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I have a weird problem with my Golf 4 V6 2.8L 4motion, 1999. On exactly half tank (correct by gauge, distance and volume refilled), the car brutally loses all power if going uphill or accelerating quickly (most of the time flooring 2nd gear, all the time flooring 1st). The power loss is total and lasts for 1-2 seconds, then all power comes back instantly. I refilled the tank and the problem totally disappeared.

The car seems to have the fuel tank in two parts due to the central drive shaft, but I can't find how the two communicate, as the symptoms seem to be linked to that property.

This is the most detailed schema I could find : https://volkswagen.7zap.com/ImgsWatermark/images//Bilder/194/194201070.png

The device between 7 and 8 (right part on the picture, left side of the car) looks like a gauge but I don't see any means of flowing fuel between the two halves.

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I'd check that your lift pump / swirl pot is working. The way the GTI and VR6 fuel system works is that there is a relatively low pressure fuel pump that pulls fuel from the tank into a "swirl pot" and it's from this pot that the high pressure fuel pump draws fuel. If this pump isn't working, you won't have anything constantly refilling the swirl pot so in certain situations, the high pressure pump will be scavenging from air instead of fuel. Filling the tank masks the issue because the fuel is never at a level where the pickup will be exposed to the air gap at the top of the tank.

If you remove the rear seat base you ought to expose an access hatch that can be unscrewed to expose the top of the lift pump and fuel lines. If the pump in this doesn't prime when turning the ignition to it's run position, check that it's receiving voltage. If the pump does prime, consider removing it (it comes out the top and I'm pretty sure they're a bayonet fitting) and make sure that the pickup is clear and the strainer isn't full of debris.

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It might be as simple as a blocked fuel filter after the pump. The filter can restrict the higher flows necessary for higher RPM. I have had carburetor cars act the same way , of course the fuel pressure is much lower than for injection but the dirty filter restricts flow the same. Filling the tank puts a little more pressure to the pump supply and may help it to push fuel through the filter. And it will be a low cost item to replace the filter as a first step.

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