I have Mazda Protégé 1995, 1.5 engine and 5 speed manual transmission. There is a problem with rpm. When I start the engine rpm are fine, but about 20 minutes later the car is keeping up 1100 rpm when it`s in neutral gear.
What can be the problem?

 

I believe you have a MAP sensor, and that might be the issue. It could be getting hot, and sending a bogus signal that is affecting fuel/air issue.

 

Adding to Kevinkpk's response....
Mazda calls the thing EGR Boost Sensor, as if it would boost something.. It sits on the firewall , passenger side and there is a vacuum hose going down behind the intake manifold. Pull that hose off there and go into the nipple with a wire (2-3") push through any resistance. This spot tends to clog up and prevent the vacuum signal going to the EGR Boost Sensor.
Also read the sticky "the unofficial user's manual to the Mazda Protege". Yours truly compiled a bunch of info there.

 

just slow down the engine rpm

 

idle air control valve?