Fuel Facts: 7 Half Truths, Lies and Old Wives Tales

Petrol In Diesel Car – What Should You Do?
November 29, 2017
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From my not inconsiderable years on the tools in the Automotive Industry, spanning four and a half decades and three continents, I have heard a lot of opinions about the fuel we use to power our motor vehicles. Some of these opinions are close to the truth, some have no bearing on reality and some are just bordering on the outrageously stupid.

Here are a few of the popular misconceptions.

#1: “A few litres of petrol will clean out the system”

This one makes a small amount of sense in as much as Petrol is a solvent and will dissolve or at least water down any sludge deposits in the tank. The bad news is that it then emulsifies this sludge and transports it through the fuel system where it can now potentially block or restrict the flow in the fuel filtration system.

#2: “I always put some petrol in my diesel before an NCT test or CVRT…it stops the smoke.”

If its smoking you have an over fuelling problem if it’s black smoke or component problem if it’s white smoke. By changing the viscosity of the fuel it may temporarily disguise the symptoms of a more serious problem. Papering over the cracks as my father used to say.

#3: “Petrol will stop the diesel fuel from freezing in the winter”

Again this has a certain element of truth.

Up to about 15 years ago diesel would form wax pellets in its liquid at -5 centigrade and would cause stalling and difficult starting from cold. Indeed in colder climates they used a special type of “Winter Diesel” to combat this. Some car and commercial vehicle manufacturers recommended Benzine based fuel additives during the winter months but some driver’s substituted petrol which was not authorised or recommended.

All fuel sold in this country now conforms to European Standard EN590 which does not “freeze”  until below -15C.

#4: “Petrol and diesel will separate if left long enough”

While the two liquids have indeed got different specific gravities, it must also be remembered that they are both distilled from the one source (crude oil) by a process called cracking.

When reunited by misfuelling the two liquids are now as one again.

If you mix a 5 and a 7 you get a six.

Even if you could separate the two fuels by distillation it must be borne in mind that they have different additives mixed in post distillation such as ethanol in petrol or anti freeze in diesel which would further complicate the separation.

#5: “Diesel doesn’t burn”

If this was the case it could not propel a diesel engine. It has a much higher flash point than petrol which means it’s harder to ignite under normal ambient condition. Diesel ignites when mixed with air and pressurised. This can be in an internal combustion engine or by pressurising as in a heating system.

It will not normally ignite by introducing a source of ignition (match or flame) but it’s not worthwhile taking any risks.

#6: “Petrol will burn”

It may surprise a lot of people to know this but petrol does not burn. The vapour given off by petrol however is not just flammable it’s combustible….it doesn’t burn , it explodes and can be extremely hazardous in any situation. It can self ignite given sufficient heat and only needs a tiny spark to produce a violent explosion. The vapour is heavier than air and can easily travel within a building.

A single cup of petrol has more explosive power than four sticks of dynamite.

#7: “Ah sure fill it up ….it’ll be grand”

This advice is normally given by unqualified people when asked for their opinion following a misfuel.

Properly trained forecourt staff are NOT supposed to give an opinion but rather refer the unfortunate driver to an expert. Their job is to attend to your purchases of fuel and other items in their shop.

Other individuals who offer advice should have their credentials verified. You wouldn’t let your hairdresser perform surgery on you nor ask your surgeon to cut your hair.

If you are tempted to take this advice, ask for their phone number so you can call them the following morning when your car won’t start or worse still leaves them on the side of the motorway when the fuel has travelled from the tank to the engine.

If you ever run into a trouble and need fuel rescue feel free to call us anytime on 01 280 7661 or 086 255 1079.