A friend of mine owns a hyundai coupe s145 1.6 145hp a car that he bought used (50000km).The car has a low pressure turbo which is placed as a
pattent by the official hyundai service i think.
the car was checked by several mechanics who found no problem...

the car works perfectly in town and on highway wen not in pressure.but unfortunately after he bought the car and decided to make a long
highway trip he found that when the engine is on the fifth or fourth speed and it
requires a lower speed (the exact time that all the cars start losing rpm)the same time the yellow check engine light turns on and the cars engine starts to
interrupt its normal operation as if the car has no gas. When you change
the speed to a lower the problem stops.

the local official service could not solve the problem and the car was sent to
the official service in athens greece. They said that that they changed pistons
and the problem was caused due to bad and low octane gasoline and that
he should now use 95+octane gas.

1st The engine had not been opened so you understand pistons were never
changed.

2 The problem was not solved.My friend has tried all the types and brands
of high octane gasoline but the problem is still here, The yellow light turns on and....the engine has the same problems

that is very curious because in normal use the engine warks perfect and the
car is really fast and you can hear it works perfect. nobody can help him
and now that even the official service did what it could do then...
who you can ask?

Please reply if you ever had a similar problem or you just have an opinion about this problem.

thank you and forgive me for my bad english/not a native speaker so...
thank you again

Could it be the ECU telling the engine to cut fuel as the pressure is peaking about 12psi.

S-Coupes will go into "Nanny-mode" if they peak over 12psi and basically cut or reduce fuel going to the engine.

Get a Boost guage and fit it to the car to see what psi you are runing at.

Just a thought but hope this helps.