Very Inconsistent mpg
Discussion
All, for various reasons the missus and I have ended up driving two almost identical 2001 Honda Civic autos (1.6). One’s done c. 130,000 miles, the other (mine, bought in July) c. 70,000 miles. Mine was supposed to be a stop-gap before buying a new car, but to be honest I really like it…
We did a long motorway trip in the older Civic and on calculating the mpg got 42. Recently we did a similar trip in mine and got 42.6. So far so good. Again, in mine round town/commuting I’m getting about 36 which sounds about right. These figures are all calculated by running almost empty and re-filling the tank.
On two occasions, the average on mine has brand only dropped to a tank average of 26 mpg. I can immediately tell if this is going to be the case because usually the gauge needle doesn’t move from its stop until about 60-80 miles in. When it’s going to be a low one, it moves at 30-50. It’s the same fuel, and the overall journey profiles for a tank of fuel are the same.
The car runs very well, doesn’t use oil, and has been well maintained. The only anomaly is when mine’s parked, it sometimes makes a buzzing noise from around the fuel tank. Can’t place it, but it stops if I unscrew the cap. This is accompanied by a strong smell of petrol fumes - like the tank is pressurised.
Not normally obsessed by mpg figures by any means, but it intrigued me to see what the figures would be for two identical 23 year old cars.
Any ideas? Thanks.
We did a long motorway trip in the older Civic and on calculating the mpg got 42. Recently we did a similar trip in mine and got 42.6. So far so good. Again, in mine round town/commuting I’m getting about 36 which sounds about right. These figures are all calculated by running almost empty and re-filling the tank.
On two occasions, the average on mine has brand only dropped to a tank average of 26 mpg. I can immediately tell if this is going to be the case because usually the gauge needle doesn’t move from its stop until about 60-80 miles in. When it’s going to be a low one, it moves at 30-50. It’s the same fuel, and the overall journey profiles for a tank of fuel are the same.
The car runs very well, doesn’t use oil, and has been well maintained. The only anomaly is when mine’s parked, it sometimes makes a buzzing noise from around the fuel tank. Can’t place it, but it stops if I unscrew the cap. This is accompanied by a strong smell of petrol fumes - like the tank is pressurised.
Not normally obsessed by mpg figures by any means, but it intrigued me to see what the figures would be for two identical 23 year old cars.
Any ideas? Thanks.
Modern petrol engines will use a carbon cannister to capture hydrocarbons in vapour leaving the fuel tank, with an air circuit to purge these into the engine intake manifold when the right conditions are met. I suppoes it's possible there's something wrong with this mechanism causing the noises and pressuration, although I don't see how that could lead to such a big difference in fuel consumption.
Two things I'd check:
Two things I'd check:
- Possibly the tank isn't being filled to the same level every time.
- Check the ECU for fault code.
GreenV8S said:
Modern petrol engines will use a carbon cannister to capture hydrocarbons in vapour leaving the fuel tank, with an air circuit to purge these into the engine intake manifold when the right conditions are met. I suppoes it's possible there's something wrong with this mechanism causing the noises and pressuration, although I don't see how that could lead to such a big difference in fuel consumption.
Two things I'd check:
Thanks. I’ve checked for fault codes last week - clean as a whistle.Two things I'd check:
- Possibly the tank isn't being filled to the same level every time.
- Check the ECU for fault code.
Tank not filling consistently is a possibility, but it would have to be about a gallon short to make the kind of difference I’m seeing.
I wonder if it could be an up-shift issue with the gearbox?
vikingaero said:
On older cars you can have something simple such as an intermittent binding brake.
True. I did service the brakes when I got it, and the calipers and front discs seemed relatively new, and the pins moved freely. I've not noticed any binding noises. Binding rear brakes has been a recurring issue on the older car though.ThingsBehindTheSun said:
vikingaero said:
On older cars you can have something simple such as an intermittent binding brake.
It's a Honda, the caliper pistons corrode and stick for fun. I would say this is possibly the reason.Strange thing is - after the intial rapid drop of the fuel needle compared with normal, it's now settled down and I've got just under 200 miles at the half way point. I usually get about 400 miles per tank, best case. I have noticed the needle moves up and down slightly even on a flat road, and will often read slightly differently in the morning from when it was parked the night before. On one run I seemd to be gaining petrol, which was nice.
I'll calculate the ture mpg again tomorrow after a motorway trip, and see whether it's good, or another very bad one.
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