Skyline’s charcoal canister !

[joshuaho96]

7 hours ago, TurboTapin said:

So I decided to test the canister and figure out exactly how it works.

The top port marked "VC" that originally went to the throttle body does not react at all to pressure. It reacts to vacuum.

When there's more vacuum on the VC port then there is on the purge port, it opens allowing a vacuum on the charcoal to regen the charcoal. Without this, the charcoal would become saturated from the fuel vapor going through it and into the bottom port that connects to the frame rail. 

With this being said, unless you pipe VC pre throttle and purge post throttle I believe your charcoal will saturate rendering the canister useless. I avoided this and the concern that the canister would not handle 35+PSI and decided to remove it and pipe the tank vent into the frame rail. 

In theory as you said with differential it would also open, but you require vacuum on the purge line to suck in fuel vapor so that rules out any situation outside of vacuum. 

I need to check precisely how this stuff is all set up again but I'm pretty sure that the throttle body port for the vacuum signal is a "weak" signal, at least in the RB26. I'm not sure exactly how it works but you absolutely cannot plug in an FPR vacuum line into it for example. So even if you're running 35 psi of boost in the manifold I'm pretty sure it doesn't just immediately pop the bottom cap off and shoot the contents towards your feet as if you shoved 150 psi into one of the ports.

[BK]

6 hours ago, joshuaho96 said:

I need to check precisely how this stuff is all set up again but I'm pretty sure that the throttle body port for the vacuum signal is a "weak" signal, at least in the RB26. I'm not sure exactly how it works but you absolutely cannot plug in an FPR vacuum line into it for example. So even if you're running 35 psi of boost in the manifold I'm pretty sure it doesn't just immediately pop the bottom cap off and shoot the contents towards your feet as if you shoved 150 psi into one of the ports.

Correct, the RB26 throttle 1 line to the canister is a pin prick. Still have mine connected at 33psi + on the 2.8 blue 32, no dramas.

[TurboTapin]

7 hours ago, joshuaho96 said:

I need to check precisely how this stuff is all set up again but I'm pretty sure that the throttle body port for the vacuum signal is a "weak" signal, at least in the RB26. I'm not sure exactly how it works but you absolutely cannot plug in an FPR vacuum line into it for example. So even if you're running 35 psi of boost in the manifold I'm pretty sure it doesn't just immediately pop the bottom cap off and shoot the contents towards your feet as if you shoved 150 psi into one of the ports.

 

35 minutes ago, BK said:

Correct, the RB26 throttle 1 line to the canister is a pin prick. Still have mine connected at 33psi + on the 2.8 blue 32, no dramas.

 

Yea agreed, it's an orifice that drops the positive pressure or vacuum way down. In my case with a DBW throttle body, it's a different story as I don't have this nipple. As originally mentioned, I could tig a nipple onto my intercooler piping pre throttle with an appropriately sized orifice. 

[Neostead2000]

Your alternatives are to simply add a one way check valve at the end of the rubber hose and have the valve secured somewhere under the driver side wheel well. This is what I do to my cars at least. The check valve keeps outside dirt out and the valve is only really going to crack open once a significantly high tank pressure is reached. Way before that, your gas cap would have vented all the excess pressure from the fumes anyway.

By adding the check valve and securing it under the wheel well, the airstream helps immediately disperse any potential fumes. And yes, you could just avoid the hard lines and do this setup right off the reservoir. Also some people are simply TOO sensitive to gasoline fumes so that may be why you're being affected so much.

On my 33 it came without an evap canister so I had to do the check valve mod anyway. Have done it also on my Mitsubishi and can't feel a whiff of fresh fuel.

[TurboTapin]

3 hours ago, Neostead2000 said:

Your alternatives are to simply add a one way check valve at the end of the rubber hose and have the valve secured somewhere under the driver side wheel well. This is what I do to my cars at least. The check valve keeps outside dirt out and the valve is only really going to crack open once a significantly high tank pressure is reached. Way before that, your gas cap would have vented all the excess pressure from the fumes anyway.

By adding the check valve and securing it under the wheel well, the airstream helps immediately disperse any potential fumes. And yes, you could just avoid the hard lines and do this setup right off the reservoir. Also some people are simply TOO sensitive to gasoline fumes so that may be why you're being affected so much.

On my 33 it came without an evap canister so I had to do the check valve mod anyway. Have done it also on my Mitsubishi and can't feel a whiff of fresh fuel.

Check valve won't do anything to help against dirt unfortunately, you would require some sort of pressure for it to close. What is the second valve you mentioned? Evap purge valve? 

[Neostead2000]

13 minutes ago, TurboTapin said:

Check valve won't do anything to help against dirt. What is the second valve you mentioned? Evap purge valve? 

It's a one way valve, it lets stuff one way but not the other. So if you have it set up with flow from  reservoir to outside air, then have another foot of hose after it venting to atmosphere, neither dirt or water can get through it and into the lines. But in-tank fuel vapor can still exit through that check valve if it gets too high.

The other valve I mentioned is built into the fuel filler cap. That also has safety mechanism which will let off vapors if they get too pressurized for whatever reason.

[TurboTapin]

9 minutes ago, Neostead2000 said:

It's a one way valve, it lets stuff one way but not the other. So if you have it set up with flow from  reservoir to outside air, then have another foot of hose after it venting to atmosphere, neither dirt or water can get through it and into the lines. But in-tank fuel vapor can still exit through that check valve if it gets too high.

The other valve I mentioned is built into the fuel filler cap. That also has safety mechanism which will let off vapors if they get too pressurized for whatever reason.

I understand what you're saying, but by design whether you have a ball or flapper check valve, road dirt will never close it... Unless your check valve is normally closed in the direction vapour comes out which I would never do as that vapour should free flow out without restriction. 

 

Just remove it. It serves no purpose and might be causing more harm then good. 

[r32-25t]

How much dirt do you think is going to get the whole way down a 6mm hose that runs the length of the car 

[Neostead2000]

It's not so much there to keep dirt from entering, I'm aware it has a minimal impact (although grime buildup is still something to be considered in damp/dusty environments! The factory set up has a check valve in-line before or sometimes after the charcoal canister on like 99% of road cars. What I was suggesting to OP was, if he has trouble finding the OEM setup or if he does not want to clutter up his engine bay, the easiest and cheapest way to cure his strong fumes problem is to retain (or add) this check valve. If he lets the line vent freely he will continue to get strong whiffs of fuel. Adding the valve keeps them inside the fuel tank where they're meant to stay. And in the events of over-pressurization, both the check valve and the gas cap can do their jobs and vent it out. But he would have opened his cap to refuel way before such an event occurs anyway.

[TurboTapin]

16 minutes ago, Neostead2000 said:

It's not so much there to keep dirt from entering, I'm aware it has a minimal impact (although grime buildup is still something to be considered in damp/dusty environments! The factory set up has a check valve in-line before or sometimes after the charcoal canister on like 99% of road cars. What I was suggesting to OP was, if he has trouble finding the OEM setup or if he does not want to clutter up his engine bay, the easiest and cheapest way to cure his strong fumes problem is to retain (or add) this check valve. If he lets the line vent freely he will continue to get strong whiffs of fuel. Adding the valve keeps them inside the fuel tank where they're meant to stay. And in the events of over-pressurization, both the check valve and the gas cap can do their jobs and vent it out. But he would have opened his cap to refuel way before such an event occurs anyway.

Your statement is incorrect. Your vent needs to well vent, so gas can come out the other end. You absolutely do not want to hinder that in anyway. Unfortunately with venting comes fuel vapours...

Try and pour gas out of a jerry can with the vent closed. Same result. 

[Neostead2000]

Okay so let's get technical here, I guess my degree in automotive engineering might come in useful after all.

 

The fuel system in your average car is quite simple. There is a fuel tank, a fuel pump, a line and the fuel rail. We wont focus on things like regulators and injectors. The fuel tank is a sealed unit however it is not under perfect vacuum. At any one time there is both fuel inside as well as air mixed with fuel vapors.

As the pump runs, it constantly sucks fuel out of the tank and pushes it to the rail. Any excess fuel is returned via the return line back into the tank. The less fuel there is, the more fuel vapors develop. And the longer they sit inside especially during warm summer days, the more chance they have to start pressurizing but UP TO A POINT.

Usually, they cannot go past the fuel pump because the pump is already filled with fuel and usually has an internal check valve which is there to keep it primed at all times. The vapors also cannot go back through the fuel return line because at the end of it sits the regulator diaphragm. So the only way for the vapors to escape is through your gas cap OR any emissions system such as the charcoal canister.

The charcoal canister is an emissions system which manufacturers were made to start adding to their cars. The goal is to capture the hydrocarbon vapors emitted from the fresh fuel sitting in the tank. Before that, the system was much simpler with either just a regular open end hose right off the reservoir venting to atmosphere, or if your car was fancy you'd have a small fuel filter and/or check valve. Again, the effect was the same; at a certain pressure the valve would crack open and release the built up vapor pressure. An important thing to note, the pressures that can form inside are very small, usually less than half a psi.

Your analogy with the jerry can does not work here because we aren't relying on gravity to help the fuel  leave the canister. We are relying on a positive-pressure pump to produce the flow to the rail, and since these cars use a return system, fuel is constantly in circulation also aided by the venturi effect in some fuel tank setups.

[BK]

1 hour ago, Neostead2000 said:

The other valve I mentioned is built into the fuel filler cap. That also has safety mechanism which will let off vapors if they get too pressurized for whatever reason

I will point out that's not how the factory fuel tank cap works. It doesn't release fuel tank excessive pressure out to atmosphere, it opens to draw air into the tank as the fuel fluid level decreases.

Excess pressure is only ever released via the tank vent line.

[TurboTapin]

45 minutes ago, Neostead2000 said:

Okay so let's get technical here, I guess my degree in automotive engineering might come in useful after all.

 

The fuel system in your average car is quite simple. There is a fuel tank, a fuel pump, a line and the fuel rail. We wont focus on things like regulators and injectors. The fuel tank is a sealed unit however it is not under perfect vacuum. At any one time there is both fuel inside as well as air mixed with fuel vapors.

As the pump runs, it constantly sucks fuel out of the tank and pushes it to the rail. Any excess fuel is returned via the return line back into the tank. The less fuel there is, the more fuel vapors develop. And the longer they sit inside especially during warm summer days, the more chance they have to start pressurizing but UP TO A POINT.

Usually, they cannot go past the fuel pump because the pump is already filled with fuel and usually has an internal check valve which is there to keep it primed at all times. The vapors also cannot go back through the fuel return line because at the end of it sits the regulator diaphragm. So the only way for the vapors to escape is through your gas cap OR any emissions system such as the charcoal canister.

The charcoal canister is an emissions system which manufacturers were made to start adding to their cars. The goal is to capture the hydrocarbon vapors emitted from the fresh fuel sitting in the tank. Before that, the system was much simpler with either just a regular open end hose right off the reservoir venting to atmosphere, or if your car was fancy you'd have a small fuel filter and/or check valve. Again, the effect was the same; at a certain pressure the valve would crack open and release the built up vapor pressure. An important thing to note, the pressures that can form inside are very small, usually less than half a psi.

Your analogy with the jerry can does not work here because we aren't relying on gravity to help the fuel  leave the canister. We are relying on a positive-pressure pump to produce the flow to the rail, and since these cars use a return system, fuel is constantly in circulation also aided by the venturi effect in some fuel tank setups.

I was trying to be polite but clearly you must of used your engineering degree to go into project management or something else non technical or you made that up because you have no idea what you're going on about.

All fuel tanks are vented. Every single tank on every single car that has ever existed. Not including most tanks in general that hold liquids. That vent is required to equalize with outside pressure. For thermal expansion, so your fuel pump doesn't pull vacuum, so you can pump in gas at a gas station, etc. 

Cheers. 

 

[GTSBoy]

4 hours ago, TurboTapin said:

so you can pump in gas at a gas station, etc

Well, perhaps less so this, because as BK said above, the cap is a suction relief (which is the only "vent" on the tank), and the carbon canister is effectively a pressure relief (ie, a vent on the overall tank system, but it's not so much on the tank). So when you're pumping in, even if the pump nozzle somehow managed to completely seal on the filler neck, you couldn't really build pressure in the tank anyway.

US (and I guess others') emission regulations worked towards preventing fuel vapour spill out through the filler neck because it was a significant fraction of HC emissions wrt cars, trucks, etc. The carbon canister was supposed to catch them at fill time as well as when just sitting around.

I'm not disagreeing with you - just pointing out that there is not a single two-way vent on the tank.

[Kinkstaah]

I still don't know how this works.

When I fill up my car, I crack open the fuel tank filler cap and I hear hissing, which I assume is air filling up the gas tank as I release what is now a vacuum of sorts due to the fuel level being depleted while sealed.

Is this normal? If something is fked, should I just notquitetotallybedoingmyfuelcap all the way?

Nobody ever seems to know the answer.

[GTSBoy]

Small vacuum in the tank is normal, and shows that everything (or at least most of the purge system, at least) is working normal.

[soviet_merlin]

How does temperature affect the vacuum in the fuel tank? I'm curious now. Because normally leaving the car in the sun is what makes it happen even when not driving the car and using fuel.

But heat expands substances so shouldn't it be the opposite?

[Neostead2000]

9 hours ago, TurboTapin said:

I was trying to be polite but clearly you must of used your engineering degree to go into project management or something else non technical or you made that up because you have no idea what you're going on about.

All fuel tanks are vented. Every single tank on every single car that has ever existed. Not including most tanks in general that hold liquids. That vent is required to equalize with outside pressure. For thermal expansion, so your fuel pump doesn't pull vacuum, so you can pump in gas at a gas station, etc. 

Cheers. 

 

You are very polite, we are having a technical discussion like adults and I don't see a problem, compared to some other forums I've browsed through before. I'm not trying to argue for the sake of it or make it seem like I'm a know-it-all, but I believe I've got a good understanding of automotive fuel systems in general so that is why I'm standing by my explanation.

One key factor I forgot to mention is related to safety. In the event of a roll-over, fuel can go up through lines it shouldn't, such as the fuel filler and vent hose. We all know fuel fillers are required to also have a one-way check ball or flap, such as the one that flaps inward when we insert the fuel gun. Likewise, the venting system also requires such a safety mechanism because when you think about it, that vent line most always goes at the very bottom of the car until it reaches the engine bay, which then goes into the charcoal canister. So for the most part, its location is lower than the highest part of the fuel tank. Fuel can and will fill that line in the event of a roll-over, which is another reason why there would be a check valve before or after the canister for this very reason.

The least somebody could do after removing the canister entirely is to keep a check valve at the end. In motorsports this is a mandatory requirement as well as a strong check-ball in the fuel filler neck if one is fitted to the fuel cell.

On my Mitsubishi I've had the canister removed since 2016 and have ran the factory valve at the end of the purge line like that, venting under the wheel well. My reservoir has not caved inward, or split, or blown apart. My fuel pump runs great and maintains adequate pressure. The gas cap produces a hiss when unscrewed which is normal. And I do not get strong fresh fuel odors around the engine bay or car for that matter, and I can pump fuel in quick without it going up the filler neck.

P.S I never stated that the fuel tank should not be vented or that it is completely sealed. In fact I said the opposite. Having a check valve with a low cracking pressure at the end of the vent line does not make it permanently sealed. While filling up fuel the air can be pushed out through the existing vent on the fuel filler neck and through the check valve on the purge line.

Edited December 22, 2022 by Neostead2000
Added post scriptum

[TurboTapin]

2 hours ago, Neostead2000 said:

You are very polite, we are having a technical discussion like adults and I don't see a problem, compared to some other forums I've browsed through before. I'm not trying to argue for the sake of it or make it seem like I'm a know-it-all, but I believe I've got a good understanding of automotive fuel systems in general so that is why I'm standing by my explanation.

One key factor I forgot to mention is related to safety. In the event of a roll-over, fuel can go up through lines it shouldn't, such as the fuel filler and vent hose. We all know fuel fillers are required to also have a one-way check ball or flap, such as the one that flaps inward when we insert the fuel gun. Likewise, the venting system also requires such a safety mechanism because when you think about it, that vent line most always goes at the very bottom of the car until it reaches the engine bay, which then goes into the charcoal canister. So for the most part, its location is lower than the highest part of the fuel tank. Fuel can and will fill that line in the event of a roll-over, which is another reason why there would be a check valve before or after the canister for this very reason.

The least somebody could do after removing the canister entirely is to keep a check valve at the end. In motorsports this is a mandatory requirement as well as a strong check-ball in the fuel filler neck if one is fitted to the fuel cell.

On my Mitsubishi I've had the canister removed since 2016 and have ran the factory valve at the end of the purge line like that, venting under the wheel well. My reservoir has not caved inward, or split, or blown apart. My fuel pump runs great and maintains adequate pressure. The gas cap produces a hiss when unscrewed which is normal. And I do not get strong fresh fuel odors around the engine bay or car for that matter, and I can pump fuel in quick without it going up the filler neck.

P.S I never stated that the fuel tank should not be vented or that it is completely sealed. In fact I said the opposite. Having a check valve with a low cracking pressure at the end of the vent line does not make it permanently sealed. While filling up fuel the air can be pushed out through the existing vent on the fuel filler neck and through the check valve on the purge line.

We have roll over valves near the fuel tank above the diff. The 5/16 vent line goes to it before continuing to the engine bay. They are a normally open, and close when upside down. 

[alexj]

I tested the rollover valve in mine, and maybe unsurprisingly after 30 years it wasnt functional so i put in a motorsport one.

Until then, for as long as i had my car, it had the canister removed and a bolt blocking the pipe that went to the canister in the engine bay. I hadn't checked and it didnt cause any noticable problems. It was pressuring the tank though.