‘77 280z smog test failed on fuel evaporative control funtionals

[240zadmire]

Greeting,

 

took the 280z 1977 for the smog test and everything passed except the fuel evaporative controls functional.

 

The technician say something about pumping nitrogen into the gas tank and take measure for 2 minutes… he said there must be a leak somewhere that didn’t hold pressure.

 

I don’t smell any gas btw.  
Is this problem relate to the canister or so I need to replace all fuel lines?

 

 

much appreciated

 

[Dave WM]

odd, "gross" for result is not pass or fail. the tank has a restriction valve from the overflow tank (not sure if that is the right name for it, its small tank located near the filler) that takes fumes from the tank and passes them thru the carbon canister. so the tank is always vented but fumes are captured in the charcoal. 

not sure how it was tested, unless they just put pressure into the tank and thru the filler hole and measured the PSI to see how rapidly the pressure would blead off (the same way a compression leak down test done on cylinders can tell the % of blead down. 

Did you observe how the test was performed? 

 

I suppose if the restriction valve was not functioning correctly then it could be the problem. Maybe a simple fix would be to insert a restriction into the line to the carbon canister, prob being you dont know what amount of restriction they are looking for.

again this assume you have not modified the fuel vapor emissions systems at all.

IF they are simply seeing if the tank holds pressure with the engine off, then they don't understand how the system works. Its not computer controlled with purge valves operated by a computer.... the car predates such things.

 

Edited March 5, 2022 by Dave WM

[Dave WM]

 

[240zadmire]

Thanks for the video.  Very informative.  I believe he clamped a hose near the canister.  Pump nitrogen through the tank filler.  The car was off while he was doing it.  Seem like he took a long time testing it…

I did replaced the hoses on the overflow tank.  It probably has a kink on it.  I’ll open it up and will take a look again.  

Edited March 5, 2022 by 240zadmire

[Dave WM]

ok yes if he clamped the correct hose and it did not hold, then you have a problem. I would start by smoking out the system, you should be able to feed back thru that same hose and look for smoke. If i recall correctly you can back feed that line. However a better solution to duplicate his test would be to clamp the same hose an rig up a way to smoke from the filler since that is what he did. maybe just a smoke hose and some damp rags to fill around the opening.

fyi the correct canister hose would be the one that goes to a hard line and heads back to the tank.

Edited March 5, 2022 by Dave WM

[Dave WM]

you may want to take look under car, rear pass side, look to make sure the rubber line from the overflow tank is even hooked up. On my car the hard line was rusted closed so to keep the tank from over pressurized the rubber hose had been removed (venting directly into the air). Maybe someone did the same "fix" to yours...

IF the hard line is rusted closed you may want to look into removing just the bottom most section, that is likely where the rust is. Much easier to just patch in a replacement there than trying to replace the entire hard line.

[240zadmire]

This car was “restored” by me, so all hoses and bolts were connected or tighten something 😉

 

im trying to build a smoke machine.  Saw some one build on YouTube using bicycle pump…. Very effective.

 

if I were to disconnect the hose that connected to the charcoal and pump smoke to that rubber hose, is that the same as from the gas filler neck? 

[SteveJ]

I made my own smoke machine, too. Buy some tire valve stems, and remove the valves inside. I bought the soldering iron at Walmart. 

Edited March 5, 2022 by SteveJ

[240zadmire]

Tire valve stems, that’s ingenious

 

thanks

 

[SteveJ]

25 minutes ago, 240zadmire said:

Tire valve stems, that’s ingenious

 

thanks

 

And if you have an air compressor or tire inflator, you can use it for the air source. Just make sure you dial the regulator down on the compressor.

[Dave WM]

2 hours ago, 240zadmire said:

This car was “restored” by me, so all hoses and bolts were connected or tighten something 😉

 

im trying to build a smoke machine.  Saw some one build on YouTube using bicycle pump…. Very effective.

 

if I were to disconnect the hose that connected to the charcoal and pump smoke to that rubber hose, is that the same as from the gas filler neck? 

I think so, but it seems more logical to use the same approach as the tech did, since its that test that will count. IIRC the restriction device works both ways, but more restriction on way than the other. So I would stick with the hose in the filler neck, with a damp rag sealing it up while the same canister hose clamped off as done during the test. 

I must say i am impressed by the through testing done in your state.

[SteveJ]

11 minutes ago, Dave WM said:

I think so, but it seems more logical to use the same approach as the tech did, since its that test that will count. IIRC the restriction device works both ways, but more restriction on way than the other. So I would stick with the hose in the filler neck, with a damp rag sealing it up while the same canister hose clamped off as done during the test. 

I must say i am impressed by the through testing done in your state.

OTOH, pushing in from the vent line would indicate whether or not there is an adequate seal on the filler cap. Unless the tech had the right adapter, that could have been the source of the leak.