Under hood canisters

[Stitches upholstery]

Hi. I was wondering if anyone knows what effects, if any, there would be from unhooking the vacuum lines from the canisters behind the driver side headlight(my car is a 78 deville)

Thanks

Trevor

[Jon S]

The canisters are vacuum storage devices that enable the vacuum system to maintain vacuum pressure when the engine is off.  Disabling them would cause headlight covers to open (if so equipped), parking brake releases not to work unless the engine was running and havoc with the heating/ventilation system.

Why would you want to disconnect the canister?

[Stitches upholstery]

What I was concerned with was them being worn out/broke and creatin a vacuum leak.

[Jon S]

What I was concerned with was them being worn out/broke and creatin a vacuum leak.

You could test them with a vacuum pump and see if they hold pressure.  I've never had one go bad.  They are glorified/modified Campbell soup containers.

["Cadillac Kid" Greg Surfas 15364]

Trevor,
You are not talking about the Emmissions device that is connected to the fuel tank and the carburetor are you?
Greg Surfas

[Stitches upholstery]

I was thinking that's what those are

["Cadillac Kid" Greg Surfas 15364]

Trevor,
On the '78 motors the connections to that canister are complex and tied into three systems.  I would recommend you leave it in place and renew all the vacuum lines needing replacement.
Greg Surfas

[Stitches upholstery]

What all are they tied into?

[bill06447]

Appears you are referring to the fuel vapor recovery system. Vents the fuel tank and the carburetor bowl. Without the system intact, best case is stinky gas fumes in the car, worse case the car stops due to fuel starvation and all sorts of things in between. The system is easily diagnosed if there's a problem suspected, its been on pretty much every vehicle since the late 60's and doesn't affect performance good or bad.

Bill

[Scot Minesinger]

Greg is right that canister needs to remain and all original vacuum lines replaced.

In general most car manufacturers did not install not needed items as that ate into their profit.  There may be a few things that can go, such as air pump, that served only for emissions, but this is not one of them.

[Stitches upholstery]

Thanks for the info! Here's another question, there's a switch of some sort mounted next to the carb on the left side. It looks like it may have had a tab that on it that would move when the throttle was moved. It looks to be on the same bracket that the return spring hooks to and has a 2 wire plug going to it. Any ideas what it is?

Thanks

[35-709]

That is the transmission kick-down electric switch.  When you give the engine hard throttle the TH400 transmission downshifts electrically into "passing gear" by means of that switch.  Sounds like you need a new or good used one if that tab is broken off. 

You would be doing yourself a big favor to get a 1978 Cadillac Shop Manual for your car.  In your case you would need the 1977 Cadillac Shop Manual and the 1978 Supplement --- easily found on eBay Motors.

The 1977 manual ----
http://www.ebay.com/itm/1977-Cadillac-DeVille-Eldorado-Seville-Service-Shop-Repair-Manual-Plus-Wiring-/171715426843?hash=item27fb097a1b&vxp=mtr

The 1978 Supplement ----
http://www.ebay.com/itm/1978-Cadillac-Service-Shop-Repair-Manual-Book-Engine-Drivetrain-Electrical-OEM-/140815135794?hash=item20c93c7432&vxp=mtr

While you're there get the Fisher Body Manual covering your year also.  All invaluable help.