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If your RPMs stay low with the speed of the car then it sounds like you could have a severely restricted exhaust. If the engine revs but does not move like it should-then you may have a trans problem. Plugged catalytic converters will not always set a trouble code-certain makes and models will set catalyst efficiency codes quite readily but mitsubishi is not necessarily one of them. Anytime you step down all the way on the gas and the car doesnt go and the rpm's dont rev up-its a very good possibility that the exhaust is restricted.
You can feel the exhaust coming from the tailpipe when the engine is reved up-it should be strong and steady. From the information you gave it sounds like a classic case of exhaust restriction-if this does not make sense to you or if there are more details concerning this problem-please reply and I will give you a more precise answer. The more details the better. Let me know what you think.
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Like I said it sounds like a restricted exhaust-classic symptoms. What type of diagnostic tests were done?
Yes to determine if there is a restricted exhaust issue-a manifold vacuum test needs to be done. Vacuum readings at the intake manifold at idle and under steady acceleration needs to be monitored. If the vacuum decreases under steady high rpm's then there is an exhaust restriction.
An exhaust backpressure test could also be done to determine excessive backpressure due to a restriction.
Temperature testing of the catalytic converter can also determine if that is the problem. The cat should be 100 degrees hotter at the outlet than at the inlet if it is operating properly. Any heat pocket at or before the inlet and a temperature differential that is cooler at the outlet side indicates restriction.
OK let me know. Have there been any other repairs done recently? Does it correspond to the symptoms I was describing as far as the low rpms go?