So I’ve been struggling with fuel economy problems for months now.

After I changed the transmission filter in my Volvo 740 Turbo, I went from getting 11.5L/100km to 15L/100km over a whole tank.

I’ve been investigating a lot of things, but obvious answers like “oxygen sensor” or “coolant temp sensor” haven’t panned out.

The story thickens.

Yesterday, I topped off the tank. Right to the top. I drove 16km.

I woke up today to see the fuel gauge had moved significantly. I go straight to the petrol station, and manage to put 5.82 litres in the tank.

Something highly strange appears to be going on. I suspected siphoning for a while but I’ve got a locking fuel cap on the car. Perhaps this could be picked but it seems like a long shot. What could it be? Leaky injector?! I’m seriously stressing out at this point.

One factor that I’ve not yet mentioned is that when I changed the transmission filter and was taking the car off its jack stands, I accidentally lowered the car down improperly such that a ramp got jammed under the fuel tank and I had to jack the whole car up to get it out. My next area of investigation will be for leaks around the fuel tank area tonight.

Any help is very appreciated. Driving me mad, this car.