My LS is overheating — what should I check first?

First question: when does it run hot? At idle and low speed the fan does most of the work, so overheating in traffic points to airflow: the fan, the shroud, or the fan not switching on. At highway speed, moving air does most of the work, so if it's fine in traffic but hot at speed, look beyond the fan — radiator capacity, water pump, or the tune (Speedway's overheating guide makes the same split for race cars: hot under yellow = airflow; hot under green = look elsewhere).

Checklist before buying a bigger radiator (Speedway):

• A shroud that covers the whole core (the most common problem they see).
• Fans actually switching on at the right temperature.
• Water pump condition and flow.
• Air in the system — the LS thermostat sits low, so the steam line has to be plumbed to purge trapped air.
• Radiator fins not bent or blocked, and no internal damage.
• Ignition timing and air/fuel ratio.

Shroud: without one, the fan only pulls air through the part of the core directly behind the blades. Speedway's example: a fan covering 228 sq in on a 406 sq in core. A shroud that covers the whole core makes the fan pull air through all of it — critical at idle and low speed. Some performance shrouds have rubber bypass flaps that open at speed.

The tune: a lean air/fuel mixture makes cylinders run hotter, and ignition timing that's too far advanced or too retarded also adds heat. Speedway notes improper timing or air/fuel can make an engine run hot even with a properly sized cooling system.

Only after those, consider more radiator. See the cooling Q&A on steam lines.