What is this and do i need it ?

[Fester's Quest]

Hi all, i'm hunting down an idle bounce problem i've got with my 91 L and i've ran into some parts i can't identify.

The idle bounce only occurs about half of the time the engine is running. Otherwise, it idles and runs clean. ( 35 mpg average ) When it happens, the idle fluctuates from 550 rpm to 800 rpm.

Today i noticed a clicking noise that happens when the idle drops low. The clicking noise was coming from something that mounts to the same thing that the windshield wiper motor bolts to. It has an electrical connection and two small air lines running to it. I disconnected the electrical plug and the clicking stopped. The car didn't run any different with it unplugged. One of the two air lines runs to the right side of the intake manifold and the other one runs to a metal canister on the right. What is this vacuum system and what does it do ?

[KingFish]

That is your canister purge valve. The metal can is full of active charcoal that holds onto the gas vapor from your fuel tank. When the computer tell it to, the valve opens and lets the engine vacumme suck up all the fuel vapors.

[KingFish]

You could try pulling the vac line that goes to the manifold and plugging it with a cap to see if the problem goes away. The evap system is not monitored by the ECM and it will not cause a check engine light.

[Fester's Quest]

Thank you KingFish !

I'm thinking about removing all the evap system components since i'm going for lightweight on my build. Do you think it would effect gas mileage ? If so, i might just leave the system in and bypass the electronic valve by running a hose straight from the evap tank to the intake manifold.


As a note for my original post, i had mentioned that the car didn't run any different with the electronic valve disconnected. After driving it all last night i discovered that it does actually run alot better with it disconnected. It cut the idle bounce by about half and it no longer shakes & feels like it could stall when it dips to low rpms ( 580 rpm ) .

[WmWatt]

You give up burning some gas vapours mostly on startup but I doubt it will affect gas milage on a normal healthy engine. It's just there to keep fuel vapours from polluting the atmosphere. Mostly they come from gas evaporating after the engine stops.

[Movin]

The evap solenoid shouldn't be running (clicking) at idle.
The ECM must be getting false info from something like
the TPS. I don't have the test procedure handy on the weekend
but maybe some one else does.
The solenoid should be capable of blocking the vapor flow
completely. It also has a orifice to regulate flow so
that when it turns on during warmed up cruise conditions
it will not have running problems.
The system is also the fuel tank vent and barring other
leaks could cause enough vacuum in the tank to cause
problems if the system were capped off. Old everything
on these things means the tank would find somewhere
to vent. If you have a worn out solenoid and bypass it
put a small orifice in line to keep vapor flow at manageable
levels.

[Huli]

Re: What is this and do i need it ?

I was all curious because I had the same clicking and idle issue. Took a look at my solenoid and canister. Was not connected, just sucking air!!! So I gravity tested it.... nope, can't fly.

Plugged the hole

[Fester's Quest]

The evap solenoid shouldn't be running (clicking) at idle.
The ECM must be getting false info from something like
the TPS. I don't have the test procedure handy on the weekend
but maybe some one else does.
The solenoid should be capable of blocking the vapor flow
completely. It also has a orifice to regulate flow so
that when it turns on during warmed up cruise conditions
it will not have running problems.
The system is also the fuel tank vent and barring other
leaks could cause enough vacuum in the tank to cause
problems if the system were capped off. Old everything
on these things means the tank would find somewhere
to vent. If you have a worn out solenoid and bypass it
put a small orifice in line to keep vapor flow at manageable
levels.

I was thinking about cutting the gas tank vent line and removing the canister, hoses, and solenoid. Do you think i should put a small cone filter on the cut vent line so that no junk and debris can get inside ? Or maybe just leave it exposed.

I was curious whether it could be the throttle position sensor as well. I'm clueless on how to test it, or maybe i should just take it off and clean it ?

[nitrofarm]

Canister purge is a great invention and it robs no power. Dont delete it,you'll be wishing you didnt remove it in the summer.I personally cant stand smelling the fuel vapors anymore. Plus your tank can/will literally spew fuel out of your "Shade Tree" vent after you fill your tank on a hot summer day.Which can be a big safety issue in the right circumstance.Keep you purge system its a good thing.My 2 cents.......

[Movin]

I agree completely with Nitro! If you try open vent, then a filter would at least keep the line clean. Think of it as free fuel that you couldn't use otherwise.

[bravekozak]

I found a sexy red fuel cap at the junk yard, operated on it to vent it, then removed all the clutter inside. Here is what it looks like now.

[Fester's Quest]

Canister purge is a great invention and it robs no power. Dont delete it,you'll be wishing you didnt remove it in the summer.I personally cant stand smelling the fuel vapors anymore. Plus your tank can/will literally spew fuel out of your "Shade Tree" vent after you fill your tank on a hot summer day.Which can be a big safety issue in the right circumstance.Keep you purge system its a good thing.My 2 cents.......

Yea, i'm starting to think that cutting the line would probably just make my gas evaporate faster.

Unplugging the solenoid didn't seem to affect gas mileage though. both measurements below were taken after buring a full tank of gas and filling up at the same gas pump.

Mileage with evap tank solenoid plugged in : 35.63 mpg
Mileage with evap tank solenoid unplugged : 35.59 mpg

[Huli]

Re: What is this and do i need it ?

I am sure if all the parts are there and the engine vacuum is not sucking right past it like mine was then no harm...lol. any shade tree fixes known since my canister was found broken?

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