Hi
Please, someone, help me out. I have a 2007 MK4 Mondeo 1.8 TDCI, and the problem is that it loses power and won't rev past 2500-3000 rpm when the tank is more than half full. However, as soon as the tank is half full or less, it runs like a dream. I would greatly appreciate any assistance you can offer.
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Would you mind verifying that the problem is actually the reverse of what you've described?
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I couldn't agree more! That is precisely the issue I am facing; I realise it's a strange one.
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I just do not understand how this could be a problem. However, I say we try it out anyway.
Loosen the filler cap and give it a go while driving.
Loosen the filler cap and give it a go while driving.
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Because I believe he will have "easy fuel" on an MK4 Mondeo, I don't think the "filler cap loosened" rule applies.
I don't know anything about diesels, but I'm assuming it has something to do with the EVAP system, a faulty purge valve, or a clogged tank vent pipe.
My 1985 Honda Accord had a similar strange problem in the past: it wouldn't idle correctly when the tank was full of gas. I eventually figured out that the problem was related to the tank becoming porous from rust in some spots. I guess a hole isn't likely to occur in a plastic tank, though.
I don't know anything about diesels, but I'm assuming it has something to do with the EVAP system, a faulty purge valve, or a clogged tank vent pipe.
My 1985 Honda Accord had a similar strange problem in the past: it wouldn't idle correctly when the tank was full of gas. I eventually figured out that the problem was related to the tank becoming porous from rust in some spots. I guess a hole isn't likely to occur in a plastic tank, though.
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Do not install EVAP on a diesel vehicle.
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I looked up your symptoms online and one of the suggestions was a clogged or malfunctioning fuel pump. This makes sense, because the pump is under more pressure when the tank is full than when it is half full. Precisely the laws of motion.
Without a doubt, a starting point for your process of identifying and fixing problems
Without a doubt, a starting point for your process of identifying and fixing problems
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Joined: Mon Apr 06, 2026
My leaning is toward the idea that the fuel tank contains a hose or some other part that gets distorted and begins to limit the flow of fuel as the tank level rises. What happens when the fuel pump is under pressure, the fuel level is slightly higher than the ideal, and the pressure differential squeezes the fuel hose?