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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 (the same split works for race cars: hot under yellow = airflow; hot under green = look elsewhere).

Last updated

Checklist before buying a bigger radiator:

  • 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. 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.

What we recommend

Start with the electric fans: confirm they actually run once the engine is hot, then check the relays, fuses and wiring. Next, purge trapped air with the fill neck at the high point of the system and make sure the head steam line is connected to a high point in the cooling system. We recommend pressure-testing the system before assuming a head gasket, and running a proper 50/50 coolant mix, since a mostly-water fill can contribute to steam pockets. Don't diagnose by pulling the thermostat out permanently; thermostats fail open or closed, not typically leaking.

Check this first:

  • Fans turning on at temperature
  • Steam line routed to a high point
  • Cooling system pressure test
  • Coolant mix is 50/50

Careful: Continuing to drive an overheating LS risks head-gasket and engine damage.

Based on what we've seen work across our own builds, our friends' builds and the wider LS community, reviewed by our lead shop tuner (over 20 years of LS and LT tuning experience). Every combination is different, so use at your own risk and check our safe tuning limits before any wide-open pass.

Go deeper

Check this first

  1. Verify coolant is full and fans kick on at temp
  2. Check thermostat operation and radiator for cold spots (blockage)

Try this, then this

  1. Top off coolant, bleed air via steam vent — most common cause. Worked if: temps stabilize
  2. Replace thermostat and verify fans — next most common. Worked if: temps stabilize

How you know it's fixed

  • Holds 195-210 F in traffic with A/C on

Still not fixed?

  • Pressure-test for head gasket; check water pump impeller

Every option we know of

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Maker / productPart #FitsPriceWhereStatus / notes
GM
replacement thermostat 187F		131-151all LSabout $15Buy
parts store		In production
DIY
burp/purge cooling system of air		n/aall$0garageDIY
LS engines trap air at steam vents

Prices are approximate as of September 2026. We list every option we know of, including small shops and used parts we can't link to. Confirm fitment with the seller.

Time and cost: 30 min - 2 hours

Should I run a 160°F, 180°F, or stock thermostat in my LS1?
What we recommend

For a naturally aspirated or bolt-on street LS1, we recommend the stock thermostat (86°C/187°F) or a 180°F. A 160°F thermostat really only makes sense for aggressive tunes, marginal pump-gas compression, or some power-adder combinations. Some people run 160s with no problems, but a bolt-on LS1 doesn't need one. If you do go cooler, have the electric fan settings reprogrammed to match. Don't use the 1999+ F-body dash gauge to judge temperature, because it's heavily buffered and not a precise reading.

Check this first:

  • Plan a fan-setting change in the tune if you go below stock
  • Use a scan tool, not the 99+ dash gauge, to verify coolant temp

How you know it's fixed

  • n/a — reference

Key numbers

WhatValue
Stock LS1 thermostat187 F (Murray 37987)

Every option we know of

Maker / productPart #FitsPriceWhereStatus / notes
Murray
187F thermostat		37987all LSabout $15Buy
O'Reilly		In production
OE temp
Stant
180F thermostat		14279all LSabout $15parts storeIn production
runs like stock
various
160F thermostat		n/aall LSabout $20SummitIn production
needs fan retune; may set P0128

Prices are approximate as of September 2026. We list every option we know of, including small shops and used parts we can't link to. Confirm fitment with the seller.

Time and cost: Reference only

What are the symptoms of a blown head gasket on an LS?
What we recommend

The classic signs are steam from the exhaust, oil in the coolant, milky sludge under the oil cap, water in the drained oil, and hard starting or heavy bogging under load. Before tearing anything down, we recommend running a compression test, then a leak-down test, and checking for coolant in the oil and any visible external coolant leaks. The factory MLS gasket is hard to kill, so confirm the diagnosis first. If it is the gasket, plan for milling plus new bolts and gaskets while the heads are off.

Check this first:

  • Run a compression test
  • Follow up with a leak-down test
  • Check for coolant in the oil and any external coolant leaks

Careful: Driving with a failed head gasket can overheat the engine and contaminate the oil, leading to bearing damage.

Check this first

  1. Cooling system pressure test: watch for pressure drop with no external leak
  2. Combustion-gas sniffer on the coolant (block tester fluid turns yellow)
  3. Compression + leakdown to find the weak cylinder(s)

Try this, then this

  1. Pressure test + sniffer — finds most failures. Worked if: positive — heads come off
  2. Compression + leakdown — pinpoints cylinder. Worked if: weak cylinder found

How you know it's fixed

  • All tests pass; coolant level stable; no overheating

Still not fixed?

  • Pull heads; check deck flatness

Key numbers

WhatValue
No single definitive testuse multiple tests together

Every option we know of

Maker / productPart #FitsPriceWhereStatus / notes
DIY
block tester (combustion gas sniffer)		n/aallabout $40parts storeDIY
DIY
cooling system pressure tester		n/aall$0parts store loanerDIY

Prices are approximate as of September 2026. We list every option we know of, including small shops and used parts we can't link to. Confirm fitment with the seller.

Time and cost: Diagnosis: 2-3 hours

How can I stop the upper radiator hose from kinking where it comes off the water pump on my 5.3 LS swap? The intake sits right on top of it and bends it, cutting coolant flow.
What we recommend

What we've seen work is running a short section of bent steel exhaust pipe through the kinked area as a crush-proof spacer inside the hose. Have the ends flanged so the hose clamps seal and don't leak; an exhaust shop can bend whatever diameter you need, such as a 2-inch ID piece. We prefer this over an internal hose spring, which can still collapse under load.

Check this first:

  • Hose inside diameter so the pipe fits snugly
  • Flanged or beaded ends on the pipe for clamp sealing
  • Clearance between the hose and intake after the fix

Careful: A kinked or collapsing upper hose restricts coolant flow and can overheat the engine.

Check this first

  1. Verify coolant level and look for external leaks
  2. Confirm fans come on at temp and thermostat opens
  3. Purge air via steam vents; pressure-test the cap
  4. If all else passes, test for head gasket (see HELD-246)

Try this, then this

  1. Level, thermostat, fans — most common. Worked if: temps normal
  2. Radiator flow + air purge — next most common. Worked if: temps normal
  3. Head gasket tests — last resort. Worked if: confirmed or ruled out

How you know it's fixed

  • Holds 195-215 F in traffic with A/C on

Still not fixed?

  • Radiator upgrade; check water pump flow

Every option we know of

Maker / productPart #FitsPriceWhereStatus / notes
DIY
systematic checklist		n/aall$0garageDIY
do it in order

Prices are approximate as of September 2026. We list every option we know of, including small shops and used parts we can't link to. Confirm fitment with the seller.

Time and cost: Diagnosis: 1-3 hours

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