Vacuum Valve for the Charcol Canister

[Justin]

What does it do, and why?

I've seen a few vehicles with C.C.s hooked up in the rear (A 90's Chevy I did an amp in yesterday for one).

I highly doubt there is a vacuum line ran all the way back in the vehicles that utilize it in the rear...


Just wondering

 

[jetjock]

On a 7M? Which valve are you speaking of?

For other cars that are OBD-II there's a purge valve that's used to auto test the EVAP system for leakage. It's part of the emissions monitoring software that all newer vehicles must have. The MKIII doesn't do it...

 

[92TealSupra]

On a 7M? Which valve are you speaking of?

For other cars that are OBD-II there's a purge valve that's used to auto test the EVAP system for leakage. It's part of the emissions monitoring software that all newer vehicles must have. The MKIII doesn't do it...

One valve is fro the EGR I thought the other is for the evap correct?

I have two extra sets of those things sitting around lol.

 

[jetjock]

Ah, I see. He must mean the BVSV. It's used to disable off-idle EVAP purge until coolant temp is hot...

 

[Justin]

Ah, I see. He must mean the BVSV. It's used to disable off-idle EVAP purge until coolant temp is hot...

Yes, that's what I mean.

Why is it a bad thing to have off idle EVAP purge while the coolant temp is lower?

Basically what I'm getting at is why not have a Charcoal Canister in the back instead of up front?

 

[92TealSupra]

Yes, that's what I mean.

Why is it a bad thing to have off idle EVAP purge while the coolant temp is lower?

Basically what I'm getting at is why not have a Charcoal Canister in the back instead of up front?

Product placement? What if you need to replace or test this unit? That wouldn't be to much fun.

 

[jetjock]

The hose going from the cannister through the BVSV (mounted in the radiator) to the TB is ported vacuum. Means they'll be no suction on the cannister at idle but there will be when the throttle plate is open. The problem with that is with the engine cold sucking on the EVAP cannister off idle (or on idle for that matter) will cause excessive richness because the ECU is in open loop warm up enrichment. Once the engine is hot and in closed loop it's not an issue because the ECU can correct for this richness using the O2 sensor.

So we need two ways to control EVAP purging: 1) We need to disable it at idle whether hot or cold and 2) we need to disable it off idle when cold. The ported vacuum from the TP does the first job while the BVSV does the second. This is all in the book btw.

Many EVAP cannisters are in the back these days and some are even more buried. I've never seen a car without some type of connection to the engine no matter where the EVAP cannister is located. How else would the HC stored in it get combusted?

 

[92TealSupra]

The hose going from the cannister through the BVSV (mounted in the radiator) to the TB is ported vacuum. Means they'll be no suction on the cannister at idle but there will be when the throttle plate is open. The problem with that is with the engine cold sucking on the EVAP cannister off idle (or on idle for that matter) will cause excessive richness because the ECU is in open loop warm up enrichment. Once the engine is hot and in closed loop it's not an issue because the ECU can correct for this richness using the O2 sensor.

I was curious, the BVSV isn't it a bi-metallic switch? The coolant has to warm up enough to make the vacuum port open for the Tb to get the air? it can't operate until it gets hot enough or is it opposite?

 

[jdub]

An alternative is sourcing a JDM M70 charcoal canister and mounting it in the passenger front wheel well, behind the rear well liner. Gets it out of the engine bay and works just as well as the USDM round canister.

 

[jetjock]

Teal: Right, it's a bimetal valve that opens when the coolant gets hot. The temp escapes me at the moment but it's pretty low. The valve is what keeps purging from occurring during warm up even with the throttle plate open.

Justin: I'll be the first to admit modern evap systems are complex but unless they're changed the technology behind how the cannister works (it full of coconut charcoal) I don't see a way of getting rid of the HC without sending it to the engine. The pressure in the cannister has to be reduced to reverse the adsorption of HC and without using the engine that would mean a vacuum pump.

 

[92TealSupra]

Teal: Right, it's a bimetal valve that opens when the coolant gets hot. The temp escapes me at the moment but it's pretty low. The valve is what keeps purging from occurring during warm up even with the throttle plate open.

Justin: I'll be the first to admit modern evap systems are complex but unless they're changed the technology behind how the cannister works (it full of coconut charcoal) I don't see a way of getting rid of the HC without sending it to the engine. The pressure in the cannister has to be reduced to reverse the adsorption of HC and without using the engine that would mean a vacuum pump.

If you were anal enough, and I know I am not, you could run temp test on it and see.. lol

 

[jdub]

Teal: Right, it's a bimetal valve that opens when the coolant gets hot. The temp escapes me at the moment but it's pretty low.

Below 35 deg C - BVSV closed
Above 54 deg C - BVSV open



(for those that care)

 

[Justin]

Many EVAP cannisters are in the back these days and some are even more buried. I've never seen a car without some type of connection to the engine no matter where the EVAP cannister is located. How else would the HC stored in it get combusted?

Ahhh. See I was under a false impression of how the system operated. I was under the idea that the vacuum lines were strictly for opening and closing a valve or something, I did not realize that fumes were ingested by the engine.

I thought the charcoal in the canister neutralized the harmful particles in the fumes and were expelled out the tube in the bottom.

Now that we all have had our lesson for the day I understand

 

[WhtMa71]

^I thought the same thing.