So I bought a used 5 speed standard 2003 elantra about a month ago. It worked great no problems, 78000 miles. I have put about 1000 miles on since. Now the acceleration is crap for highway/high speed driving. I struggle to maintain speed going up hills, let alone accelerate up them. On flat ground i cannot accelerate higher than 65-70. Took it into my mechanic, he though the catalitics were clogged. He checked both and they were in good condition, and another mechanic confirmed this. The timing belt was also another though to be problem but was further determined to be fine

I was wondering if anyone knows what possibly would cause my problems.

 

Here's a what i did to trouble shoot mine. Unplugg 1 sensor at a time. Start with the Throttle position sensor. Its at the begining of the intake manifold. there will be a black plastic tube from the air cleaner running to it. Make sure the car is running while you do it. If the car stalls or changes idle speed it's good if not change that. Then go to the mass air flow sensor. Those are the 2 electronics that have any major acceleration connections.