for a couple inches, probably to block it off. My question is, considering the current state of all the emission stuff removed, do I actually need the vapor return line to the fuel tank on the driver side? Since it was obviously not being used, I'd like to just cap the nipple it went to on the tank, eliminate that line, and remove the vapor canister that was also not being used. I don't know if that will adversely effect the engine performance. I wouldn't think so since none of it was connected anyhow, but I'd like an opinion from a more experienced mind.
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C
Well the vapor canister collects gas tank fumes when using the stock "unvented" cap then draws "purge air" through the canister into a "ported" carburetor connection to burn off the collected fumes. If the carburetor "ported port" connection is plugged then there is no effect when the carb and engine are well tuned. But once pressure builds up in your fuel tank the unvented fuel cap will relieve pressure and fumes possibly into your garage and smell it up. I don't think the charcoal vapor cannister (CVC) has much effect but it should be connected to a ported cab connection when used.
Now the PCV works similarly but has a much greater effect on the carbs tune and engine low RPM running. But we are discussing that here unless you request so.
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M
Now that you mention the PCV, I forgot to mention that there was none on the engine. It had the two crome caps on the valve covers which I replaced with a PCV on one and a small filter on the other. I used the small filter because there was no place to hook a tube the the air filter. I have found out, since doing this little detail, that the filter housing does have a knock out to put a connection onto, but I already had the small filter by then and decided to just leave it that way. You mention the stock, unvented fuel cap. Is there a vented version available or can the stock cap be modified? Thank you for the reply.
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Michal said:
Now that you mention the PCV, I forgot to mention that there was none on the engine. It had the two crome caps on the valve covers which I replaced with a PCV on one and a small filter on the other. I used the small filter because there was no place to hook a tube the the air filter. I have found out, since doing this little detail, that the filter housing does have a knock out to put a connection onto, but I already had the small filter by then and decided to just leave it that way. You mention the stock, unvented fuel cap. Is there a vented version available or can the stock cap be modified? Thank you for the reply.
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Yes, I'm using vented cap on my tank which came with a un-vented cap. Major vendors like ZIP or Corvette Central should have them but to avoid shipping I'm sure AutoZone, NAPA or O'Reilly's can look one up and may have them in stock.

So the way the Positive Crankcase Ventilation system works is uses a clean filtered air source into one of the valve covers. Then the other valve cover will have a grommet holding the PCV valve which connects to the carburetor. I recall this is not a ported connection on the carb. So the carburetor is somewhat tuned for crankcase gases that are somewhat metered by the PCV valve. You will read about many owners having problems getting the correct PCV valve such that the engine will idle and low speed RPM correctly/without studder. You can even buy adjustable PCV valves and I've read that was a solution for some owners. I haven't had a problem with this yet but you should beware of it.

Good luck.
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M
That sort of sounds like the small filter in the second valve cover should be sufficient. As to the PCV being correct or not, I'll find out when I can start the engine again. Thanks for the info.
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