77orange

Need a little help - working on a 77 with automatic, air, and cruise control. The evap hose from the tank goes into the canister, then out to the cruise control transducer, then up to the brake booster. Is this right?? Seems like routing the canister up into the cruise transducer then into the brake booster would not be wise. Also, PCV valve hose does not have a "T" but instead only runs from the top of the valve (located on top of the valve cover of course) directly into my Qjet.

Any help on this one? Thanks. Jim

Alan 71

Hi Jim,
I believe a more typical routing would be for the smaller hose from the canister to run directly to the carburetor and for the larger hose from the canister to run to a 'T' at the pcv valve and then on to the carburetor .
Regards,
Alan

77orange

Thanks Alan. My canister only has two ports on it so I think one of them has to go to the gas tank. My concern is where the other one "should" go as right now it goes into the CC transducer then on to the brake booster. My thinking is along your lines that the port not connected to the gas tank should go to a "T" connection at the PCV valve. But then there is the cruise control connections...

The Servo is missing from my system - didn't have it when I got the car. So I am missing part of that system. Maybe the cruise control sends two vac lines to the servo?? That would explain this issue...

Jim

77orange

UPDATE: OK, after looking very closely at the diagram from the assembly book I think I have some of this resolved, but some not. Would like a confirmation on this but I think the cruise control gets its vacuum from the manifold via a connection that comes from BEHIND the brake booster (I originally posted I thought I thought it was the brake booster but I was wrong). The CC transducer then has a port connected to the servo AND a port that connects to a some kind of connector that is inline with the servo. I didn't know this 4 way plastic fitting existed - cause there isn't one on my car!

OK. So if that's the case I can just plug off the vacuum line going to the cc transducer (still working on getting the parts like the servo and now a 4 way white plastic fitting so i dont have cruise anyway right now) and that will fix that.

NOW, what to do with the canister. My assumption is someone removed the hose and T that should route it to to the carb via the T at the PCV (which of course is also missing). So the end of that vacuum run is at the cc servo.

Can someone confirm this? Thanks again for everyone's help.

bmotojoe

No, not from throttle plate booster fitting. On your auto car there is a 3 port fitting on the rear right riser on your intake manifold. Top goes to vacuum modulator valve auto trans, the two side ports vacuum for headlights the other is your main vacuum for cruise.

bmotojoe

On your 77 the canister connects directly to the carburetor, no tee to the PCV. You can cap off any vacuum dead ends until you find your parts.

If you need the 4 way plastic fitting, get ready for an eye opener...
EBAY: https://www.ebay.com/itm/NOS-GM-Crus...wAAOSwPe1T1-~b
At least you will have the Gr.# and part #

77orange

Wow - liking your color version of the diagram

Roger the advice on capping the vacuum for now.

Ouch! $100 for a t fitting?? Is there anything special about that fitting or just a connection bus?

And finally, now on the connecting the canister directly to the carb. Need to look up which fitting that is on the old Qjet.

Jim

7T1vette

One of the lines connecting the evap canister to the carb MUST be a 'ported/timed' vacuum connection. And it should connect to the evap canister as the 'signal' line which 'tells' it when to purge its contents into the larger line going to the base of the carb.

You do not want that canister to PURGE when the car is idling, as it will change the A/F mixture at idle and it will either stall or run very rough; so that 'signal' vacuum line cannot be connected to "manifold" vacuum.

bmotojoe

All 4 hoses are 5/16", nothing special as long as there is clearance between the two bottom hoses. You can also run a 5/16" tee short vacuum line and another tee, essentially you have a 4 way fitting.

I like them grungy...

My spare 17057204

77orange

I'm a little confused about the signal line. My canister only has 2 connections as far as I can tell. I am attaching 2 diagrams: one from the assembly manual and 1 from the service manual. I will go out shortly and take a picture of it if my phone will fit in there...

bmotojoe

Yes, just two on top. There will be a 90* fitting on the bottom of the canister but it is open to atmosphere.

77orange

Here is a pic of my 79 carb (it's my spare to the 77 which was having issues

)

77orange

so my canister port looks like the smaller capped one on the right of the carb (looking at the picture), right above the idle mixture screw opening.

Also, confirmed my canister only has 2 connections on top. Looks like most other years had 3, 4 5, even 6! For once the old 77 had a simpler set up.

bmotojoe

I'm not 100% on this but I think in 79 they tied into the EGR ported line for the vacuum source to the canister. The upper large port that you capped off also went to the 79 canister as a vent line.

The small capped port above idle mixture screw is for the vacuum advance.

77orange

So, here lies the problem. Since it appears that in 79 the evap canisters used 3 or even 4 lines, which port do I hook my vacuum hose to from my 77 2 port canister? My Qjet is from 79 on my 77 vet with 2 hose canister. Could use the bowl vent port on the top, or a smaller vacuum port on the bottom. Or, I could T into the PCV line. This canister only has two lines and the manuals show one goes to the carb... but where??


Thanks. Jim

7T1vette

Someone would need to analyze both the '77 and '79 evap canister systems and carbs to provide you with the info you need. Apparently, in later C3's the evap canister design/system changed considerably from earlier years (with which I am more familiar).