Hi,
I have a 1999 opel chevy pickup 1600cc and I have a weird problem with it, mechanics don't know what it is.
Problem: after running about 300Km the engine loses power.
Power loss is due to that the spark plugs become dark and dry (like the upper spark plug on this image:

If I replace the plugs with a new ones, I can drive another 300km. Maybe more or less, depends how often I push on the accelerator. If I push too much, the engine loses power more quickly ( I drive in a mountain area at around 2000 mts above the sea) Compression is 125 psi on all 4 cylinders.
We replaced the MAP, temperature sensor and oxygen sensor. Didn't help.
The mechanic says it could be the injector or the engine control unit. The suggestion was to change the from 87 octane gas (which I am using right now) to 92 octane gas and see if the problem disappears.
I need to know for sure, where the problem is, but we can't find it. All the engine indicators are ok. How can I find what is wrong?

Will appreciate any suggestion.

Thanks

 

Looks like it's running too rich, and fouling the plugs.
Check this NGK Spark Plugs USA

 

Looks like you already have that plug reader link. It's definatley fuel related, or wrong heat range plug. I'd try a next heat range higher plug.