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fuel problem when tank is half full

883 Views 52 Replies 7 Participants Last post by  Goodbar
I am still having the same issue starting the car when it has around 170 miles after a full tank. It acts like fuel is not available. I have to crank it for about 10 seconds before it starts. There is no problem when the tank is full. Fuel empty light also comes on about this time. I replaced the fuel pump when this issue started. There is some issue with the fuel tank sending unit and the fuel gauge. Car runs normally after starting, around 24 mph.

What does this add up to, and what to fix ? Could the faulty fuel tank sending unit be causing the whole problem ? They don't make or sell that part anymore at Mazda.
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Another casualty of the repair work. Should not have used the trailing arm for a temporary jack stand position.
But the bent one was not difficult to replace, and cost about $ 50 for the part. Also finally got around to installing new lift struts for the trunk. The new ones are way better.

Metal Fashion accessory Tool Titanium Wood
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Oof. That sucks. Good that they still make replacements. Did you do the final torque with the car on the ground?
Just curious how would cars be affected if there were no trailing arms ? The rear suspension is already fixed in place by having the cross member bolted to the frame.
The trailing arms locate the rear hubs in the fore-aft direction, and transmit the rear braking force to the body of the car.

Without them, as soon as you tapped the brakes, the hubs would tear clean off the subframe. Well, maybe not that, but everything would bend backward and the tire would slam against the back side of the wheel well.
Unfortunately when I fill up complete at the gas station (hose shuts itself off), there is still some gas that leaks out the top of the fuel tank. If I jack up the left side of car where the filler is, the gas will come over the top on the right side. I replaced the gasket where the fuel lock ring is, so that should be okay. I have posted photos of my gas tank already here. Anyone can see from those photos where the gas might be leaking from ? It is just a small amount after filling up, then it stops and does not leak.
This may be a dumb question, and I can easily check it, but if the 2 EVAP line hoses coming from the top are reversed, could that cause a leak from the top ?
I also see the check valve line, but where does that hook up to the fuel tank ? Off the top of the fuel tank there are just the 3 EVAP hoses. the 2 fuel hoses, and the electrical connector.
Any comment why gas leaks out the top of the fuel tank when it is filled up ? When you let the gas hose run until it clicks off, there must be extra gas in the filler tube. That must cause enough pressure to push gas out the top until the filler tube is empty. It only leaks for a short time and not a lot of gas comes out the top. Would this be considered normal, and the solution is just not fill it up until the gas hose clicks off ?
No a leaking gas tank is not considered normal, if liquid gas can escape than gas vapors can escape, both are mild fire hazards.

All hoses, fittings and connections to the fuel tank should be clamped sealed and hold pressure.
It sounds like fuel is leaking out one of the EVAP (Fuel vapor valves) seals or the sending unit seal on the top of the tank.

Looking at the picture of your tank:

Based on this picture it does not look to be the rear Evap or the dirt around it would be cleaned away by fuel.

In this picture it possibly looks like fuel has leaked or ran around the front evap and leaked down the front of the tank, did it come from the Evap seal or sending unit?
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The check valve line does not connect directly to the tank it connect to one of the ports on the filler assembly.
If you put multiple WSM pages together you can decipher how the hoses are run and where they go.

1- Location of charcoal canister
Organism Font Parallel Engineering Diagram

2- Operation of check valve
Schematic Font Parallel Engineering Technical drawing

So both fuel vapor valves connect together and pass through the check valve on the way to the charcoal canister, but both vapor valves connect to the fuel filler assembly.

2B - simplified view:
Rectangle Font Parallel Diagram Engineering

3- Fuel lines and tank hose routing:
Rectangle Font Parallel Slope Diagram

3B - Same:
Map Font Parallel Slope Engineering

(Engineers should be forced to assemble and work on the crap they design, bet the system would have far less components and function the same if they where.)

So in this complicated and mildly hard to follow diagram we can see that the filler assembly has 5 fittings,
1 the main fill pipe
2 the pressure port (connects to the yellow elbow on the tank and sends air/fumes into the filler neck to keep fuel being pumped in from building pressure and shutting off the pump nozzle.
3 & 4 both fuel vapor valves
5 the check valve that runs back to the charcoal canister.

Based on images 2B and 3B the fuel vapor valves join together in the filler assembly piping and come out port A in image 3B.
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If I jack up the left side of the car, upon complete fill up, fuel will leak out the top and pour over the right side of the fuel tank for a short time. If the car is not jacked up, it will leak over the back side of the fuel tank. It is definitely only coming from the top, nowhere else. I replaced the gasket where the fuel sending unit is. I guess it must be one of the two rubber EVAP ports that don't hold the pressure that comes with the little added gas in the filler tube upon complete fill up. Impossible to tell where exactly it leaks from since the top of tank can't be observed.

I see the rubber grommet and vacuum check valve for both EVAP ports are still sold by Mazda. Maybe I can buy those and install them next time I drop the fuel tank.
The rubber gromments that cover the EVAP ports just seem to be put in there like any other grommet. If so, it is understandable why they could not hold much pressure. Another thing noticed is the 626 years 1999 and up have much bigger black floats for the fuel sending unit. That looks like an improvement and would make the float more stable and not vary but give a better reading of the fuel level.
I just received the new grommets and valve for EVAP system. They are clearly shown on the fuel tank also....but they only fit inside that larger opening. Is that larger opening just part of the fuel tank.....or is that another part that could be replaced ?

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