Problem with oxygen sensor or other sensor ..?

Greetings. I have a chevrolet this year 1998, the car presents a problem that does not maintain the constant minimum when starting, without it cold it starts well, but when it reaches a certain temperature the fault begins, the revolution marker rises and down constantly above and below the 1000 revolutions until it goes out, begins to throw a lot Filter of gasoline, spark plugs and cables are new. The IAC, TPS, MAP and ECM sensors were reviewed by a technician checking its good condition and the continuous problem. Recently the mechanic has told me that the problem could come from the oxygen sensor, but I have looked for it at the exhaust of the engine and the exhaust tube and I can't find it. Could it be that there are models that have no oxygen sensor? I have led the car to scan and do not report any failure or turn the ECM engine light. Does anyone know if the vehicle necessarily has to have the oxygen sensor or does this model have? Any suggestion regarding the failure? I have already taken it to 4 different mechanics and who know well of vehicles but have not found the fault.

It can be the Lambda probe, injectors covered so it is well in high revolutions, fuel pressure, also covered escape although I doubt this last, if it helped you help me a lot, but you have to keep looking

In oxygen sensor it must be in the admission multiple possibly covered, so you do not find it, remember that the black smoke is only fuel then you drown, if it can be a covered injector or such veins you have to regulate the butterfly, it is best to remember that it moved or change to the car to be with those symptoms

usually in what I understood .. a regime under the engine does not work well .. It must be the injectors because at high regime the injectors are forced to Valga the rear, inject fuel with more force .. if that is not that check the gasoline pump ..

I will try everything that they have recommended and I tell you how it went, I still can't locate the oxygen sensor, it gives me the impression that this model of the vehicle does not use it. Thanks for your answers