Q: I have a 2006 Volkswagen Polo 1.4 auto BKY with about 30,000 on the clock and running perfectly. I mostly drive short runs, but the car is well maintained in terms of oil and filters.
Over a couple of years, I have filled up with fuel from a pair of major supermarkets in Cornwall as their prices are excellent. On each occasion, usually on the trek back up the M5 or M6, seldom above 60-65mph, there’s a lot of heat radiating from the area in front of the cat and exhaust manifold. All dashboard instruments show normal readings and the car runs normally. MPG is unchanging at 32mpg in the country and 40mpg on motorway runs.
Two friends have also noticed “a hot smell” about their petrol cars when using discounted supermarket fuel. This situation does not occur with fuel obtained from regular filling stations. Checks made while travelling from the north to the south, using fuel obtained from my usual supply, show no sign of any excess heat, other than that which would be expected.
I have no problem using the supermarket fuel and will continue to do so, but I do wonder if the fuel supplied from these outlets is perhaps burning a little slower and raising exhaust temperatures, thus producing an extra shot of heat to the manifolds and cats? I hasten to add that I have no interest in any fuel supply factor and wish the supermarkets well, but I do like to find out the cause of something out of the ordinary. As fuel is being changed over time – for example, using bio additives, etc – are cars affected by a slightly different fuel formula when obtained from different suppliers? Colin Thirlwall
A: Without knowing the exact specifications of the fuel, it is difficult to say exactly, but I would guess that the amount of ethanol in the fuel was the main factor behind the change in the exhaust temperature. Different supermarkets use different blends and possibly a higher content of ethanol, which could be making the difference. Most modern vehicles, including your Polo, have been designed with a biofuel mix in mind, and running on E5 or E10 fuel will cause no problems.