My engine (2L straight-6 turbo, coil-pack ignition) ran fine for a long time. Then it developed some misfiring under heavy acceleration, once the RPMs got over about 4500 (redline is 7200).
I got a new battery, got new factory spec spark plugs (platinum, gap 1.1mm), and cleaned my coil packs.
I now notice that it misfires a little, at WOT only, when I'm using
95-RON petrol , and does not misfire when using 98-RON petrol. (However it isn't long since I did that coil pack cleaning, so I could be mistaken about this).Can the octane rating or the composition of the petrol affect whether it misfires or not?
By 'misfire', I mean that the car will suddenly lurch a little, the exhaust pitch drops for a moment, and the RPMs do not increase quite as quickly as they were. These 'pops' will occur at up to 3 per second if I keep the foot flat, and they seem to stop happening if I keep accelerating past 6000 RPM. (But I don't do this often either as I am worried that they might be damaging the engine).
I also had a friend say that he had something similar happen, then reduced his spark plug gap to 0.6mm and the problem went away. Should I try this and what would it indicate?
The only other thing I can think of trying is to adjust my timing (ie. adjust my cam-position sensor) and see if the problem gets worse or better. It's hard to test in a workshop because it doesn't happen when the car isn't in gear.
I suppose that this could also be indicative of a vacuum leak. My hoses all look OK to the naked eye, is there any way I can test if they are leaky or not? eg. put some goo on all the joins and then drive for a while, and see if the goo has bubbled anywhere?