00 Kia Sephia won't rev above 3500 rpm


So, since last December I have experienced a gradual worsening of my car’s acceleration problems. First it started as simply a weaken acceleration, then a failure to accelerate on steep hills, then small hills, until now the power is so low it can’t get above 3500 rpm period! The idle is so bad the car feels like it is about to stall at any moment (though it hasn’t yet). I noticed that before the problem got too bad, I could press the gas pedal up to a certain point before it would cut the power and the engine would feel like it was choking. Now I can barely touch it.

So far I’ve changed the fuel filter (which was very bad!), then the fuel injectors, then the O2 sensor, and finally the fuel pump, all without success. The OBDII codes for misfires went away after the new injectors, while the O2 sensor took care of its OBDII code. Since then I have not had any codes show up, but the problem has only gotten worse.

I haven’t checked ignition related things (like spark plugs, timing, ignition coils, etc…) because the car has never had a problem starting, nor have there been misfires since the injectors, just a lack of power.

There are several little sensors and valves I can check, but I can’t think of any one big thing that would be causing the problem. Any advice would be much appreciated!

Could be a clogged catalytic converter. This can be easily checked with a vacuum gauge.

If the converter is clogged then the reason for that must also be resolved; oil consumption, long term running in an overly rich condition, very high miles on the car, etc.