I know we don't buy an AMG to be bothered about fuel consumption but, recently my average fuel consumption has increased by around 4-5 mpg per tank, 23 MPG compared to the 27-28 MPG I used to get. I do the same journey every week, at the same average speed as it's mostly motorway driving. I calculate the MPG at every tank fill instead of going by the trip computer. The car is a 2014 E63 with just over 90k miles. Being in the UK it's RWD not 4-Matic
I have scanned the car a few times (iCarsoft CR Max, not Xentry) and there are no codes. I have changed the engine oil and filter and air filters. The brakes all feel the same temperature across the same axle and not hot when I've finished my journey. When coming to a stop I have been selecting N to make sure the car still rolls and isn't dragging.
I think this started after I went through some water a couple of months ago. The road home was flooded in a few places. I may be clutching at straws but, if water managed to get in the exhaust system through the wastegates would it affect the Cat's without throwing any codes?

Has anyone got any suggestions for other things I should check?

Hi there,
It's a lot colder lately here, so the car takes longer to warm up, meaning increased fuel injection to compensate for condensation. At least in our region.
The water would have gone through your air filter first. You said you replaced it and found no dampness?
Pictures of the Rufford ford comes to mind. Hope you didn't pull something like that.
With the engine running, water won't reverse flow from the tailpipes into the engine.
My advise would be to check the average fuel consumption reading on the display. How low is the mpg when you first start and how long does it take until the engine oil is warm and fuel consumption drops to normal. At such low speeds in the UK, I'd think this can take a while.
I just recently had a 50 mile trip and it took at least 30 miles for the engine to completely warm up, going speeds at 50mph or lower with temps in the high 20's F. My average fuel consumption was pretty high too.

Had increased FC for my car, workshop found a leak in the intake manifold.

What kind of leak? Fuel? Air? Shoulda been a DTC.

I’m up to 70 mph within 5 to 10 minutes of starting the car. The engine warms up pretty quickly and the display will be reading around 22 mpg. After 150 miles yesterday the average on the display was 26.4 mpg. Before I went through the flood water, I would see anywhere between 28.5 and 30.5 mpg on the display after the 150 miles.
I don’t think any water will have/could have gone up the exhausts. I’m just wondering if any may have managed to get in through the turbo wastegates. I didn’t check the air filters at the time. I’ve changed them now to help with troubleshooting.
It’s been getting steadily worse since the water.

I’m hoping not to have to delve too deep in to the engine to find the cause😬 But, if that’s where my troubleshooting takes me then that’s where I’m going!

There aren’t any DTC’s and I don’t think I’ve got a leak but, who knows?

Here in the States, testing conducted by the US Dept of Energy show a reduction in fuel economy of approximately 15% in colder weather (20F compared to 70F).

I’d wait until warmer weather to see if it goes back to what you’re used to seeing vs digging into a problem that might not even exist.

now that you mention the standing water... we now have a possible signature for poor mixture.

Read both your engine lambda and cat oxygen sensors.
Do that couple times to compare results for a lazy participant.

Combustion sensors are super heated ceramics and don't like thermal shocks.
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Cold weather and Winter vs summer fuel blends could be the answer.

No DTC. M156 engine, only found out when I needed to calibrate the new clutch but the RPM kept hovering slightly above the required speed.

I’ve just been out for a run in the car. The voltage of both R/H and L/H oxygen sensors drops, what looks like randomly but both at the same time, to 0 volts. In my iCarsoft they are identified as Oxygen Sensor Downstream of TWC[KAT]. This can be for anywhere between 2 seconds and 12 seconds.