Charcoal Canister venting options - seek opinion

[jmarkel]

Hi all, I have everything hooked up and running great after a long, almost 5 year hiatus. I need some advice on what to do with my fuel vent. My jeep is running the Howell TBI an has zero emissions stuff hooked up to it. The valve cover has a plug in the rear and the front one has a vent cap. My air intake/filter does not have an inlet to plumb anything to.

When I made my custom lines, I ran stainless from the tank up to the front. Fuel on the passenger side, and fuel return and vent lines up the driver side. The vent line is sitting there right at the firewall, currently open. The charcoal cannister isn't going to do anything if I hook it up because the rest of the system is absent. What is the safest thing to do to terminate that vent line coming from the rear tank then?

 

[AK-RWC]

The charcoal canister WILL do something without any other I/M equipment present. It just takes three vacuum lines (plus the line from the tank). Presumably, you have a PCV valve (this barely counts as emissions equipment). You need to install a T-fitting and run one line down to the canister. One vacuum line comes from the intake manifold. The last vacuum line comes from the TB. Done.

Like an idiot (because I didn't really understand what I was doing when I was 21), I deleted the canister in my CJ-7 when I did a 304 swap 22 years ago. I didn't really understand why my dad told me to keep it when I did the 350 swap in my other CJ-7 when I was 19, so I hung on to the canister stuffed into a closet. I spent the next five years wondering why my Jeep ALWAYS smelled like gasoline, especially when I parked it inside a garage. In my late 20s, I decided I was glad I kept it, and that I should re-install it. I finally reinstalled it 9 years ago, and shockingly, the smell went away. There was zero change in performance, and my Holley Pro-Jection 4Di never blinked by adding it.

 

[jmarkel]

I am not opposed to putting a canister in here...I don't want the smell, and I think it is a safety item. But I'm really kind of confused by it and what it will do. So follow me and see if someone can answer these questions.

1. I don't have a PCV valve installed right now. If I wanted to do so, I'd need to plug one of the other valve cover lines. That's fine. But I haven't seen a need to do this yet.
2. So then there are three things to contend with. Fuel Line Vent, Ported Vacuum Line, and the third is the line that goes into the carb base or intake. Again...that's understood. The Ported Vacuum opens the system, purges the tank into the carb base or intake and burns the vapors collected from the fuel vent (or PVC if hookedup correctly). Ok...get that.

The question is that if I hook the canister up to ported vacuum, and now the canister is "open" all the time when it is running (other than idle I suppose), is it not going to constantly be drawing vapors from the fuel tank when running? And if it's doing that, isn't it going to be sucking air in the gas tank filler cap and thus introducing unmetered air into the system causing a lean condition? This is what I'm confused about? Is the stock tank cap vented or not vented?

I may or may not use a PCV on my jeep...really depends on a few conditions, but I do want to use a canister. I can always Tee it in there at some point, but my biggest concern right now is making sure unmetered air is not introduced to the intake.

 

[ag4ever]

Air in the gas tank has no bearing on how rich or lean the engine runs. That is determined by air through the throttle plates and fuel through the carb. (Assuming you don’t have any open vacuum ports.)

 

[jmarkel]

Air in the gas tank has no bearing on how rich or lean the engine runs. That is determined by air through the throttle plates and fuel through the carb. (Assuming you don’t have any open vacuum ports.)

If there is unmetered air coming in the gas tank (unvented tank cap) , and the fuel vent is feeding into the canister, and the canister is open to atmosphere when running, then yes, it will impact how the engine runs.

 

[ag4ever]

Here is a basic description of the system from the ‘82 factory service manual.








Note the part that reads, “..fresh air enters from the bottom...”

 

[jmarkel]

I agree with everything that's in these pictures, but the issue still stands, particularly with a fuel injected vehicle. On the stock setup, the purge signal is controlled. There's a CTO valve or some sort of trigger that makes the ported vacuum initialize. When that happens, the vapor from the tank and carb bowl gets sucked up into the manifold, from the canister, to be burned. I don't have that signal, so when the engine runs, it will always be triggered. The question still remains...what meters the air going into the manifold? The filter itself? The diameter of the hose? Obviously the engine will run like garbage if you take the line off the manifold input from the purge port canister. But how much air is still injected if hooked up vs just plugging the manifold?

I suppose there are ways to meter it if I needed...I could put an orifice to slow the draw. On the stock canister it seems like A LOT of air is pulled through the system just by sucking on the purge line. There is no way that much air won't impact the way the thing runs.

Anyway, I purchased CP3051 Canister. It has 3 ports. Ported Vacuum to trigger it which will go to the ported line shared with my distributor. The tank vent, and the purge line which will go into the intake manifold. On the bottom, the air comes in. So, if I'm getting too much air through the purge line and into the manifold vacuum (I have a few places I can do this with my setup) I can put a metering orifice there to slow it down so I don't create an overly lean condition.

If I want to add my PCV in the future, I'll just tee it directly to the purge line. Does this all seem reasonable and correct?

 

[sdsupilot]

I missed that you had Howell TBI efi the first time I read this. That runs and meters differently than anything jeep had. The GM computer runs a port on the base of the TBI, opening it when the correct parameters are met. This picture shows where the port is. It also shows how the pcv circuit is ran to the canister.



The charcoal canister on my donor vehicle was wet with gas. I also chose to use an in tank fuel pump and replaced all the fuel lines. My vent line has a rollover valve and a filter to keep dust/debris out of the tank. All that said, I haven’t used mine and my garage never smells like gas like it used to with the old jeep I6.

 

[spankrjs]

Post #538:

https://www.cj-8.com/threads/spankrjs-1984-long-term-project.32726/page-27

I use the above vapor canister, available new from NAPA, on any TBI Jeep I mess with.

I have used the above canister on both 258's with Howell TBI, and a Chevy 350 with OEM TBI.

Some OEM GM TBI computers/vehicles have purge solenoids. Early models do not. The above canister is simple to hook up, just like GM did it on the early TBI vehicles:

One nipple gets the vapor line from the fuel tank
The other nipple goes to the dedicated vapor vacuum connector on the TBI

 

[jmarkel]

This helps, so thank you. However, my system doesn't include most of this stuff. The Howell didn't come with wiring or stuff to handle these elements either. I don't have an EGR Valve, EGR Solenoid, or Vacuum check valve. So if I remove those from the circuit, I'm now talking about the orange line which is where I have the concern anyway. PCV....i did go ahead and order one, so I'm going to hook that up. But coming from the canister on the orange line, what exactly meters the amount of air that is introduced to the system then? The PCV is easy, it is based on how much air is generated from the engine as an air pump. Its sucked out of the crank case, and introduced into the port to burn. On the charcoal canister, that is another "air leak" because as soon as you apply the vacuum, air is sucked into the system and goes to the same port as PCV.

I'm questioning, and maybe wrongly, how much impact this would have to tuning? Seems to me that it would cause the system to run leaner as you introduce air into the manifold, which is after the fuel mixture is made in the carb.

 

[spankrjs]

The Howell just uses the simple two vacuum line hook up for the vapor canister.

As or the PCV system, just look at how a 1988-1991 GM truck is routed.

 

[Randyzzz]

Fuel injection systems should have latitude in their adaptive self-tuning to account for a small vacuum leak-a vapor canister for example.

 

[Spieg]

Is the stock tank cap vented or not vented?

Not vented unless the seal has failed. I had a problem with a clogged vent line years ago and the tank would build enough pressure to spray gasoline about 10 feet when removing the gas cap.

 

[jmarkel]

Thanks for all the input. The canister will arrive on Friday and I'll get it hooked up and report back.

 

[spankrjs]

The stock gas cap is not vented, BUT it is made in such a way that it allows air in. If not, as the engine is pulling vacuum through the canister, the fuel tank would start to collapse inward.

 

[jmarkel]

The stock gas cap is not vented, BUT it is made in such a way that it allows air in. If not, as the engine is pulling vacuum through the canister, the fuel tank would start to collapse inward.

I don't think that's right. When the signal is thrown (running engine) air is drawn from the purge line, and it is sucking air in from the bottom air filter element as fresh air intake on a stock. It shouldn't be creating a vacuum from the tank else when the tank is full, it would end up sucking in raw gas.

 

[spankrjs]

Besides fume venting, as gas is burned, the volume inside the tank is changing. Fuel out, air in through the gas cap.

 

[spankrjs]

From the 1983 FSM:



The vacuum is caused from the vapor system and the engine burning fuel, emptying the tank.

I had the FSM on 1988-1994 TBI GM trucks. Same fuel cap situation. That's why I felt confident using a charcoal canister made for a GM TBI on a Jeep using the Jeep fuel system, they are compatible.

The TBI vapor port in the throttle body is a specific port that is always pulling low vacuum through the fuel system. That is why I used the GM canister on TBI engines.

The Jeep 4.0/Mopar MPI kits use a different canister that is purged through manifold vacuum. I use an OEM Jeep 4.0 canister in these applications, 91-95 Jeep 4.0. Some of the later TJ stuff is different.

Swapping on fuel injection is easy, all the details will get you. I always match what the OEM engineers did.