LS swap cooling: radiator, fans and steam line

How we put this guide together

Of the 2,012 LS swap questions we pulled from forum threads, 141 were about cooling: which radiator, where the steam line goes, one fan or two, and why a swap that idles cool runs hot on the highway. We already answer about thirty of those one at a time. This guide ties them together: what's different about an LS, how to decide between keeping your radiator and buying a swap radiator, how to pick and wire the fans, where the transmission cooler goes, and how to track down overheating in the right order.

Step 1: Know what's different about LS cooling

Three things catch people who are used to a small-block Chevy:

• Both hoses are on the passenger side. Most LS water pumps put the inlet and the outlet on the passenger side, so an LS swap radiator has both ports on the passenger-side tank. A radiator plumbed for an old V8 has its inlet on the other side and needs a hose adapter or creative routing. Truck pumps also point the upper outlet straight up, which matters under a low hood.
• The thermostat sits low, in the inlet. On an LS the thermostat lives in the housing where the lower hose meets the water pump, not up top. That's why the engine needs a steam line.
• The steam line isn't optional. With the thermostat low, air collects at the top of the heads. GM vents it through small steam ports in the heads, joined by a crossover pipe, and runs that line back to the cooling system. Cap it off and you get air pockets, hot spots and a system that won't fill. Join the steam ports and run the line to the radiator's steam port, a tapped port on the water pump housing, or a hose adapter with a steam barb. More: LS steam vent: do I need it?

Hose sizes: the lower connection is 1-1/2 in on every LS. The upper outlet on the pump is smaller than a small-block's: measure yours (published figures are 1-1/4 in and 1-3/8 in depending on the pump), because the most common hose adapter steps 1-1/4 in up to a 1-1/2 in radiator inlet. Which pump you run is decided by your accessory drive spacing, so settle that first.

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Step 2: Keep your radiator or buy a swap radiator

Both routes work. The question is whether your radiator is big enough and healthy, and how much hose routing you'll put up with.

1. Keep a healthy V8 radiator. If it came from a V8 (not a V6 or four-cylinder version of your vehicle) and it isn't plugged, it can cool a stock or mild LS. Add an LS hose adapter with a steam port (about $25 to $45): it takes the steam line on a 1/4 in barb and steps the LS upper outlet up to a 1-1/2 in radiator inlet. Expect a long upper hose that crosses the engine.
2. Universal double-pass radiator. A double-pass design puts both hoses on the same tank, which is exactly what an LS wants, and usually cools better than a single-pass of the same size. Budget for brackets and some trimming. Examples: Speedway universal LS radiators, Griffin's universal cores.
3. Direct-fit LS radiator. Made for your chassis with LS hose routing and a steam port: Cold Case, DeWitts, Champion, BRP, Novak and others list them by vehicle. They bolt in and hook up like a stock part, and they cost the most ($370 to $1,500 depending on chassis and fans).

Cap and core. A 15 psi cap is the usual choice; every psi raises the boiling point, and 15 psi adds about 45°F. How big? A 2-row aluminum core is worth it even for a stock engine; 3 and 4 rows suit towing and real power. More rows aren't automatically better: each row adds thickness (measure the space for core, fans and shroud; thick cores have hit pulleys and steering boxes), and the back rows see warmer air and do less. Core area beats core thickness. A 5.3 needs a V8-sized radiator: the 4.3 radiator in a GMT400 isn't enough, the 5.7 tow-package unit is.

Step 3: Pick the fans and the shroud

Clutch fan or electric? A mechanical fan works if it fits: on a truck that means the Gen III Vortec truck pump, its fan clutch and a shroud. On most swaps the engine sits too close to the radiator (or too far forward, like Tejas-plate squarebodies) and electric fans are the answer.

One fan or two:

	One big fan	Two smaller fans
Good	Simpler wiring, one relay, less current	More airflow, can be staged, one keeps working if the other dies
Bad	May not cover a wide core; its deepest point can end up closest to the engine	Needs a relay per fan, more wiring and a strong charging system

The shroud matters as much as the fan. Without one, a fan only pulls air through the part of the core behind its blades: one example was a fan covering 228 sq in of a 406 sq in core. A shroud that covers the whole core makes the fan work all of it, which is what keeps an LS cool at idle and in traffic.

Junkyard fans are fine if you size the wiring for them. LS1 F-body fans, 1993-97 F-body fans, Taurus, Volvo and late Jeep JK fans all show up in swaps in the table below. Check the fan's current draw before you pick the relay, fuse and wire.

Step 4: Wire the fans so the computer runs them

1. Give each fan its own relay, triggered by the computer's fan outputs or a thermostatic controller. Two fans on one standard relay can pop its 30 A fuse when both start at once.
2. Size it for the real draw, including start-up. A 16 in fan runs about 17–18.5 A; a pair of 12 in fans runs anywhere from about 9 A each (light duty) to 25 A each (high output). A fan can pull about twice its running current for a moment at start-up. A 40 A relay per fan is common, high-output fans may want 70 A, and 12 AWG is the minimum wire (10 or 8 AWG for big fans and long runs). Fuse a little above the running draw: 30 A for a 25 A fan. Use the wire gauge chart for your lengths.
3. Let the computer stage them. GM computers have two fan outputs: low speed first, high speed when it gets hotter or the A/C pressure climbs. Stock Gen III calibrations turn the fans on late (Fan 1 at 226°F, Fan 2 at 235°F); most swap tunes bring that down to about 190-205°F, which the tune handles.
4. No computer fan control? An adjustable thermostatic controller runs one or two fans (one common unit is rated 25 A combined), with an A/C override.

Pin numbers, relay diagrams and the computer's fan outputs are in the fuel pump and fan wiring tool.

Step 5: Plan the transmission cooler

The cooler built into a radiator tank is fine for a mild automatic. For a higher-stall converter, towing, a 4L60E making power or a hot climate, add an external cooler: higher stall makes more heat, and the tank cooler dumps that heat into the engine coolant. Mount it low in front of the radiator, in line with the grille opening, and run 3/8 in hose or hard line with bulkhead fittings where it passes sheet metal. Some direct-fit radiators (Cold Case's G-body unit, manual-trans versions) have no cooler at all. More: trans cooler in the radiator or separate?

Step 6: First fill and overheating, in the right order

First fill: fill slowly with the steam line connected, then run it with the cap off (or the bleeder open) until the thermostat opens and the level stops dropping. An LS that won't take coolant, or spits it, almost always has trapped air: check the steam line first. The first-start checklist walks through it.

When it runs hot, ask when:

• Hot at idle and in traffic, fine on the highway: airflow. The fan isn't switching on, it's on the wrong speed, or there's no shroud.
• Fine at idle, hot at highway speed or under load: look past the fans. Radiator capacity, a tired or wrong water pump, a shroud with no bypass flaps choking ram air, and the tune: a lean mixture or timing far from right adds heat that no radiator fixes.
• Climbs from cold and won't settle: air in the system or a stuck thermostat.

Fix them in that order before you buy a bigger radiator. More: my LS is overheating: what should I check first?

Common mistakes

• Capping the steam ports. It's the number one cause of a swap that won't fill or runs hot at the top of the heads.
• A fan with no shroud. The fan works, the core doesn't.
• Two fans on one small relay. Burned relays and blown fuses, usually in traffic on a hot day.
• A thick core that hits the pump pulley. Measure the space with the fans and shroud in place.
• Blaming the radiator for a tune problem. If it's cool at idle and hot at speed, check the air/fuel ratio and timing before buying cooling parts.
• Mixing water pump and drive parts from different spacings. The pump has to match the balancer. More: accessory drive guide.

Find your vehicle in the table below for the radiators and fans that fit it.