1998 Infiniti I30 Overheating Under Load
You said you checked your fans and thermostat. I understand seeing if the fans are running is a great check. But how did you check your thermostat? Replacement is a great check, and a rather cheap preventative maintenance. If you have not done so, I would replace the thermostat. Theres not much to go wrong with a water pump. Usually they are replaced when they leak. I could see if your system had never been taken care of, maybe the impeller is all rusted up and not being efficent. You can replace your pump if you desire, but you will probably find the one you take off is pretty close to the one you put on. Again it is not a hard thing to replace, and relativley cheap.
If your thermostat is getting old, and starting to stick little, I could see this being a little problem. Also if the thermostat sticks open, the fluid may pass thru the radiator too fast to cool it enuff for when your car is under a load.
Check the fins on your radiator/condensor and see if you are getting good air flow across the radiator coils. They make flush kits to back flush your radiator and try and clean it out in reverse. It might not be a bad idea to replace the thermostat and flush your radiator and see if that will get your system back to handling the load under extreme conditions.
Suggested Repair
Replace the thermostat, flush the radiator and cooling system, and inspect the radiator and condenser fins for restricted airflow. A water pump is less likely unless it leaks or a neglected impeller is corroded and inefficient.
Conversation Summary
The owner of a 1998 Infiniti I30 reported near-overheating after about an hour under load in hilly areas; turning off the A/C lowered the temperature, while idling and short trips were fine. Fans and the thermostat were said to be working. The expert recommended replacing the thermostat as a cheap check, flushing the radiator, and verifying airflow across the radiator and condenser. A sticking-open thermostat or restricted cooling was considered more likely than a water pump unless the pump leaked or its impeller was corroded. A later commenter with a 2000 Infiniti I30 reported the same symptom after replacing many parts, including the water pump and thermostat, but received no further diagnosis.
