What Causes Cars to Stop

A car pulls into a garage and fills up with petrol or diesel. The driver pays and starts driving down the road. There’s a funny noise around the engine. A squirrel is trapped inside and the car is travelling like a kangaroo. Eventually, the car limps into a service garage and an investigation begins.

The mechanic invariably blames the fuel – the smell or colour is funny, problem sorted. Is he right? Only a fuel oil analysis can determine if the oil is responsible. And there will be less animosity all around if the facts of the case are highlighted as quickly and accurately as possible. It is a truism in the oil industry and life, that the earlier a problem is identified and defined, then the better for all concerned. Real or imagined injustices can fester over time and lead to expensive court cases. Let us look at the main causes of oil contamination, their prevention and how they can be dealt with.

  1. Free water
  2. Suspended water
  3. Sediment and slime(emulsion fibre)
  4. Cross product contamination
  5. Washed diesel
  6. Biodiesel
  7. Bioethanol in petrol
  8. Sampling and analysis in Dispute situations


Free water is one of the main contaminants in oil. This can enter an oil tank from a variety of sources. Rain water is an obvious one but there are other less obvious sources. Sea water can squeeze through gate valves at oil terminals, entering their tanks and end up at the bottom of garage tanks. Groundwater can enter through the cracks in underground oil tanks, and it can be assumed that all underground oil tanks will leak after ten years. Condensation from damp air is another source, as is tap water, squeezing through the hatches on the top of oil trucks when they are being washed.


Suspended water is the cloud you often see in diesel or petrol in cold weather – the water is dispersed throughout the oil in the form of tiny droplets. (In very cold weather, it is sometimes wax formation in diesel – this can be determined by the cold filter plugging point test) Filters in the car remove the water and wax, but when the temperature drops below zero, a sheet of ice or solid wax can form in the filters. Detergent, seeping under the hatches of oil delivery trucks, can also lead to high suspended water – the detergent makes the water more soluble in the oil. These obvious problems that arise, have led the Irish and British authorities to set a maximum of 200 ppm for the suspended water content.


Sediment can block filters and cause problems with flow and combustion of the fuels. The original crude oil contains a lot of fine sand and this can find its way through the system to the end user. For example, it is not uncommon, when cleaning out a heavy fuel oil tank to find about two tons of sand. Other sources are common dirt, carbonised oil and rust flakes.

Slime is formed by the action of bacteria on oil. They can eat the oil, and when they die, a brown greasy slime is formed. It is common to see filters blocked up with this slime. Good hygiene practices and regular draining of the oil tanks will reduce this problem. If a bad infection occurs, tank cleaning is the only solution, but biocide can provide temporary relief by stopping the growth of the slime and bacteria.


A cross-over contamination occurs when a wrong connection is made, or oil is not properly drained out of an oil truck compartment. Petrol in diesel, or diesel in petrol will cause serious problems. The diesel will simply not combust in the petrol cylinder, and the petrol will provide zero lubrication for the diesel injectors, leading to burn-out damage. Testing in the laboratory is the only real way to determine if this has occurred – a mechanic saying he can smell petrol, kerosene or diesel is extremely subjective and won’t stand up in court.


Washed diesel is the result of mixing acid with marked gas oil(green central heating oil) to remove the dye. Depending on how sophisticated the process is, the quality of the final product varies greatly. At its worst, the oil can destroy an engine; at best, it performs satisfactorily.

The product is, of course, illegal, and the Customs have regular roadside checks to determine if the diesel being used is washed. In the absence of the marker dye, the Customs rely on the sulphur level tests to identify washed gas oil from good quality diesel. Marked gas oil has sulphur levels of 1000 ppm, diesel has to have a level of below 10 ppm since Jan 1st, 2009.

Another important point to be made in the case of washed diesel concerns the warranty, or guarantee by the car manufacturer concerning the vehicle. If an illegal product has caused damage to a car, then the car companies declare the warranty void. This means that the purchaser of a new car in this situation has to cover all the repair costs.


Biodiesel useage is becoming more and more common. Oil companies are allowed to put 7% biodiesel into ordinary diesel but there are whole fleets of cars and trucks running on one hundred per cent biodiesel. Its use also carries special problems: higher water content, solids and wax formation. The raw material for making biodiesel is so variable – from pure plant oil to tallow from meat factories – that the quality of the final fuel runs the full gamut. It is always advisable to carry out testing to determine its quality – short and long term damage to expensive engines can be very costly.


For the last couple of years, bioethanol has been added to petrol to make up E5(5% bioethanol) and E85(85% bioethanol). This addition improves the octane number(ignition quality, resistance to knocking) of the petrol but it also makes it more miscible with water. Thus, if water enters the tank, it can leach bioethanol from the petrol, leading sometimes to a 50% bioethanal/50% water mixture at the bottom of the tank. Persistent water dipping in garage tanks and good housekeeping at terminals are the best way to prevent this problem.


The best way to handle dispute situations is for both parties to be upfront with all the facts so that the responsibility can be attributed fairly. Let us look at a typical case: a customer fills up with diesel or petrol and sometime later the car malfunctions and it is brought to a garage. The mechanic takes a sample of oil and gives it to the car owner, telling him that it is to blame for the damage done to the injectors, engine etc.

Firstly, the oil sample should be taken in an unused container and labelled appropriately. Second, a sample should be taken from the pump where the car owner claims he filled his vehicle. It should be decided at this stage who will pay for the sampling and analysis.

Both samples should be despatched to an independent laboratory where a qualified chemist will analyse them. The results of the analysis of each sample can be printed on the one report. An interpretation of the results by the chemist can apportion the likelihood of the oil having been bought at that particular pump – in short, do the results from each sample line up.

It then falls on both parties to reach an agreement on how the costs and the damage are to be managed. You will notice that I haven’t mentioned solicitors – in our experience, it is generally better for all parties to agree on a mutual cost solution, rather than each go to a solicitor, where the courts can become horrendously expensive.

Again, the warranty factor applies in contaminated oil. If contaminated oil – that is, an illegal product, is used in the vehicle, then the supplier motor company will not stand by the warranty. Thus, getting accurate information as early as possible is the best way forward. Good, representative sampling, impartial analysis, and trust in the results, are the key to the solution of these problems.