Hot fuel

Car Mechanics (UK) - - Help! -

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.

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