Bravo 1.8 HLX pinking on normal unleaded?

I have a Fiat Bravo 1.8 HLX N reg 1996 only done 38000,the car has always been ran on normal 95 ron texaco unleaded,and the last 3 months or so the engine has started pinking (pinging) it's been to a local garage who's had
it on a bosch kts diagnostic computer and found no errors or faults,the only thing out of spec was we took the car for a drive with the KTS connected and went hard through each gear ie about 5-5500 rpm's and he noticed the injector pulses were only 11ms for which he said should be 14ms min and more like around 18,i've been told since from other people that it's just a red herring with injector pulses as they constantly change? i did however a few days ago try texaco super unleaded and the pinking vanished,what's the reason for it not liking normal unleaded anymore? it's not carbon,it's had a treatment of ecotek's powerboost foam,hardly nothing came out the exhaust,it's had treatments of redex and stp complete fuel system cleaner,but it just won't run on unleaded anymore.all sensors have been checked,he said ignition timing can't be adjusted because the ECU controls ignition and air/fuel mixture ratios,i changed plugs a couple of weeks ago to no avail,then decided to change plugs again but for champion instead of NGK and noticed small yellow deposits on the plugs ceramic insulator,does this indicate anything?
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This may not be that easy to get to the bottom of.
Pre-ignition (pinking) is affected by fuel mixture being too lean (or badly atomised), too much ignition advance or local hot spots and fuel octane rating. Hot spots can be cooled by more fuel, cooler and continual air intake and colder running spark plugs. Systems with 'knock' sensors use the sensor to provide feedback on the ignition advance.
The recent evelvated ambient air temperature could be the cause if your car was running close to the limit to begin with. New plugs of the same type would eliminate plug degradation if the condition persists. This would only leave timing, fuel mixture, fuel octane, knock sensor and other hot spot causes.
I don't believe your car has a knock sensor but I could be wrong.
On the ignition timing front it is not unkown for the TDC sensor to be incorrectly set. This will never show on any diagnostic kit as this kit uses the TDC pulse as a point of reference. To check the TDC you need to either get physical and/or us a traditional strope timing light.
The fuel mixture Lambda sensor could be defective sending incorrect fuel/aix miture readings to the ECU. This can be check by the usual MOT emission tests which measure the fuel/air ratio. If this is Ok then we are down to the delivered fuel based on the Lambda signal to the ECU.
With fuel mixture/atomisation one has to assume that the injector pulse is correct and that the injectors are varnished or otherwise clogged and need a good clean to restore the all important 'mist' spray as opposed to a possible non atomised jet. You could try running a fuel injector cleaner through the system a couple of times to see if that cures the problem. A clogged air filter can also effect the mixture.
A defective air flow meter would also upset the fuel mixture. Not all fuel injection systems have a 'flow meter' but use other methods to determine air flow rate. e.g. the ECU calculates the flow rate base on engine volume, rpm and air temperature. Here you are really at the hands of the diagnostic equipment. Same goes for the knock sensor.
If the problem still persists the one could try colder running plugs.
As you say this problem has only recently occurred the only other variables (beyond car variables) are the air temperature and the fuel octane rating (try other 95RON makes like Esso, Shell etc.)
I'm sure I've missed something so hopefully somebody else can pick up on that :-)
Nick /////

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