Fuel tank venting, EVAP systems, etc

[Lars]

Big blind spot for me. I've been reading CB and other forum posts for hours, I've found that getting solid info isn't easy. At least for me. Doug, if you are reading this, your posts on CB were far and away the most helpful. Hoping you will chime in here.

I have dual tanks. EFI. Both tanks are now NWMP, but what I'm going to describe occurred with the stock aux tank which I had until a year or two ago. Early 1970 model thus no EVAP system, and I've never seen an intact EVAP system on an early Bronco so I'm totally unfamiliar with them. Rear filler neck is the original with shallow cap. Front filler neck now has the deeper cap.

Problem is that, for the last 20+ years that I've run EFI, the tank in use pressurizes eventually. A lot. I've never installed any sort of venting. Every time it's happened (in over 20 years, a lot) I've thought "self, it's time to address this" but then I climb in my Bronco and immediately forget. Supposedly one of my caps is "vented" and the other one isn't, but I've never found a straight answer on what exactly "vented" means. I suppose I should cut a cap apart to figure it out, haven't done that either.

So I'm pondering some sort of passive EVAP system, in other words one that doesn't require a CANP valve controlled by my A9L ECU. Which isn't an option because the wires have been long removed from the harness.

So. thoughts on venting? Doug, if you are reading and have made it this far, I understand that you've successfully installed EVAP systems on some of your EFI Bronco projects without the CANP valve. Would love to read your insights. And/or thoughts of alternatives.

[904Bronco]

Well a Timely question...

I was gifted a 97 Explorer harness, minus a few connectors and in need replacing a few broken ones.
As luck would have it, I found a 96/97 in the Jalopy Jungle pick in pull yard in Nampa, ID during my visit. I have everything I need, but OMG there are extra wires!!! ??? Garry - EFI Guy explains mid year changes in early 97, 97 1/2 to late 97, then in 98.
Many text messages to Garry, review of Classic posts, and I have watched his videos over and over... I am getting there, So I completely understand the hunt for knowledge.

So if you do not want to/or can't add a canister purge valve then all that is left a passive method. Problem with that is that it works great as long as you are flat level ground. Off road you can have fuel going where it is not suppose to be.

There needs to be a some sort of check valve.
Areo tanks can be ordered with a rollover vent valve that screws into the top of the tank. That makes things easy.
There is the 77 Bronco roll over/check valve that is mounted out board to the Passenger frame. The valve is available from Deloran motors, the bracket is not... as highlighted in a recent Classic post. Add to that the design of those mid Seventies tanks to prevent vapors from getting into the atmoshere.

The easy part of all this is to use a Fox body mustang CC mounted up high in the engine compartment like a 76-77 Bronco. Two small ports, Two large ports. The large ports have the familar mushroom caps > vents. One small port is from the tank(s) the other small port has been attached to a fitting added to the air inlet (around the filer). This fitting takes advantage of the venturi action of air being drawn into the motor. Cuising speed is actually when you want the vapors drawn out...

People have stated this works just fine, without a check valve. I question that.
Some have said they looped the vent line up high out of the tank (to keep fluid out and allow fuel to gravity feed back into the tank) and then back down to the frame >then engine compartment and the CC. I don't know???

All these solutions may work most of the time, but one has to plan for the what if...

Fuel caps - what year bronco? Someone did a great explaination of non vennted vs. vented caps on Classic. I won't try and repeat what was said in Doug-speak. Basically you need the air to get in as the fuel level decreases. Which was no big deal in the early Bronco years. But you don't want the fumes to get out

Great the Site timed me out as I write this! Crap.

in the later years. I am going to say there is something built into the later model tanks that allows this to happen.

So there, some more to ponder...

Paul, aka Dirtdonk has my Peacock blue 68. It has an Areo tank with the top rollover valve on it. Single rear tank, metal line plumbed up to the engine compartment as stated. The tank is vented to the Charcoal canister and that is it. No line to the air intake. I had no smell issues, like there were with the factory tanks. I am sure he would comment if asked... He has not said anything to me about it, so I assume all is well.

That is my suggestion, do that 1st. If it does not help things get better then you can add the passive vent line. Check valves can be added later, depending what you find as you wheel yours. If you need some pictures, let me know.

[Lars]

Thanks for the detailed reply, Doug! The overthinking continues. Eventually I will want to stop losing sleep over this and I will try something.

[DirtDonk]

As said, no big fume issue. But there is still the occasional "whiff" of gas just caught for a bit, then gone. However mine sits outside 24/7 so it might be somewhat more than detected.
I have planned, and still plan to connect the outlet of the charcoal canister to the air intake system somewhere convenient. Near the air filter most likely, but I don't think it has to be there as long as it's not behind the throttle body where it would get full manifold vacuum.

In early Broncos from later in '70 up to '75 there was the single small (1/4 or 5/16 inch maybe?) line from the tank(s) to the single small fitting on the canister, then the two large hoses up into the engine compartment. One hose was just for atmosphere/ambient/whatever to allow for better flow out of the other one that was attached to the air-cleaner housing just outside of the air filter.
Early models had a simple plastic ring that held the ambient hose open (they were very prone to crushing and staying crushed, with their metal spiral reinforcement), while later models utilized the "mushroom" cap that was in use for many years after.

The pre-'76 models as described had a low-mounted charcoal canister on the frame rail up front, but kept liquid gas from flowing into it by using a plastic tank mounted up high inside the cabin behind the driver's left shoulder area. Basically behind the B-pillar/door striker pillar. This "condensing tank" let excess vapors turn to liquid and flow back to the main tank, but also acted as a high-mounted catch-tank for those long nose-down trail rides.
Very important to keep liquid gas out of the charcoal media.

The way I understand gas caps is, as Doug said, that the early ones were vented to allow inward flow of air to compensate for fuel used, and to allow for heat expansion. But with no mechanism for stopping fuel vapors coming out too, the later models were changed to allow only inward flow for fuel usage and any outward push from pressurization was taken care of by the EVAP system in the form of the charcoal canister.
Modern ones on computer controlled vehicles have their own pumps that test the system now and then, have greater capacity I would think for handling much of the need with zero chance of fuel vapors escaping, and a positive way of controlling how much and when the engine would suck up the excess (the CAN-P system mentioned earlier).
They made the necks incompatible with earlier caps so people would not automatically find it easy to toss their canisters and retrofit older fully vented caps. Not sure how easy it is to work around that aspect, but the most obvious way is to get one of the later filler necks that are the only ones available new. That's what I'm doing on the '68 "when I get around to it" sometime in the future.

That's about all I think I know about it. Luckily the static systems are pretty straightforward and don't need much thought other than how to do the plumbing in a clean and tidy way.
On Tim's '73 with EFI we put the newer style plastic charcoal canister up under the front wheel well just behind the shock hoop and mounted fairly high up. It's clean and very out of the way and hard to see unless you're really looking for it.
He's got an Aero tank, so presumably he's got an anti-rollover valve on the top. But I don't remember that detail in particular and pretty sure he did not put any other external rollover protection in the system.

Paul

[DirtDonk]

Well, I see by your other thread about the A/C install that you will have to continue your creative bent to find a spot to mount a charcoal canister. Most of the normal locations are already tied up under your hood and in your passenger wheel well with "stuff" of other functions.
Still, should not be an insurmountable obstacle. But maybe you'll end up being better served by the early design with in-cab condensing tank? As long as they don't leak, you don't have a fume issue in the cabin. But then, you probably have something already stuffed into that corner too!

Paul

[pippinmader]

I worked on evaporative emissions regulations for a while, mostly for ATVs and dirt bikes. Here is my $.02:
Gasoline vapor from a tank comes from at increase in temperature and the corresponding change in head space vapor volume (PV=nRT) and hydrocarbon concentration (based on partial pressure of a given RVP fuel at a a given temperature). FYI a full tank produces less emissions than an empty one because it has less empty space (head space).

• Emissions from a fuel tank occur during:
  • Diurnal operation due to the natural diurnal change in temperature
  • Running operation due to heat from the exhaust, road, and fuel pump.
• A passively purged carbon canister uses the volume reduction from the tank cooling off to back purge the canister and it can control about 60% of diurnal emissions (in your garage for example)
• Running loss emissions are the ones that you are talking about. In your case I would bet most of the heat is from the fuel pump. That is one of the reasons new cars use return-less EFI systems, so they don't add all that heat created by running a pump sized for peak HP against the regulator all the time. The OEMs realized the cost of ever-larger carbon canisters was higher than the cost of controlling fuel pressure using the pump (vs a conventional regulator and return system). The problem gets worst when we all put in larger capacity fuel pumps in to support more horsepower (power = flow*pressure). You did upgrade your fuel pump to support that HUGE new engine, right?
• Some of the more expensive aftermarket fuel systems handle this with a fuel pump controller that adjusts pump speed based on RPM or TPS or calculated HP or some other witchcraft they decide is best. I have always through in theory it would be fairly easy to use the AC shutoff output from the ECU and a DC/DC converter to underdrive/overdrive the pump when the throttle is at 80% or more. It is on my list of ideas that I haven't done yet but want to someday. There are also aftermarket solutions like this one: https://aeromotiveinc.com/product/bi...ed-controller/

If you want to use a carbon canister

• Almost any carbon canister will work as long as you get one that has a built in liquid/vapor separator or buy a separator like this one: http://www.perkofuelsystems.com/comp...el_separators/
• You can actively purge the canister using the stock computer (it will only purge during closed loop operation so that it can correctly compensate for A/F and it won't "learn" during purging so it doesn't effect your fuel tables). Or you can count on the effect of the fuel cooling down after you shut the engine off to passively back-purge the carbon. The passive option will work for shorter trips but the canister will eventually "break through" and stop working on long trips. I would suggest that you need something with a carbon bed volume of between 1000 ml and 2000 lm for your combined 35ish gallon fuel tanks.

Hope that helps!

[Lars]

Paul and Pippin, thanks for the input. Most helpful. I'm not sure what I'll do yet. Initially I'll probably try some sort of primitive venting solution, but eventually I'll likely go for an active system. Technically the wires for the CANP valve are in my harness, just cut short and buried which means breaking into it and digging them out. Annoying work but not unbearable. Then changing some lines in the input file to enable it. In any case I now understand better how the EVAP systems work.

And Pippin, yup. Big fuel pump. 225 liters/hour. Walbro GSL 392.