Help needed here.

I think maybe I am one effective working fuel tank vent away from a healthy fuel delivery system. When I installed the new tank, I cut the filler tube in two, and had the lower half welded in. Then the upper is connected to the lower with a clamped rubber hose of the proper fuel handling material. This coupled with the plug I applied to the factory vent in the filler tube created a pretty well sealed tank. Unlike the factory unit, which always smelled to high heaven of fuel.

I don't Really know how this is supposed to be designed.
As it turns out, I don't really understand how the modern evaporative, and purge systems are supposed to work. I did try to apply a vent to my tank, but I now think I did it wrong, and it does not work. I sent a new Mopar tank to Tony's Rod shop in El Paso, where they installed a sump and modern in-tank pump. I think this tank I sent them was patterned after the 70 cars as it looks just like the one that was in my old 70 Dart.

My Questionable design.
This particular tank had four small, lateral nipples coming out of the upper driver's side front corner of the tank. On my 70, these nipples had four hoses which went to an expansion tank up in the trunk. On my '69 I plumbed these four nipples up onto a manifold which I constructed of brass nipples and fittings. I took what I thought was going to function as a vent tube from a fifth nipple on this manifold, and ran it to the front, and into the air cleaner.

The Aha Moments
My first clue that this design was not working was that the engine became fuel staved after a half hour of driving. Fuel pressure dropped, and AF ratio went lean from 14.8:1 up to 17:1 or worse. It still ran, but not well. In subsequent investigations into what I had done two years ago, I traced this all out. When I pulled one of the hoses off the manifold to sniff for gas, thinking maybe it was plugged somehow, a strong stream of gas ran out.This was not just a trickle. I quickly reapplied the hose to stop the flow. But, it occurred to me that if this hose is full of gasoline, the other four must be also.

Trying to Understand this set up.
I did not pull them all off to check, but liquid does seek its own level. Each hoses have a low spot in them as they must come under the frame and back up to the manifold which is in the left front side of the trunk. So it seems that the hoses either fill up as the tank is being filled, or when the car corners. The four nipples from the tank are coming out of the side, and not the top. And, if they are full of gasoline, they will be blocking any venting action. I don't understand why the gas would stay up in the manifold though. This fuel ran down out of the nipple that comes down out of manifold. I think. It all happened pretty fast, and I plugged it back up pretty fast as I was under there with a trouble light, and I did not want my trouble light to make the fuel go boom.

What is the right way to do this?
So how do the factory tank vent systems work? Do they have a high chamber in the tank where vapors only can collect, and which will function as a vent to release the vacuum as well as allow vapors to travel to the charcoal canister?

Here is the good news. There is a factory vent up high on the filler neck, which I capped off. Can I not simply run a vent hose from there to the front?


And finally, what were those four tubes for? Are they for expansion? If so do I need to make a more substantial expansion tank for the trunk, much as my 70 Dart had? Isn't a canister full of fuel vapor in the trunk pretty dangerous? Does anybody out there have a fool proof system I can apply here? I guess the filler tube is going to be full of vapor anyway. Huh!

Is the light at the end of the tunnel here?
Thanks, as always. I have been chasing this ghost up the wrong alley for a long time. I think I might be getting closer to catching it. I am hoping one of you clever guys out there will not only know how the old Mopar system worked, but how modern ones work as well.

Sam

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Venting the fuel tank to the air cleaner will not only create a safety hazard, but will also create random fuel mixture problems. Definitely not the way to do it.

First, about that 4-vent tank: The system you're asking about is present on '70 California cars and '71 50-state-plus-Canada cars. '71 was the first year all cars in North America (not just California cars) were equipped with evaporative emission control systems (ECS). This first-two-years system is more complex than the relatively simple '72-up system.

The '70-'71 setup uses a complicated fuel tank: there is a small inner tank at the top of the main tank and connected to the main tank chamber by a very small passageway, so that the main tank chamber fills up with the secondary chamber still mostly empty. The gas pump nozzle detects a full tank and clicks off, you hang up the nozzle and drive off. Over the next little while, the secondary tank fills up from the main chamber, dropping the fuel level in the main chamber so as to prevent fuel leaving the tank when things warm up and expand.

These '70-'71 ECS fuel tanks have four vent fittings, one at each corner of the top of the tank. These are brought together in the trunk to a more-or-less vertical vent cylinder leaning against the left wheelhouse. This vent cylinder has five hose fittings: one for the hose from each corner of the tank, and one for a hose connecting to a line that runs up to the right rear corner of the engine bay.

Inside that vent cylinder, each of the four fittings from the corners of the tank continues vertically, each to a different height. The fitting to the engine bay continues inside the cylinder almost to the top. The idea with the 4-corner venting and the different-height pipes inside the cylinder is that no matter what angle the car is at, in what direction, there will always be at least one vent fitting above the liquid-fuel level, so there'll always be a vapour vent, thus preventing pressure buildup that would force fuel vapours out into the atmosphere.

The '70-'71 ECS is also unique and interesting in that it uses the engine crankcase to store fuel vapours piped forward from that vent cylinder in the trunk and from the carb bowl. The interface point is a fitting on the fuel pump body on slant-6 cars, or extra fittings on the engine breather cap on V8s. When the engine is started, the collected vapours are drawn off out of the crankcase via the PCV valve. This system does a reasonably effective job of containing the vapours, since the crankcase is well sealed, but it can cause hot start/hot idle problems, since there is no control over when the vapours are extracted from the crankcase. Most of the unburned hydrocarbons wound up out in the atmosphere anyhow, having been first put through the engine (and emitted as black smoke).

For '72, things got very much better. At the back of the car, the inner tank was deleted; by clever shaping of the top dome area of the tank, the same function was achieved by means of a controlled air pocket at the top of the inside of the tank. The '72+ tanks also have only one vent fitting. At the front of the car, the crankcase was no longer used to contain fuel vapors. Instead, a canister containing activated charcoal granules was mounted at the front corner of the engine bay. Lines lead to it from the carb bowl and the fuel tank vent. There is a purge valve mounted atop the canister, with a small diameter vacuum line teed into the distributor vacuum advance line, and a large diameter line from the underside of the purge valve, teed into the PCV valve hose. There's vacuum in the distributor vacuum line only above idle, so the purge valve only opened above idle. Voila, no more hot start/hot idle problems, since fuel vapors were only drawn off and burned above idle. This system works fine.

You're starting with a 4-vent tank. Ideally you'd duplicate the '70-'71 setup with the angled liquid/vapour separator in the trunk; it should not be difficult to get hold of the necessary separator. It's on every '71 A-body and all '70 California A-bodies. This will give you a single vent line to work with and will serve to keep the liquid fuel in the fuel tank where it belongs, something your brass manifold can't do. I am assuming the four vent ports on your tank were not blocked off or otherwise altered in the process of tank modification.

You'll want to add a charcoal can. Bracket's easy: every '72-up A-body has it at the right front corner of the engine bay. Or fabricate your own bracket. Original types have three ports: TANK (tank vent), CARB or BOWL (carb bowl vent), and PURGE. If you find a replacement with only one vent port ("tank", these are common on fuel injected cars that don't have a bowl vent) that'll work fine for your fuel injected application.

There needs to be a canister purge valve that only opens when the engine is above idle. Some canisters have these built in (looks like a round "flying saucer" atop the canister itself), but many do not, including Mopars after '72 and most late-model ones in which purging is controlled by the computer via a solenoid. Fortunately, standalone purge valves are readily available and inexpensive. NAPA Echlin # 2-28011:


On a carbureted application, the small fitting gets teed into the vacuum advance hose (which should have no vacuum at idle, full vacuum above idle). No suitable ported vacuum on an EFI setup, but I am guessing/assuming you have a closed-throttle switch of one sort or another (some throttle position sensors have a binary closed-throttle switch function built in). If you don't yet, add one. A lever-type momentary-break (normally-closed) switch is perfect, though if you can find only a normally-open, that can be made to work by means of a relay. Put it on a simple bracket with slotted holes for adjustment and set it so that it closes (NC) at a throttle position corresponding to about 1400 rpm. Then power a vacuum solenoid through this switch so that the solenoid is open above 1400 rpm or so, closed below that. Plumb the system such that the purge valve receives its trigger vacuum (to the small port) above 1400 rpm or so.

The purge valve's big fittings go inline with "PURGE" hose from canister, which gets teed into the PCV hose.

Also, you will need to change your fuel filler pipe to accommodate a pressure-vacuum fuel cap. These were used starting in '70 (California) '71 (50-state/Canada) but you will want to use a '72-'76 fillpipe because the caps are much easier to find for them. If you were using the whole filler pipe, any 4-door or Scamp/Swinger fill pipe will work, but not one from a Duster-body car. Your "half and half" setup, though, probably won't care what car the upper half comes from. I'm not entirely clear on why the whole fillpipe can't be used in your setup. If you use a fillpipe from a '76 unleaded-fuel A-body, you'll be rid of the problem of fuel spilling onto the quarter panel during fuelling. While you are making this mod, please pay careful attention to how your two halves of the fillpipe are connected. Clamped hose works fine until a violent crash; if you can't use a whole, intact filler pipe, see if you can engineer a more robust setup that would tend to better resist pulling apart and spewing gasoline everywhere should the car get hit hard anywhere near the fuel tank.
Re-opening this vent will work as a get-by method, but if you do it, you need to run a hardline from that vent vertically up the aft of the wheelwell, then make a tight-radius U-turn and come back down and through the trunk floor to under the car. The upside-down U-bend serves as a mediocre prevention against liquid fuel egress. An open tank vent like this means the garage will stink again. remember your fuel in the tank is going to be running warmer since you're cycling fuel from the engine bay back to the tank (if I'm not mistaken; if there's no fuel return in your system at present, get busy and add one!). This warmer fuel argues even more strongly for a proper closed tank vent system w/charcoal can as described above.
No. Its internal fuel/air mixture is always way too rich to ignite.

And since we are talking about fuel supply system safety, does your fuel pump control circuit include an inertial cutoff switch to kill power to the pump if the car is hit hard? If not...add one!

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Yes, the '76 fillpipe works pretty well, as long as the fuel tank filler pipe seal is tight and the pump jockey doesn't try and put one last "squirt" into the tank after it clicks off.

Variation to the rule above, my duster is tight enough that if they insert the nozzle all the way into the filler pipe it'll bump off at the slightest 'hiccup'/bump and they have to hold it and "listen" for the fill level.

FYI,

-D.Idiot

Thanks Dan. That was precisely the kind of info I was hoping someone would supply. And, I kind of suspected it would be you. Thanks again for your time and expertise in such matters. I can see that the early challenge of automotive engineers was getting a vapor chamber that did not become blocked by liquid fuel.

There is indeed a return fuel line. There are five parallel "things" under the floor, 1. Fuel delivery 2. Fuel return. 3. currently ineffective tank vent/purge 4. Battery ground cable to engine block. Battery is in the trunk. 5. Rear brake lines.

When you combine your comments about the garage smelling again if there is a tank vent with no charcoal canister(which is not a part of the current system), with the fact that I can not smell gasoline around this car when it is parked, I think it safe to conclude there is not currently an effective vent on this tank. Prior to putting this tank in, it was always stinky.

So here is the most fundamental, down to basics question I still have. If you get an effective purge system for handling evaporative emissions, does this also function to allow air to enter the tank to eliminate the vacuum block? A dumb question, but I have learned it is better to ask those early. I have also learned the hard way not to assume anything, or take anything for granted here. I can see that there is likely some hard learned experience behind what looks like simple engineering problems/challenges for automotive companies. Wasn't it a simple O ring failure that caused Challenger to explode? It wasn't a failure of all the high tech hardware and software.


Parts such as the expansion chamber you describe are getting pretty hard to find these days around here. Most of the yards around here have crushed these old Mopars by now. I guess I could make one if I cannot locate one. When I restored my old '70, I simply replaced everything back to the way it was before, with no idea what it did. Does anybody out there have such a part lying in their parts piles?

So if I understand it correctly, these four nipples each run to one of the four corners of the tank. I don;t know for sure, but it seems as if Tony's Rod Shop would have had to work at it it to disable these. And, the external nipples still seemed to be fastened in pretty strongly, suggesting they have full support from the rest of the tube inside the tank. On my 70 parts car, which is now gone, and was also a slant, there was a nipple on the crank case breather cap which I kept for awhile and then tossed at some point.

I like your idea of configuring a hard line inside the trunk to use the existing filler neck vent tube. I see the logic of running it further up and then bending it back down. It is already pretty high up, but I can likely get it a little higher and then turn it back down. This would be simple, and unlikely to fail in some unexpected, complex way as my supposed attempt did. And it would circumvent the possibility that Tony's Rod shop did indeed chop one of the internal lines when cutting the top of the tank to install the sump.

The line that I am currently trying to use as a vent likely has liquid fuel in it along the way somewhere. I suppose it is safe to blow it out with compressed air before hooking it back up.

I also own and drive a Miata. It has a compact charcoal canister and an electric purge solenoid. I could easily just copy their parts and mounting for the Dart. Would there be any problem with having the solenoid activate with key on? Asian and European parents often have pretty handsome children.

I do not have an inertial fuel pump cut off. Do you have a part number for that? I think it is smart to get the system working reliably before introducing that variable. In that way, if the inertial cut off switch happens to be defective, I can see the cause and effect clearly. For that matter, is there a generic supply house charcoal canister one can buy?

What I have learned in the past 3 years now, is that with the forum for support, I am just kind of dumb. Without this forum for support, I can be really dumb. I had my head immersed in high tech while it was really something quite low tech (think space shuttle O rings) that was getting in the way. So, when this car ends up working well, you guys can all take pride in it.

Sam

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Y'welcome.
I agree.
Yes -- the charcoal can serves as a tank vent, and the pressure-vacuum fuel cap serves to relieve overpressure or vacuum situations that may develop under some conditions even with a charcoal can vent.
But since nobody ever asks for them, anybody parting a '71 (or '70 California) A-body will be able to supply one for you, and they probably won't want much for it. You may have to get it from away; this guy probably has one for you if you can get in touch with him. Post a parts-wanted ad here on this board.
Correct.
There is no rest-of-the-tube inside the tank.
Yes. You don't want purge at idle and low RPM; it'll screw up idle quality in ways too random to compensate for.
Grab one out of the trunk of most any '90s Ford at a wrecking yard. Looks like this:


Not as such, but they are rather universal in function. See if you can find one from a late M-body (Diplomat/Gran Fury/New Yorker Fifth Avenue). That way it will be the right diameter to fit in the A-body charcoal can bracket.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Thanks again Dan. I did the quicky, short term fix of bending a piece of tubing that goes up into the window surround, and then back down to the tube that goes to the front. When I blew out the tube with compressed air, a bunch of liquid fuel blew out of that line.

So when I sucked on the cap vent from inside the cap, I could suck, but could not blow through the same orifice. It seems that this should have allowed enough air in the tank to compensate for the vacuum created by fuel use. So maybe the lack of a vent was not causing my problems. Or maybe it was. The vent line was plugged for sure, but the cap was not. Is this wrong thinking. Is the cap vent too small to allow enough air to enter? I know that when the cap was not vented in older models, they depended on the much larger vent in the filler tube to vent the tank. And, explain again please how this cap functions with the charcoal filter. I appreciate your patience with me on this. I am being pretty slow here with this I know.

Sam

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There is no vent on a fuel cap for a pre-'71 (50-state) pre-'70 (California) car. Not sure what kind of cap you're using; it also raises questions about just what kind of fuel filler pipe you've got. The pressure-vacuum cap alone is not sufficient for tank venting, and will not pass an adequate volume of air to prevent partial vacuum in the tank.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Interesting. I always thought the early (at least '68-down) caps were all vented, since the vent tube that goes up near the mouth of the filler pipe does not exit to the atmosphere directly. Maybe I am misinterpreting your '71-down statement above.

I'm pretty sure I once bought a cheap parts store (Stant, ugly, non-locking) cap for one of my cars ('64 I believe) that said vented on it.

Lou

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Home of Slant6-powered fun machines since 1988

I have three caps. Two are the chrome after market variety. One is vented, and the other is not. The vented one has the safety prongs to keep it from kicking out too fast when released. The third one is a locking cap and it is vented as well. In each case, the vented cap sucks but won't blow, from the inside. Lou, I was under the impression the cap with no vent is for the older cars. It seems my 69 was solid. I have lost several of those by leaving them at gas stations, so cannot check. Maybe some one with a stock set up out there can verify. I think the upper part of this filler neck must be the older style since it has the nipple for the vent on it.

It is encouraging, Dan, to have you say the vent alone will not prevent vacuum lock on the fuel supply. Maybe I have solved the problems of fuel flow. Right now, I am baby sitting my grandkids. As soon as Mom and Dad come back from their day out, I will fire it up and give it a test.

Right now I have the vent line running to the air filter. Eventually I will get it plumbed to a charcoal canister. Just out of curiosity, could you plumb the purge hose from the canister to the air filter without messing up the mixture? And would this not be better than no charcoal canister at all?

Sam

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Nope...the caps are completely sealed through '69.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

That's a '72-up (actually late-'71 up) pressure-vacuum cap, but from your earlier description it sounds defective.
Yeah, but then you'd have no purge action on the charcoal can.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Does being able to suck in but not blow out make it defective? That is how both vented caps are behaving. How should the vented cap respond?

So the canister needs vacuum on it to be purged. The Miata is plumbed right into the intake manifold. So what is the logic of the solenoid. Only when TPS is above a certain level? I can see why the ported vacuum switch is the no brainer here. Unless Mazda designed their purge switch to open only above a certain voltage. I will see if I can determine that.

Sam

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Well, keep in mind that the pressure-vacuum cap has spring-loaded valves in both directions -- it's not supposed to be free-flowing in either direction, and you can generate greater suction than pressure with your mouth.
Yup, probably so, though with rapid enough feedback control of the mixture it becomes less crucial to constrain when purge occurs because its effect on mixture can quickly be compensated on the fly.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

I went out and test drove it tonight, but there was a bunch of tuning needed, and I spent more time on that then anything else. The jury is still out on the vent effectiveness, but it seems better.

One thing that confuses me is that the pressure regulator allows 43 psi at idle when it is cold, and the vacuum reference is supposed to cut it down to 37 at idle. It does not seem to do that until it warms up. Which it did tonight. However, it did not drop down further as it did when Lou was here when it dropped down to 30 and then was unstable. I did not take it out and slam on it as we did then, when we saw the fuel pressure drop as low as 10 PSi under load and boost. Until I try that I won;t know if it is really better.

I must block off the idle air control valve on the TB before I get into further tuning. I suspect the solenoid is drifting on me, and fouling up the idle because it idled at 1200 RPM tonight until I closed the Throttle blade some. Last Sunday when Lou was here we set it for around 900 RPM. So, that is the next small task, cutting an aluminum plate to block off the chambers that the idle solenoid connects. The back to tuning.

Stay tuned. (HA!). I have posted an Evans coolant question on another thread.

Sam

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Sam, are you using 3/8th or 5/16th inch fuel line tank to engine? If stock 5/16th inch you may not be getting enough volume of fuel to allow for higher pressures.

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67' Dart GT Convertible; the old Chrysler Corp.
82' LeBaron Convertible; the new Chrysler Corp
07' 300 C AWD; Now by Fiat, the old new Chrysler LLC