Test the radiator cap and the new thermostat (You'd be surprised how often new parts are still bad). Check your ignition and mechanical timing as well if the exhaust gasses don't leave until after expected that heat is absorbed by the engine... so retarded valve timing or advanced ignition timing will cause an increase in heat.
Missing the undercover reduces efficiency it doesn't do so much that a car will overheat without it... The fan cowl is more important but even that is more effective at low speed.
__________________
If something breaks or you need to contact a member of the administration please post HERE. Unless it's a private or administration matter please post it on the forum. It benefits no one else if car related questions aren't posted for future users and takes away from the time I'm able to spend helping on the rest of the forum.
If you're so inclined I'm always more than happy to accept tips via PayPal. 
Tip Jar ---> 
|