underside of car is overheating

i have a santro - 2002 model. the car was juddering a lot, so got spark plugs changed and also the clutch plate and wire replaced. after this, it worked well for a day. after that, i took it out and after about 2 kms, car was refusing to pick up and the underside got hot, so hot that the area near the gear stick was very hot and i could smell burning. and once i parked the car, i could hear noises from under the car as if little metal pieces were rattling around in there. what is the problem? someone please help.

Welcome to Carwale Rabel.

Have you checked the Coolant level in the radiator? Where was the needle of Engine Temp? 

The symptoms you've mentioned indicates the engine has overheated because of the lack / absence of coolant. It happens if the radiator / hose pipe has got leakage.

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the coolant level is fine. the needle on the engine temp is very close to cool. so it does not appear to be a coolant problem. 

If the needle is near cold even after 5-10 mins of driving then the engine temperature sensor has quit (it is a sensor located somewhere on the coolant pipe close to the engine - ask the mechanic where it is). The first thing to check would be this - without correct temperature reading from this, ECU will not start the radiator fan and engine will overheat. The catalytic converter looks cooked as well due to all that overheated exhaust - this is approximately under the gear box lever. Looks like this will have to be replaced.

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