Bought a 96 Elantra with ~100.000 miles/150.000km on it last week. Car ran fine until now with a lot of rain and cold/wet weather. It hesitated and ran a little rough this morning. Also low power.

Electrical system seems like the first thing to check. Car has new plugs and plug wires. Also I noticed the gas cap can't possibly be original, it doesn't screw in place and is made of plastic. Perhaps some moisture in the gas because of this...

Where do I start diagnosing? Is the 96 ODB2 or ODB1?

 

OBD-II was mandatory for all cars sold in the United States in 1996. Check the connector to be sure. The 2 are very different:

OBD-I:


OBD-II:

 

Thanks, although I was aware of the difference connector. On my 96 Chevy LT1 engine it is ODB2, but the 95 models came with ODB1 computer BUT the ODB2 connector so for those cars you can't know for sure by looking at the connector only.

Also, I have a European car, I'm not sure if that makes any difference, hopefully they used the same engine management system across markets. Hopefully it is the ODB2 computer with the ODB2 connector shown in your last picture, then I can connect with my laptop and read the codes at least.

Right now my prime suspect is the distributor cap, but I also noticed I have a small exhaust leak. Hopefully the leak is downstream of the "first" O2 sensor or that sensor will send erratic data to the PCM, I would guess the PCM would sense a lean condition and give it excess fuel.