I have a 2010 Honda City, 1300cc, MT(mileage around 42000km). Around a week and a half ago, I got my fuel tank full. The fuel needle rose to 'full'. After re-fueling, I drove the car for another 40km. Afterwards the car was left standing for around 3days(around 90 Hrs to be precise). After 3days when I started the car the fuel needle had dropped around 3.5 units(City Fuel meter has 10 units from 'E' to 'F'). I clearly remember that the needle was still on 'F' when I last left the car.
I did some googling and was able to establish following:
1) Car manufacturers design fuel gauges so that they don't drop immediately after 'full' for psychological reasons.
2) Fuel measuring equipment is quite basic in nature and it can be in-accurate.
3) Fuel needle does not drop linearly with fuel level, due to angular movement of the float. This is the reason last quarter(1/4 th) for fuel appears to be used up more quickly.
4) There is a fuel drain plug/bolt under fuel tank which can be used to drain/steal fuel in many cars(I am not sure if city has one)
But reasons 1-3 can not explain the behavior observed in my case. One could expect a sudden drop of around 1 unit, but 3.5 units is just too much. Also if only needle was in-accurate, I should not have reached 1 unit above 'E' after only driving for around 260km.
So I could think of following reasons:
a) Car is eating too much fuel + fuel gauge is somewhat in-accurate.
b) Fuel is leaking.
c) Somebody stole petrol from the tank while car was left standing for 3 days.
d) Bad quality petrol. Since I got the tank full in days of petrol shortage from a small caltex petrol station.
e) Some other reason I can not think of.
To eliminate reasons (a) above. I got my car refueled yesterday(this time only 1000Rs.). The fuel needle rose to just under 'half mark'. Over next 24hrs drove car for 26km and needle only came down by half unit(there are 10 unit on city fuel meter). This is around expected, because this drop shows use of around 2 liters which is expected for around 26km. I also confirmed that odometer reading was accurate by cross-verifying distance traveled using google maps. Also the route traveled is the same route I travel everyday(to and from my office), so traffic conditions and speed can be assumed to be the same.
I think I can eliminate reasons (b) because if I lost even 2 units of fuel over 90 hrs. The rate of fuel loss would be around 0.022 unit/hr. Which should result in loss of 0.33 unit over 15hrs. And loss of 0.533 units over 24 hrs. I confirmed the fuel gauge was at same level after car was left for around 15hrs(0km traveled) and the needle dropped 0.5 units units after 24hrs(but car traveled 26km as well). Also I was not able to detect smell of petrol fumes around the fuel tank.
So now I think I am left with reasons c,d,e.
1) Has anyone else experienced this behavior?
2) Can fuel be stolen/drained from locked Honda City(doors, hood and fuel door all locked) with engine off?
3) Can bad fuel cause this kind of behavior?
4) Can you think of any other reason why this could be happening?