Dear rjp,
Sorry no one responded sooner! Since you have a 1991 Elantra, now 17 years old, how many miles on this engine? When did you last get your cylinder compression checked? This is not hard to do, if you have a helper and this kit:
http://www.harborfreight.com/cpi/ctaf/disp...temnumber=95187Low compression will cause a low idle.
Have you done your routine maintenance items...spark plugs, ignition cables, clean air filter, and fuel injector cleaning service?
Byron