Erm, well how it works is the Corsa has a wheel called the 60-2 (sixty minus two) wheel on the crank, it's the crank pulley. The crank pulley turns stuff like the aircon, power steering, alternator whatever. Anyway this wheel has 58 teeth and then it's flat, 2 teeth are missing hence the term 60-2.
Anyway right next to that wheel is a sensor called the Crank Angle Sensor (CAS), whenever the 2 missing teeth move past the ECU detects a change in magnetic flux and this is how the spark plugs and injectors are timed. That wheel is aligned to the no. 1 cylinder at TDC (top dead center).
The only way that the spark plugs could not be firing correctly is if the wheel is misaligned, that wheel has a fit that allows it to go in only 1 way, the correct way, hence it's fool proof. But what sometimes happens is it is not correctly torque'd by whatever idiot changed the cam belt or whatever and where the wheel fits in it starts to wear down, causing the wheel to move, it causes mayor change in performance tho, if that wheel is out by 1 tooth the car will have trouble pulling off, knock, use excessive amounts of fuel, etc. Also that wheel need only be removed when changing the cam belt and stuff like engine rebuilds, so no reason to suspect it just yet.
The engine timing (which is basically how far before TDC the spark is fired) was determined at the factory by Opel, there is absolutely no way to change it apart from making a new wheel. You can only flash the ECU with a Opel Tech2 and even GM service agents don't have the equipment to make new maps, GM PE (the GM head quarters in SA) are likely the only people in SA that can make new maps for those ECU's.
The only other way timing could be out is if the crank gear and cam pulley are misaligned which also could happen after a cam belt change by a n00b. Car most likely won't even start and worse if it's far enough out the engine will bend the valves because that engine (C16SE) is interference design.
The same counts for the mixture, but what they do allow is for you to set a single variable which is a multiplier of sorts, they measure the CO2 at the exhaust and it has to be a certain level, if not they adjust this variable until it is correct. Because they determined when making that map that the CO2 level should be at that level for that given RPM the rest of the map when adjusted by that multiplier should also be correct. Hence when the CO's are out the entire map is usually running either too rich or too lean. In this case it would be lean.
Cars don't shudder unless they are starved for fuel or running so rich that the flame front is being drowned by petrol, but that is unlikely to be the case here as the mixture cannot easily be set that rich.
Finally, the car works on a wasted spark setup, what this means is that the coil pack only has 2 coils and mosfets, when the spark for cylinder no.1 fires so does no.4, when no.2 fires so does no.3. It's quite popular because it simplifies the design and when No.1 is on it's power stroke, no.4 is on the exhaust stroke so that spark does nothing to the exhaust gasses (hence the spark is wasted). Only downside of the design is that if the coil for No.1 fails it also does for no.4, etc. and it wears down sparks plugs faster.
Anyway what happens sometimes is the Coil Pack starts to overheat stops working, then once it's cooled down it'll work again. I've had a car die at me on the highway @ 120km/h, and in gear it was dragged all the way down to 0km/h, absolutely no spark from that coil pack, yet 5 minutes later it's running again like nothings happened. The backfire could be caused by this because if a engine fails to spark the fuel that was supposed to be ignited is ejected into the hot exhaust manifold and it explodes there, hence the backfire.
It does not however explain the shudder, that shudder is in no way related to the coil pack.
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