Test results on fuel which is thought to have caused problems for thousands of drivers in the south and east of England are expected to be released.
Petrol sold at Tesco and Morrisons supermarkets is being tested
The petrol has already been traced to a storage depot in Essex. The fuel industry is testing its own supplies.
And Cambridgeshire Trading Standards officers have sent a sample from one affected car for tests at a forensic laboratory.
Reports say silicone has been found in some of the affected cars' fuel.
Tesco confirmed that it is testing for the substance, but previous analysis by the supermarket chain have failed to find any contamination.
It is thought silicone may have found its way into a batch of petrol in transit by ship into the UK or in storage containers.
Silicone is a compound containing the element silicon. Its uses include insulation and lubrication.
The problems seem to be centred on oxygen sensors which determine the mix of oxygen and petrol supplied to the engine.
Automotive expert Professor Malcolm Fox, from the University of Leeds, said all the signs pointed towards some form of liquid silicone having got into the fuel.
He added: "The analysis from the suppliers shows that the fuel is okay, so it must be a very minor, low concentration contaminant.
"When it burns, it will form silicon dioxide and will deposit a very thin sheet on the sensor - the sensor is in the exhaust - and that probably gives a false signal to the engine computer."
Chris Hunt from the UK Petroleum Industry Association (UKPIA), said silicone was one of many substances being examined as a possible cause.
The UKPIA said the problem could have arisen because of the way a single batch of fuel was blended or stored, and that petrol across the UK was generally not faulty.
Contamination by water or ethanol has now been ruled out.
Tesco said the problems appeared to be linked to forecourts in the south-east of England served by the Vopak Terminals depot in Essex.
Both Tesco and Morrisons get fuel from an independent oil company called Greenergy, which insists its supplies meet industry standards.
Vopak Terminals managing director Colin Scott said: "Verifications are continuing. At this moment we have not established any deviation from normal procedures."
The Trading Standards Institute said samples of unleaded petrol had been taken from station forecourts in Cambridgeshire, Kent, south Essex and Milton Keynes.
A spokeswoman said spot checks were also being undertaken by local authority trading standards teams in other parts of the country.
The penalty for supplying fuel which does not meet industry standards is a £5,000 fine.
The BBC has received several thousand e-mails from motorists who say their vehicles have juddered, misfired and suffered a loss of power.
Garages are running out of replacement parts in some areas as they are swamped with motorists. Repair bills are topping £1,000 for some drivers.