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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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I took the 2 clamps off the rubber filler connector hose, checked around, then put the clamps back on......fuel still leaks out after a fill up at the gas station......but for a very short period. I see no parts available for the filler flap.
Were you able to verify if the flap moves freely and does not get stuck?
I haven't taken the rubber fuel filler hose completely off yet, since I don't have a replacement if it gets damaged or torn. After I get a spare, I will do that, or drop the fuel tank. I found the leak after filling up was from the small hoses. The shop neglected to put back on 2 of the small quick release clamps. I realize they are in an awkward spot right above the rear suspension, but that is just shoddy work. All those small pipes and the looping around the filler neck are all for the return of the fuel from the fuel pressure line / FPR ?
After a few more tests, the fuel leak looks to be coming from the top of the fuel tank. It only leaks when the tank is almost full, and then you put in the last 1/4 gallon or so. That tells me the likely culprit is the fuel tank pump opening....likely no gasket put in by the shop or a damaged gasket. There are none to buy anymore, so I will have to get some liquid gasket material and see if it works. Finally will have to drop the fuel tank to do this, and then view what is going on with the fuel sender indicator.
Continuing discussion here so as not to clutter my build thread.

I measured the rubber fuel filler hose and pipe again. Pipe diameter from the filler neck is 5 1/4" OD. Rubber hose is 7" long. I did not measure the pipe or hose coming from the fuel tank. It seems a little odd the flange coming out from the fuel tank has little length past the raised portion. From what I have seen, you are supposed to clamp a fuel hose prior to the raised portion. But on this fuel tank, there is no space to do that. The clamp has to be put on after the raised portion next to the tank. I think I have a fuel leak where the fuel pump bung is. They don't sell these gaskets anymore.
I'm not sure what you're referring to by 5-1/4" diameter. Is that the fuel pump opening diameter? There's no piping that is that wide. I haven't disconnected anything further up than the rubber filler hose that attaches directly to the tank. Haven't needed to in order to drop the tank.

If you think your fuel leak is coming from the fuel pump opening, you can get a replacement rubber o-ring and lock ring for about $6; I did:

Do you have the factory workshop manual? It was very helpful for me.
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I see Rock Auto has the lock ring and gasket for the 1995, but not the 1994. I will order that.
What is the hose and opening on the left side of the lock ring on the fuel pump photo ?
Hopefully the leak is coming from the main lock ring area, and not the 2 on either side of the main lock ring. Those have rubber seals too, but the shop should not have done anything to those.
I mean the fuel filler neck pipe is 5 1/4" circumference, so it looks to be 1 1/2" diameter. I didn't take the rubber hose off to measure that.
What is the hose and opening on the left side of the lock ring on the fuel pump photo ?
That's the "evap 2" port.
I am right in the middle of removing the fuel tank. I estimated I got it down to around 2 or 3 gallons of fuel before parking it and starting this.

the good: when I took off the rubber hose from the fuel tank filler to the fuel tank, no fuel came out. The hand siphon did not work when trying it first from the outside filler cap. It did work when I stuck it directly into the fuel tank, and about 2 gallons came out easily. Also when disconnecting the fuel hoses, only a few drops of fuel came out.

Disconnecting all the hoses to the fuel tank and electrical cord went smoothly.

the bad: there are 3 straps holding up the fuel tank, and 2 of them are locked into 1 end with what looks like no way to completely remove them. Only can loosen 1 end and let that hang down. The other strap can be completely removed. 2 straps had a bolt to remove. The 3rd strap did not have a bolt where the bolt hole is....somehow that area is welded to the piece that is bolted to the it is that bolt that has to be taken off to get the strap loose and come down. I don't know if this is abnormal or not. It does make the strap end bigger, and with no way to remove the other end of the strap, makes it more difficult to lower the straps.

the ugly: the straps won't come down unless the subframe is lowered, as there is not enough space. Also having trouble with the big heat shield that covers most of the bottom area of the fuel tank. Such troubles occur when all you have is jacks and jack stands to lift up the car.

The fuel tank still feels like it is fairly heavy without fuel. When I try to put it back on, I am going to have to figure out a way to gradually lift it up so the straps can go under and support it again. Right now 2 straps are hanging down from 1 end and the 3rd strap is removed completely. Not the greatest thing to have the straps in the way when you are trying to lower the fuel tank and move it out from under the car.
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Two of the straps have pegs that hook into sheet metal brackets at the forward end. You might be able to lift them up and out of the brackets with the fuel tank in place. In any case, the straps don't need to be completely removed to drop the tank. The heat shield comes with the tank, no need to remove separately.

You probably won't be able to get all the fuel out on account of the baffles inside the bottom of the tank. Just put a floor jack under the center of the tank, disconnect the rear ends of the straps (and all the hoses) and lower it down slowly.
Because I can't get the car very high with my jack and jack stands, the 2 remaining straps have 1 end sitting on the ground and acting as ramps.....which slide and wedges the fuel tank backward against the sub frame. It's stuck for now until I figure out what to do next. Always something comes up not in the manuals for a backyard mechanic.
If you can, try lifting the front of the two straps about 1" and then pulling them toward the rear of the car. That's how you unhook those ends from the brackets. Not sure if possible with the tank still in place though.
I got the fuel tank to drop and slide back out this morning. A little higher jack stands plus the sub frame bolts removed gave it enough space to slide down the 2 straps and out the back, with the heat shield still on. As long as no damage to where those 2 straps are locked in on the front side, I can put it back in the same way. 2 straps are locked in while the 3rd one is on a hook, and can be easily completely removed.

I will spend some time today trying to spot my 2 problems......the fuel gauge sender only showing 1/2 full....and a fuel leak, but only when the fuel tank is completely full (so it must have some in the fuel filler neck that pushes enough to cause a leak out the top of the tank for a few minutes). I don't know how I would spot a fuel leak, so I will install a new gasket around the fuel line sprocket, and put some around the outside and the 2 EVAP rubber discs.

When I did the coolant line in the engine area where the knock sensor is, I had to do the same thing as far as gasketing. I would put in the normal gasket but still get a leak. After I put in the normal gasket, and on top of that put gasket former material around the outside, I don't have a drop of water leaking anywhere.

Then reassemble tomorrow, which should go a lot quicker. The sub frame came out of align after removing the 2 bolts on either side and dropping the sub frame, so it took me a while to get it back where it should be to put the 4 bolts back in. Thankful for those nail like structures on either side that make the sub frame easier to put back.
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I think the 2 fuel issues have been solved, but won't know for sure until the car is back running.

Now I have the fuel tank directly under the car where it will be attached, and the 3 straps are completely off (2 with pin ends and 1 that goes over a hook. I really don't want to do the reverse procedure, and slide the fuel tank back up over 2 of the straps if I don't have to. 1 question:

- when the fuel tank is being directly lifted up into place, is it possible to attach the straps to their locking ends easily (not the bolt ends). It looks like you would have to get the car way up for them to be easily attached.

If that is the case, I won't have any choice but to do the reverse procedure: slide the fuel tank up the straps, take off the subframe so it hangs down, and jack stand the car very high so the back of the fuel tank will clear the sub frame and be able to go up into place.
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I don't have a floor jack, but to get this job done, I may have to buy one.
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Looks like you're missing the main vent hose (what attaches to the yellow elbow), unless you removed it.

I don't know how you'd lift the tank back into place without a floor jack. You could probably do it if 1) the tank was very nearly completely empty and 2) you had an assistant or two to hold it in place while you secure the straps.

I got my tank back in this afternoon and it was pretty straightforward with the floor jack, but I did forget that the right strap goes over (not under) the heat shield, so I had to redo that part.
Worked on it some this afternoon. I thought I had the solution when I put the car on jack stands, then a long wooden board front to back under the fuel tank, and gradually raised it up from the front with a can of grocery beans. It worked great, I was able to put the pin straps back in.....but the back end of the fuel tank would still not clear the sub frame to go up. Maybe I will take down the exhaust pipe from the 2 bolts toward the back, and see if that allows the fuel tank to be able to be pushed up forward enough so it clears the back. If not, I will get a floor jack, push the fuel tank straight up into place, and jack stand high enough to get the 3 straps on.

Other notes on this for anyone trying it:

- the fuel tank sender from a Mazda 626 year 1999 looks the same, and is put on the same way as the MX6 1994....but the wiring is not the same. To use it, you would have to cut and make a new wire connection on the 1994. Also the 626 fuel tank cover that holds everything is way different.

- the 1995 MX6 fuel tank lock ring from Rock Auto does not fit on the 1994. But the gasket can be used.
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Got the fuel tank and everything else reassembled. If you are unable to jack stand the car very high, a lot of difficulties come up not talked about, because of the 2 straps with pin type connectors on one end. But it can be done. Also with any more than 2 gallons of fuel in the tank, this would make it even more difficult.

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The black float was able to spin around on the wire, not stay firm. The shop must have bumped it when installing, and it gradually was able to spin. That looks like it explains why my fuel tank reading went from full to about 1/2 full over time, and not higher. The black float from the 626 is much bigger and I thought about changing out and installing that one, but I would have had to cut some wires. I decided to firm up the current black float and leave that fuel sender in. An improvement....but now I get 3/4 fuel gauge reading when the tank is full, not full. Not all bad as I can go for 240 miles or so before the fuel light comes on.
This was an educational experience, as I was not sure I wanted to tackle anything with fuel involved like this. But it was not that bad overall, especially if you have leisure time. A shop would charge something like $ 500 or more. There were a few casualties though:

  • 6" socket ratchet wrench gave out and needs replacement
  • scissors jack got bent when the crossmember was lowered and went out of position a few inches, needs replacement
  • to assist with easier time with quick release clamps, bought some plastic quick release tools at a much cheaper price than steel ones. they did work and got the clamps off and on much easier, but already are have signs of wearing out

It still surprises me that you can have electrical wires inside the fuel tank. And the reason given it is okay, is that there is no oxygen inside the fuel tank, only fuel vapors, so cannot ignite. I wonder who discovered that and tried it for the first time.
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