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Ford Focus Fuel Gauge Problems – Dangerous? Fixed?

I’ve been chasing a Ford Focus fuel gauge problem. The problem is the fuel gauge is just plain inaccurate. Always under reads, if it reads at all.

Quick Links: Fuel PumpA Connection?Frayed WiresVacuum Source?The Fix(ish)Unsolved ProblemWrap Up

Fuel Pump Problems

A fuel pump packed up. And then the replacement pump packed up as well.

Alarm bells were ringing with the first fuel pump because the power wires were massively frayed.

The massive fraying on the fuel pump power wires. Initially I put this down to bad materials, I couldn’t see any other explanation. Thankfully they don’t appeared to have caused sparks in the gas tank.

The second fuel pump was noisy from day one but ultimately it failed because the earth wire going to the pump came out of the connector. Specifically the terminal had snapped and the connector itself was damaged. I could not imagine how the damage happened so assumed it happened at the factory and I just didn’t notice it when I fitted the pump.

Massive damage to the fuel pump connector. Again I put this down to materials degrading over time (wrongly)

Anyway, whatever, the pump was noisy, I wanted to change it anyway.

A Connection?

The first pump had frayed wires. The second pump had damage to the connectors. How is that possible?

And then I started thinking about the vacuum that I get in the gas tank. When I changed the pumps, disconnecting the hoses from the tank, the tank would suck in air for 20 seconds.

Opening the gas cap to fill with petrol does the same thing, the gas tank sucks in a lot of air.

After going for a drive, I looked under the car and saw the bottom of the fuel tank sucked in. A vacuum has to be causing that.

Looking at the bottom of the tank and the slope in its floor. I thought this has got to be effecting the accuracy of the fuel gauge because its tilting the fuel pump. And the angle of the bend would mean the float would be lifted which would lead to the gas gauge under reading. Exactly the problem I’m experiencing.

I went for a drive paying super close attention to the fuel gauge. Needle dropping far faster than is reasonable because level sensor is being tilted as tank deforms. Stopping the car and releasing the vacuum and the needle rises slightly. Cause confirmed.

What about the frayed wire problem?

The clearance between the top of the fuel pump and the lid of the fuel tank is not great. 10-20mm.

The amount the tank was getting sucked in is easily 20mm (see pictures above).

This would explain how the top of the pump is getting damaged and how the wires are getting frayed. They are being forced into the lid of the gas tank with huge force.

Where is the vacuum coming from?

Into the engine bay.

There are three pipes coming from the fuel tank. The “in” and the “out” for the fuel. And one other hose, the fuel tank vent hose.

Tracking this vent hose into the engine bay, it goes into a circular plastic box which has what looks like a solenoid on top of it. Next to this hose, another hose leaves the box and goes to the inlet manifold.

The box
The ports on the box. From top to bottom. From fuel tank, the small vacuum signal port, the port going to the inlet manifold

Okay. We have found our vacuum source.

At the top of this box there is another small vacuum hose going to the inlet manifold. The plastic box looks like it works like a wastegate. Let’s open it and see how it works and to see what is broken, if anything.

But there is nothing really to see. Looks like an empty box with a diaphragm.

Not a lot to see in the box. The bottom half in my hand has a diaphragm which opens when engine is running. This exposes the fuel tank to the vacuum of the inlet manifold. When engine is not running the gas tank is sealed preserving the vacuum in the gas tank.

When the engine is running ie when you have vacuum in the inlet manifold, the diaphragm opens and this connects the hose from the fuel tank to the inlet manifold. This is how the vacuum gets into the fuel tank.

When the engine is not running the diaphragm closes sealing the hose from the fuel tank. This is how the fuel tank remains under vacuum when the engine is not running.

Bingo. We have found where our vacuum in the fuel tank is coming from.

Now what? – The Ford Focus Fuel Gauge Fix?

The thing that looks like a relay on top of the box.

I’m guessing, but maybe it is supposed to relieve the vacuum when it reaches a certain limit. But then again, if it did that would defeat the point of venting the fuel tank to the inlet manifold.

The solenoid on top of the box. I do not know the purpose of this. It appears to be a switch because it only has two wires going to it.

Instead of guessing what it does, I decided to blank off the hose coming off the inlet manifold.

I found a silicone hose with the same outside diameter as the diameter of the port on our “box”. I plugged one end of the silicone hose into the hose and plugged the other end of the silicone hose with a, eh, plug.

Silicone hose with a bung inserted into it. The silicone hose has the same outside diameter as the port on the box. The hose is inserted into the inlet manifold hose. Silicone first. Bung is visible, hose is inside the other hose.

I put another silicone hose (black) on the other side of the bung (and made a small hole in it so any pressure or vacuum in the gas tank can escape) and connected that hose to the “box”. Just to stop the original hose from flailing about.

On the other end of the bung I inserted the black hose in the centre of the shot. This connects back to the box purely to stop the hose from flailing around. The inlet manifold is now isolated from the box and the fuel tank. The other hoses were connected as they were from the factory. When engine is running fuel tank is exposed to ambient pressure (instead of engine vacuum).

I connected the hose from the fuel tank back into the “box”. Like it was from the factory.

We have now isolated the intake manifold from the fuel tank so no more vacuum in the fuel tank and we have plugged the hose from the inlet manifold so the engine works normally.

The vacuum signal hose that goes into the top of the “box” is reconnected as before. It will open the diaphragm when the engine is running, like before but instead of this creating a vacuum in the fuel tank, it exposes the fuel tank to ambient pressure.

And that’s it

We should no longer have a vacuum in the fuel tank, which means:

  • the tank should no longer deform,
  • which means the fuel gauge level should at least be consistent and
  • we should no longer have the fuel pump being crushed into the lid of the fuel tank causing the wire fraying and rubbing problems.

Unsolved Problems

At the start of the article I said the gas tank was collapsed slightly. Even without a vacuum in the fuel tank the bottom of the tank is still pushed in by 2 or 3 cm. I think it should be flat.

I’ll run the car with the new setup (ie without vacuum in the fuel tank), maybe fill the tank and see if the weight of the gas pushes the bottom of the tank back down. If it doesn’t, ah well, it is what it is, I won’t be ponying up for a new gas tank if it isn’t leaking.

Wrap Up

If you are experiencing inaccurate readings on your Ford Focus fuel gauge or any fuel gauge for that matter, check the bottom of your fuel tank. Is your fuel tank being collapsed by vacuum?

If so, this could be the cause of the gas gauge inaccuracy.

And it may also be worth checking out the condition of the fuel pump wiring inside the gas tank. The deformation of the fuel tank could have frayed the wires.

Hopefully this helps, if you have the same problem.

If you have some tips or experience of this problem, let me know in the comments section below (are there any issues with no having vacuum in the gas tank?).

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This page was last modified Apr 30, 2022 @ 3:09 am

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