240z gas tank ventilation

[richard1]

Some time ago I replaced the filler hose of the gas tank. Since, I observed that when the tank is filled up, I start to have a dust streak on the rear fender under the gas door as like some vapors would come from under the rubber flap and run along the fender and down to the bumper. There is no smell and the gas cap seems very tight.

Any idea about the source of this problem and how to fix?

Richard

[Bambikiller240]

When you make a left turn the gas gets forced over to the right side of the car. When this happens it's common for gas to slosh out of the gas cap after a fill up. On many Z's it is much more pronounced than the symptoms that you describe. (Lot's of gas "can" slosh out)

[EScanlon]

When you make a left turn the gas gets forced over to the right side of the car. When this happens it's common for gas to slosh out of the gas cap after a fill up. On many Z's it is much more pronounced than the symptoms that you describe. (Lot's of gas "can" slosh out)

Carl:

Sorry to jump on this one.

If you have gas sloshing out of the gas tank:

Get a new Gas Cap or at least check out the washer and whether the neck ring is mounted properly.

While I can see fumes escaping due to filling up on a cold day and then the car's warming up caused the gas to expand, venting via the cap, having it "slosh" tells me that the cap is only acting as a slower-downer and not as a cap.

Or, the filler neck has been mounted wrong such that the gas is actually venting below the filler neck cap latch ring. Or there is one or more screws missing from it.

Sorry, but that's a "if you can't cap it, don't leave a wick" boo-boo award.

Enrique

[Bambikiller240]

I didn't say that I have this issue on my car, but it was a common thing to see (back in the day) a stain on thr RR quarter panel where gas had sloshed out of the tank and road dirt/dust had collected on the wet spot. It usually only happened when the car had taken a turn really quickly. It is possible that "overfilling" (topping off) had something to do with this. Also, I don't think the cap was designed to prevent any possibility of spillage. Ever see a car upside down? They often leak.

Can you enlighten me about the Filler neck possibly being mounted wrong? Having taken my filler neck apart to replace the rubber "flap" I can't uderstand how it could be "mounted wrong". There are 3 screws to secure the metal ring, and that's basically it. (Metal ring, Rubber Flap, Filler Tube) No gasket, nothing. The screws go from the latching ring the through the rubbber flap, and rubber filler neck and then thread into the captive nuts (or whatever those are called) built into the QP.

Kyle (the moderator of the 240Z.org list) had the problem a while back and started tweaking the "ears" on the filler cap to tighten the seal. Last I heard he hadn't solved the issue. (I kinda lost interest in the mail list)

[EScanlon]

The filler neck goes through the metal cup, then through and above the rubber flap, then the cap mounting ring. This gives the flap a flat surface to be pressed against with a flat ring (the filler neck), the cap mounting ring provides pressure all around the filler neck ring and the flap evenly.

Granted that if properly tightened, the possibility of a leak through this area may seem small, but all it takes is a bit of the rubber flap to be twisted or tweaked as it's forced into the mounting ring if it's mounted above the filler neck ring. I've also seen filler neck rings mounted with only two screws, or with one loose. You know that can't be right, when there are 3 screws to mount it.

Most gas caps aren't designed to hold a bunch of pressure. I'm not sure of the pressure rating, but one thing is to contain a "splash" of gas and another is to act as the stopper when the tank is upside down. Even in a sustained left curve the pressure against the gas cap couldn't possibly be more than a pound or two...over 6 square inches (roughly 2" cap * Pi). Even if you were to up the pressure to 5 pounds the total pressure on the cap is less than a pound per square inch. Now turn the tank upside down and ONLY put in ONE gallon of gas and you're now talking 6-7 pounds on the cap which is now greater than one pound per square inch.

In my experience, most people report an "empty" tank when they have three to five gallons still left in the tank. That's a lot more pressure. Go with half a tank (7.5 Gallons) and now you're talking approximately 50 pounds of pressure. Even your radiator cap wouldn't hold that.

All this is conjecture as I don't have actual measurements nor weights, but the basic principle applies, the gas shouldn't be able to slosh out of the cap. If it does, it's not making the basic - slosh proof- function it's required to do.

I replaced my gas cap with a locking gas cap. That arrived with a nice thick rubber gasket that gets compressed when you put the cap on such that both tangs engage the neck ring. The ONLY time I've noticed the tell tale "oily smudge" was when I over-filled (I could see the gas in the filler neck, putting the gas about 2" below the cap) and I didn't put the cap on properly (one tang of the cap wasn't engaged fully).

I'll check the new Z I haven't yet disassembled and post whether the standard original cap had a rubber washer in it or not.

As far as tightening the existing cap, first check to make sure that the neck ring isn't itself worn down. Tightening the ears on the cap would make it fit tighter, but only to a small degree. I'd be willing to bet that most of those "sloshing" problems were caused by crushed rubber gaskets, or simply, missing gaskets.

What about other members? Would you check your gas cap and see if there's a rubber gasket there?

I wasn't implying that YOU had a car like that, I was simply stating that I would immediately question any signs of gas spillage and not simply attribute it to a common occurrence.

Enrique

[Bambikiller240]

The filler neck goes through the metal cup, then through and above the rubber flap, then the cap mounting ring. This gives the flap a flat surface to be pressed against with a flat ring (the filler neck), the cap mounting ring provides pressure all around the filler neck ring and the flap evenly.

I'm not sure what you mean by "metal cup". Maybe the opening built into the QP? The rubber flap is between the QP metal and the flange (or lip) of the Filler neck (right?), and the locking ring (that the cap gets secured to) goes on top of all of it and the three screws clamp it all together. Yes?

Granted that if properly tightened, the possibility of a leak through this area may seem small, but all it takes is a bit of the rubber flap to be twisted or tweaked as it's forced into the mounting ring if it's mounted above the filler neck ring. I've also seen filler neck rings mounted with only two screws, or with one loose. You know that can't be right, when there are 3 screws to mount it.

I don't think I understand the part above that I have put in blue. The flap being forced into the mounting ring if it's mounted above the filer neck ring? I found my rubber flap to be sandwiched between the underside of the filler neck flange and the underlying metal of the QP. The locking ring was in direct contact with the upper surface of the filler neck flange (lip). If all three of the screws aren't tightened properly that will (of course) cause problems.

Most gas caps aren't designed to hold a bunch of pressure. I'm not sure of the pressure rating, but one thing is to contain a "splash" of gas and another is to act as the stopper when the tank is upside down. Even in a sustained left curve the pressure against the gas cap couldn't possibly be more than a pound or two...over 6 square inches (roughly 2" cap * Pi). Even if you were to up the pressure to 5 pounds the total pressure on the cap is less than a pound per square inch. Now turn the tank upside down and ONLY put in ONE gallon of gas and you're now talking 6-7 pounds on the cap which is now greater than one pound per square inch.

In my experience, most people report an "empty" tank when they have three to five gallons still left in the tank. That's a lot more pressure. Go with half a tank (7.5 Gallons) and now you're talking approximately 50 pounds of pressure. Even your radiator cap wouldn't hold that.

Maybe I'm reading it wrong, but it sounds like you are saying that the pressure exerted by half a tank of gas is greater than the pressure that can be exerted by a full tank of gas. When a hard or quick left turn is made, centrifugal force is applied and pushes the gasoline (and anything else that is not secured) to the right. Now I am no scientist, but my understanding is that the force applied to the cap will be more than equal to (greater) than the weight of the gasoline being moved to the right. More gas = more weight transferred. (?) I don't know if the gas caps are designed to keep gas from leaking out when a car is upside down, but I do know this. Of the 3 (street) cars that I've seen upside down after accidents (that didn't appeear to have damaged the filler or cap), 2 of them were definitely leaking fuel onto the ground from the gas cap, so it does happen. Static wieght of the fuel was the only force being applied to the cap (no outside forces like when a hard turn is applying centrifugal force)

I replaced my gas cap with a locking gas cap. That arrived with a nice thick rubber gasket that gets compressed when you put the cap on such that both tangs engage the neck ring. The ONLY time I've noticed the tell tale "oily smudge" was when I over-filled (I could see the gas in the filler neck, putting the gas about 2" below the cap) and I didn't put the cap on properly (one tang of the cap wasn't engaged fully).

I have a brand new, still in the box (and sealed plastic bag) locking gas cap in my spares box. It (as yours) has a really thick rubber ring (gasket/seal/whatever)

around the inner circumferance. It was hard to find and expensive to buy, so I haven't been able to force myself to open the packaging and use it.

I firmly believe that over filling has a lot to do with the "leaking" issue.

As far as tightening the existing cap, first check to make sure that the neck ring isn't itself worn down. Tightening the ears on the cap would make it fit tighter, but only to a small degree. I'd be willing to bet that most of those "sloshing" problems were caused by crushed rubber gaskets, or simply, missing gaskets.

I would replace the damaged parts (Cap or Locking Ring) with undamaged ones before I'd go messing around with "adjusting" either one (like some people on the mailing list chose to do). It's my feeling that the parts were designed to work properly together when undamaged (unmodified) so that is the way I want them on my car.

My car has a relatively new OEM gas cap (new when I bought the car) and IIRC it has a thin sheet of rubber built into the inside surface of the cap (which would contact the locking ring) (I'll take and post pictures tomorrow)

[Bambikiller240]

Today while looking for info on 5 Speed Transmissions, I found the attached write-up on fuel leaking from the filler area.

It is from the book THE COMPLETE DATSUN GUIDE by Micahel F. Hollander

Published in 1980 by TAB Books ISBN 0-8306-2060-5

Chapter 6 "Perfecting the Z Car" on Page 124

The attached picture of a Gas cap is (I believe) a 72-73 version.

Enrique:

If I gave the impression that one should just accept a leaking fuel situation, I appologize. I did not mean that. I merely meant that the fact that it occurs is commonly known. It should, however, be remedied ASAP for safety sake if nothing else.

[EScanlon]

I'm not sure what you mean by "metal cup". Maybe the opening built into the QP? The rubber flap is between the QP metal and the flange (or lip) of the Filler neck (right?), and the locking ring (that the cap gets secured to) goes on top of all of it and the three screws clamp it all together. Yes?

Exactly. The "cup" I was referring to is from my body-working days. The quarter panel can be obtained with or without the gas filler "cup", i.e. the metal stamped "cup" that gets attached to the backside of the fender to maintain the filler opening "outside" the vehicle. The gas door closes this "cup" made in the panel.

Without this cup, the filler neck either extends OUT of the body (Jeep's, Late 50's Style pick-ups, Datsun Roadster, MG, Triumph) or is INSIDE the sheet metal and therefore inside the trunk or passenger compartment. I can't think of a single example.

Excellent post regarding the early Z fuel cap problems. I can definitely see why a hard turn in those vehicles would allow the splash that you mentioned. I wasn't aware that they had had the problem.

I understand better what you were saying originally.

I didn't figure you for the stick a wick in it type. I felt that it needed to have emphasis placed on it's not being something to ignore. How many times have you seen a where a driver lost his gas cap and just stuffed a rag into it? The perception of the danger was that .... it didn't happen....often. As a friend of mine put it, it's bad enough it's dangerous, do you have to put a wick into it for any idiot to give you a light?

Gas caps are often ignored as part of a restoration, repair, or troubleshooting. A poorly fitting gas cap can be as dangerous as the loosely fitted ring it fits onto, as your first photo attests.

Maybe I'm reading it wrong, but it sounds like you are saying that the pressure exerted by half a tank of gas is greater than the pressure that can be exerted by a full tank of gas.

Nope, I meant the opposite and when upside down. A full tank will exert more force on the gas cap when upside down than when half full and upside down.

When a hard or quick left turn is made, centrifugal force is applied and pushes the gasoline (and anything else that is not secured) to the right.

Actually, in this case, it's inertia.

Centripetal forces are what get exerted when you re-direct that force. Or the force applied to impel an item towards the center or axis of force.

Centrifugal force is what causes an object to fly away from the center of the curve once the centripetal force has been removed.

Inertia is the front half of the equation (An object in motion stays in motion)

And the second half (until a force is exerted to affect that motion) can be expressed as friction, air-resistance, drag, the ground, as well as centripetal force amongst others.

Once you've exerted AND removed centripetal force the resultant force is the centrifugal force of that item.

This is a very complex relationship that took me a long time to get right, so if I've goofed please post and correct me.

Now I am no scientist, but my understanding is that the force applied to the cap will be more than equal to (greater) than the weight of the gasoline being moved to the right. More gas = more weight transferred. (?)

Yes, but then a few other forces get added to the equation as well as limitations.

In an upside down situation, the pressure exerted on the gas cap is substantially more than it's designed to hold within. That's a given as to why it's leaking once upside down. The sole force acting on the gas is gravity. That would be it's total force and therefore total pressure on the cap.

In the change of direction example, gravity also plays a big role as a reduction factor as to the pressure that would be applied to the gas cap.

The only time that the pressure on the gas cap in a hard turn would equal (or exceed) the pressure when upside down would be when the force were greater than 1 G (as in the force of gravity). In order to do that, you're referring to the instant just before the car begins to roll. Up until that point, you have a percentage of the weight of the gas working against the centripetal force on the gas. This just means that you reduce the total force exerted on the gas cap.

Now, an additional factor comes into play. The total amount of force exerted on the gas cap, in a full G of curve is not the sum of the weight of the gas in the tank, but rather the weight of the column of fuel that is exceeding 1G, and the corresponding pressure exerted inside the fuel tank to the area of the top of the tube (actually, the bottom of the filler neck, where it connects to the gas tank) and once again you're back down in lower figures of pressure on the gas cap.

The key thing to consider here is the duration of these forces. A curve lasts only so long, whereas gravity is constant. The curve has a beginning and end (in this example) and therefore there are periods of time when the force is not enough to exceed the full G of centripetal force.

The simplest way to say it is that only at the point of the curve that the force of gravity is exceeded and only for the length of time that it is exceeded it would it exceed the gravity pull for the same item standing still. The force in a gas cap, need only sustain that "splash" for a given period of time, hence the reduced pressure requirement. If it had to hold the pressure for a long time then it would be designed differently. Instead of a simple compression gasket relying on hand applied force, a design that provided a strong constant pressure. One example of this is a radiator cap.

Except for racing systems and fuel injection systems, for the most part, carburated systems have a vented draw fuel system and not a pressurized one.

At the pressures the gas cap holds, at best you hear a subtle "pssssst" when you release it. Don't do that with a radiator under pressure.

As you pointed out, gas flying out the back is something to address.