I have been unable to keep my 88' 4runner with a 3.0 v6 auto tranny running warm enough to keep the overdrive working [mostly at highway speeds] I have even placed cardboard in the grill to block airflow, but with minimal results. Anybody have any suggestions? BTW, I have replaced the thermostat w/ a toyota one.
Any help welcome...thanks

Probably a bad thermostat. Test it or replace it. You might have installed it backward or something.