Faulty Fuel

My mum's car is in the garage with the exact symptoms, would the engine warning light come on though if the oxygen sensor had been damaged? Oxford area...

It didn't when I had a faulty sensor on my '94 BMW 525. AFAIK it 'fine tunes' the fuel/air mixture, which affects the emissions and fuel consumption. My car failed the MOT on emissions, but after it was replaced it passed. The thing is, that there was no noticeable difference in performance, between before and after. I can't see faulty Lamdas giving all the problems that have been reported. Cars failing to start, coughing and spluttering, reverting to 'limp home mode' etc. Mike.

Almost always. The O2 sensor determines how much fuel it needs to put in. If that goes wrong, then once the ECU notices it's faulty, it reduces the fuel drastically to a 'best guess' at 'lean but will sort of run'.

It's better to be lean than rich, as a rich mixture will rapidly destroy and overheat the catalyst, maybe even causing more damage to the car.

Some cars have multiple oxygen sensors. Some have ECUs which have the 'default' limp-home mode relatively far from the normal operating point of the car engine. Others have it much closer.

Well its confirmed, oxygen sensor is clogged. On a related matter my girlfriend was complaining of a bad smell coming from her car and running hot few days ago. Hasn't driven it since. Haven't gotten hold of her yet but my guess, 95% sure is she filled up at the same Tesco my mum did.

BBC news were reporting this afternoon that it was Tesco and Morrisons outlets that were affected, but they seem to have stopped saying that now, possibly the Tesco heavy mob lawyers have been in touch. Wouldnt surprise me if it was a Tesco money saving excersise gone wrong.

As all tests from Tesco on damaged cars, fueling stations, tankers, to fuel depots are negative so far, the suggestion now turns to the possibility of petrol having accidentally been contaminated with diesel.

It would take quite a large percentage of diesel in petrol to cause the problems being reported. Especially as it's affected older cars, as well as those with more sophisticated electronics. Mike.

Yes, I am aware of that. That's why it's fascinating. This suggestion was from an oil man in Manchester interviewed on BBC Radio 4 Ten O'clock News. Eagerly awaiting further revelation...

It's quite surprising to hear some of the comments being made. On our local radio station, a presenter was saying that even a litre of diesel in a tankful of petrol could cause expensive engine damage. He also suggested the problems could be caused by putting leaded fuel into a car designed to run on unleaded. As if so many filling stations sell leaded fuel, or the unleaded tanks at the filling stations have been filled with the stuff Such ignorance beggars belief. The only contaminent I can think of that might give many of the symptons is water, but that's so obvious it would have been discovered by now. Mike.

Their ridiculous expansion plans are fuelled by you all shopping there. Stop it!!

History - I'm posting with a real address. Will you email me and tell me where your mum and girlfriend buy petrol. I filled my car up last week but have only driven it a mile since then so want to make sure I'm not going to have problems.

I filled up at Texaco on Oxford Road Cowley (Formerly Q8) last Wednesday evening.

Tony

Bollocks, all ECU's default to a slightly rich mixture when the o2 sensor is knackered- thats why emissions rise, and so does fuel consumption!

Slightly rich is much safer for engine.

It wont damage the cat unless a) there is a misfire, b) the car is run in this state for some time.

Tim..

c) the faulty fuel contains lead which can kill a lambda and a cat.

Graham

Yours might just have been marginal, responding slowly or erratically. If the ECU sees an unchanging reading from the lambda sensor, it'll give up pretty quickly on using it to work out how much fuel to inject.

A failed lambda sensor stopping a car from starting is quite unlikely, I agree.

All the analysis figures are published on the Greenergy website, but I don't have the knowledge to be able to interpret them -

Greenergy Citypetrol (95 RON) Sample Analysis

Ethanol %change

Also old reports (Ethanol reading missing) available from the excellent archive.org website archiver -

Greenergy's statement -

Daytona

Very high levels of ethanol in the petrol are actually a plausible explanation for what has happened. The stoichiometric ratio for ethanol is 9:1 (vs 14.5:1 for gasoline) so running 50% ethanol is like being 20% lean. This would cause poor starting, misfires, low power, etc.

On a post-1993 car the oxygen (lambda) sensor would detect that the mixture was lean and richen it up to compensate. But the adaptation is normally limited to something like 15-20% so it could well hit a limit at which point the ECU assumes the sensor is knackered and goes back to running open-loop. Now you've poor running *and* a "broken oxygen sensor" fault logged. Of course, the garage then changes the sensor even though there is absolutely nothing wrong with it - draining and refilling the fuel, then resetting the ECU, would have been sufficient (and several hundred pounds cheaper).

Interestingly, some people have reported air meter faults, which could also occur if the ECU decides to believe the oxygen sensor not the air meter.

The idea that 5% ethanol would have this kind of effect is just nonsense, though - that would be a 2% enleanment which is well within normal tolerances. And even high levels of ethanol should not actually damage engine components, at least not over a short timescale (long term it attacks aluminium, among other materials)

Tony

This is an interesting theory.

I filled my car up on Monday from what may be an implicated Tesco. I've since driven about 30 or 40 miles without any problems. Should I assume I'm OK or start trying to siphon out the petrol? If it was 50% ethanol I'm sure I would have noticed by now. If it was silicone or something slowly gumming up the sensor perhaps the effect could be slower.

Thanks for that explanation, now on to the next theory....... :-) why is silicon (or silicone?) bad ?

Daytona