Discussion
A strange issue i'm trying to help my friend with.....
He has owned his petrol 2009 Astra 1.6 for 8 years now, and has always managed around 400 miles per tank, but in the last month it has dropped to 300 miles max. not a steady decline but an overnight change. His driving routine has not changed at all, and his driving manner is as careful as its always been.
The performance has not changed, the car drives perfectly, yet he says he has noticed the onscreen instant MPG drops to near on 0 when lightly accelerating, whereas this would normally drop to around 20'ish. I have ran an OBD scan although this came up clean as i thought it would as there was no EML, and the car has a perfect service record with clean filters, plugs etc. I asked him to run a few tanks through it just in case he had bad fuel, but 3 tanks later from different sources it is still returning 300'ish miles when he regularly got 400 before.
Logically i thought MAF or o2 sensor but like i say no codes are present, and it runs perfectly.
Any ideas where to go next?
He has owned his petrol 2009 Astra 1.6 for 8 years now, and has always managed around 400 miles per tank, but in the last month it has dropped to 300 miles max. not a steady decline but an overnight change. His driving routine has not changed at all, and his driving manner is as careful as its always been.
The performance has not changed, the car drives perfectly, yet he says he has noticed the onscreen instant MPG drops to near on 0 when lightly accelerating, whereas this would normally drop to around 20'ish. I have ran an OBD scan although this came up clean as i thought it would as there was no EML, and the car has a perfect service record with clean filters, plugs etc. I asked him to run a few tanks through it just in case he had bad fuel, but 3 tanks later from different sources it is still returning 300'ish miles when he regularly got 400 before.
Logically i thought MAF or o2 sensor but like i say no codes are present, and it runs perfectly.
Any ideas where to go next?
Edited by BuzzBravado on Wednesday 5th June 12:47
Edited by BuzzBravado on Wednesday 5th June 12:48
I'll ask him about the thermostat as i didn't look at that, but he is an observant fellow so i think he would have said.
Tyres inflated, and brakes don't bind. The sensor output might be what to look at next, which i think Torque Pro does, or i can dust off my old Solus. I'm not sure how to interpret the figures though for live data, i only used it for codes.
The fact the instant mpg readout drops to zero suggests to me that whatever sensor feeds that data will be part of the issue. Is instant mpg calculated from fuel pressure data at the injectors, or is it simply air fuel data from the o2 sensor mixed with air flow rate from the MAF??
Tyres inflated, and brakes don't bind. The sensor output might be what to look at next, which i think Torque Pro does, or i can dust off my old Solus. I'm not sure how to interpret the figures though for live data, i only used it for codes.
The fact the instant mpg readout drops to zero suggests to me that whatever sensor feeds that data will be part of the issue. Is instant mpg calculated from fuel pressure data at the injectors, or is it simply air fuel data from the o2 sensor mixed with air flow rate from the MAF??
BuzzBravado said:
The fact the instant mpg readout drops to zero suggests to me that whatever sensor feeds that data will be part of the issue. Is instant mpg calculated from fuel pressure data at the injectors, or is it simply air fuel data from the o2 sensor mixed with air flow rate from the MAF??
I'd expect it to be calculated from speed divided by fuel flow, which in turn would be estimated from the injector duty cycle.The coolant temperature sender suggestion is a good one. It would cause excessive fuel consumption, and might not be flagged as a fault if the reading is still plausible.
He hasn't done it the proper way no. But the light is coming on in under 300 miles from full when it was 400+ before. Its taking the same fuel to fill it as before give or take a litre or so.
He might accept it for what it is and give it to his wife for the local errands and get something new for his long commutes.
He might accept it for what it is and give it to his wife for the local errands and get something new for his long commutes.
BuzzBravado said:
He hasn't done it the proper way no. But the light is coming on in under 300 miles from full when it was 400+ before.
Daughter had a Misubishi Colt that did this - for 5yrs from new it would show 50 on the computer then suddenly changed to 40. Hadn't ever accurately checked MPG but where for years she'd filled the car up weekly she was having to buy fuel a day earlier each week!I looked at all the obvious things and a couple of garages, inc a Mitsubishi dealer, looked at it but without success. Car drove fine and emissions were fine. Dealer seemed quite focussed on it being a fuel leak but couldn't find one. Car ran like this for a couple of years before we sold it.
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