I have a 97 toyota tercel and wont go faster than 30 miles per hour, I thought it was the catalytic converter so I replaced it and still does the same, what can I do?

my first thought is plugged exhaust, but also there could be a problem with the mass air flow sensor. if your technically enclined and have a vaccum gauge, you could check the amount of vaccum at idle. it should be over 20 inch pounds. if not and it declines the longer the engine runs its the exhaust. I would really recommend taking it to a dealer to know for sure

The way you worded your question ---it runs up to speed cold? Then it is running rich, anything from MAF (as above) to 02 sensor(s) when it goes in to closed loop the ECU is getting the wrong signal) MAP, IAC, (Intake air temperature), TPS, even coolant temp either sensor... or crank position sensor, faulty injector, bad ECU. Too much guesswork, You really should have an OBDII scan

Take off some un-important vacuum hose and create a vacuum leak. See what happens then when warm and also unplug your MAF and see if there is any difference (don't do these at same time) 1997 1.5L also has a return-less fuel delivery and your fuel pressure could be too high and needs flow-test. One more thing...driving it like that will ruin your expensive new cat..or used whatever

When it's cold runs normal but as soon as it reaches normal warm temperature it doesn't go faster than 30mph.......thank you in advance for all your answers ; )