C4 2005 1.6 petrol red glowing manifold

[sparksie]

Hi Mahmoud

I don't think you're comparing like with like, to be fair.
Vw have traditionally made their cars with cr@p fuel in mind, though there are exceptions.
French cars are not famous for their tolerance of such things, though some are remarkably good.
A simplistic rule of thumb would be low power, low-state-of-tune, engines will be more accepting of poor fuels than highly tuned, more powerful ones.
Seems to me, assuming you have access to better petrol of course, trying a half tank would be a relatively cheap investment.
Here, in chilly Ireland, some people still shun 95 octane, maintaining that their consumption of 98 is so much better it more than compensates for the higher price.
We used to service a 205GTI, which was religiously run on 98 and it was probably the most potent 1.6 we'd ever driven! Pity the brakes weren't as good!
I got the sad job of weighing that in, after a close encounter with a trailer that demolished the whole front end, including the top of the engine!
If the experiment works and you cure the manifold problem, you could experiment further with an octane booster, an additive you mix with the fuel.
You can probably figure out a mix that suits your own style of driving, possibly costing less than the quality fuel.
To begin with, however, I think the half tank of good stuff, once you've almost emptied your present stuff, is the cheapest thing you can try!