2007 C5 2.2 HDI bizare aircon related fuel consumption

[drumvlado]

Hello folks,
Ever since I got my late 2007 C5 break 2.2HDI (175) automatic last year, it had pretty bad fuel consumption. No matter what I did, it was around 20 mpg in town, which is about the same as a V6 petrol automatic. I believe the book says something around 27 mpg. Then, I noticed that once or twice a month it would suddenly go to normal on a morning drive and than back again. Then I thought, OK it is definitely an electronic problem. The car is mechanically very sound, FAP working and clean, clean exhaust, no smoke etc. Checking for errors regularly, no such, or at least nothing connected to that.
I started with changing the most probable items - map sensor, incoming air temperature sensor, tried with friend's air mass sensor and even bought the electronic air doser brand new as I know they break at some point, but no, the problem persisted.
Then few weeks back, it was a lovely evening, not too hot, not too cold, I have switched the aircon off and drove with the sunroof off. Left it off for the night, so the next morning, similar weather, I started the car with the aircon off and I had a perfect consumption! Even slightly over 27 mpg! On the way back, too hot, so aircon on and the consumption back to 20 mpg once again.
At first I thought it was a question about aircon being on or off, but after some experiments on a straight stretch of a Croatian motorway, I couldn't see any difference on the go, so I realized that it might be a matter of if I start the car on aircon or not. Once started with aircon off, than I can turn it on in a few seconds and no problem with consumption.
I know it sounds a bit wierd, but I have been experimenting for about a month now, and it seems to work that way. So, any ideas guys? Am I missing something obvious?
I plan on going for a full inspection at the Citroen dealer, but I've heard they are not good around here, so I do not have high hopes. At least I'll try to update the software of the injection and gearbox if they are out of date.
Many thanks,
Vladimir

[xantia_v6]

The first question that needs to be asked is whether the real fuel consumption is changing, or just the indication of the trip computer?

[GiveMeABreak]

Hi Vladimir. unfortunately, the downside to running A/C is reduced fuel consumption as the A/C system puts an additional strain on the engine when running a compressor.

But additionally, you should bear in mind that if the vehicle is performing an assisted regeneration of the particle filter, and depending on the driving conditions / the engine ECU may switch on the vehicle's power-consuming equipment like the heated rear window, the cooling fan and low, or medium speed and activate the glow plugs. By increasing the resistant torque of the alternator, the torque of the combustion engine increases, as does the temperature of the combustion engine. This allows the engine temperature to increase rapidly where needed to assist with regeneration. Having said that, it shouldn't be enough to make a huge difference to overall economy ad the power consuming equipment is only operated for short periods. But this happens less frequently on motorway driving as natural regeneration can occur at motorway speeds.

But it might be worth putting some diesel fuel injector / fuel system cleaner into the fuel tank to give the fuel system a good clean. Also, if you have the option, run the car on some premium diesel for a few weeks as this contains fuel cleaning agents. It can make a big difference and keep the injectors in good order.

I usually put in a tank of premium diesel once a month or every 2 months to maintain the system.