Q&A

Check your belt alignment ensure it is properly tensioned. You definitely need good instructions and illustrations to tackle replacing the heater core on a car.There is a company that independent auto repair shops "buy" their information from that offer a very reasonable service to do it yourselvers. You get a one year subscription to year make and model specific repair procedures, service bulletins, component locations, wiring diagrams ect.... great quality information same as the professionals have ,much better than generic workshop manuals from parts stores, much cheaper than the factory manual. http://bit.ly/alldata_repair_manuals or try http://bit.ly/mitchell_repair_manuals_online and get an online repair manual subscription. Hyundai as a car manufacturer put a lot of free technical information and workshop manuals on line for Hyundai owners. Check out www.hmaservice.com lots of great information.
I am almost certain that is an interference fit engine may have bent valves, call the dealer and ask if it is an interference fit engine, if so valves may be bent.

fIRST OF ALL YOUR 1.8L ENGINE IS AN INTERFERENCE ENGINE WHICH MEANS THAT IF YOU WERE OFF WITH THE CAM TIMING YOU BENT THE VALVES. DOUBLE CHECK THE TIMING MARKS THEN DO A COMPRESSION TEST OR A LEAK DOWN TEST. A LEAK DOWN TEST IS BETTER BECAUSE YOU DON'T HAVE TO CRANK THE ENGINE, ALL YOU NEED IS THE LEAK DOWN TESTER AND AN AIR COMPRESSOR. TO MANY TIMES PEOPLE TRY TO WORK ON THEY CARS THEM SELVES, BUT MISS SMALL DETAILS THE PRO DON'T AND THEY WIND UP DOING MORE DAMAGE. HOPE ITS NOT YOUR CASE, GOOD LUCK