How to Fix a Blown Head Gasket with Sealer
Learn to chemically seal a minor blown head gasket using a professional pour-in treatment, system flush, and controlled heat cycle without engine teardown.
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🧰 Required Tools & Equipment
- ✓ Socket set and ratchet
- ✓ Drain pan (minimum 10-quart capacity)
- ✓ Spill-free radiator funnel
- ✓ Pliers for hose clamps
- ✓ Garden hose with spray nozzle
📦 Materials & Supplies
- ✓ Head gasket chemical sealer bottle
- ✓ Cooling system flush chemical
- ✓ 4 to 6 gallons distilled water
- ✓ Replacement thermostat and housing gasket
- ✓ Correct vehicle-spec 50/50 antifreeze coolant
⚠️ Safety Checkpoints & Warnings
- • Never remove radiator cap or open cooling lines while engine is hot; pressurized steam causes severe burns.
- • Catch all drained antifreeze in a sealed container; it is highly toxic to pets and wildlife.
- • Monitor dashboard temperature gauge constantly during curing idle; shut down immediately if needle enters the red zone.
📋 Step-by-Step Guide (6 Steps)
Drain the Old Coolant Completely
Ensure the engine is completely cold to the touch. Place a drain pan directly beneath the radiator petcock or lower radiator hose. Open the drain valve and remove the radiator cap to drain all old coolant into the pan.
- • Radiator drain petcock open with fluid completely stopped dripping into pan.
- • Radiator cap resting safely aside.
- • Opening the cap on a hot engine.
- • Letting coolant spill across the floor without a wide catch basin.
Remove the Thermostat
Locate the thermostat housing following the upper radiator hose to the engine block. Unbolt the housing, lift out the internal thermostat, and bolt the empty housing neck back onto the block using its gasket or temporary silicone seal. This allows unrestricted circulation of the sealer.
- • Thermostat unit sitting detached on the workbench.
- • Thermostat housing re-fastened securely to the engine without the internal valve inside.
- • Leaving the thermostat installed, which can gum up the valve and block sealer flow.
- • Overtightening housing bolts into aluminum threads.
Flush the Cooling System with Clean Water
Insert a garden hose into the radiator fill neck and flush until clean water runs out of the lower drain point. Close the petcock, add cooling system flush chemical, fill entirely with plain water, and run the engine for 10 minutes to scour oil or antifreeze residue from cylinder walls.
- • Clear, unclouded water exiting the lower radiator drain.
- • Radiator neck filled to the brim with clear flush water.
- • Skipping the flush step, which causes chemical sealers to react poorly with old silicate coolant.
- • Leaving contaminated cloudy fluid in the block.
Add Head Gasket Sealer and Distilled Water
Drain the flush water out completely and close the petcock. Shake the sealer bottle vigorously for 30 seconds. Attach a spill-free funnel to the radiator neck, slowly pour the recommended dosage of sealer into the system, and top off the remaining volume with distilled water.
- • Sealer bottle completely emptied into funnel.
- • Spill-free funnel filled 1/3 full with distilled water showing bubbles escaping as air bleeds.
- • Pouring sealer directly into an unmixed overflow reservoir instead of the radiator neck or upper hose.
- • Pouring cold fluid rapidly into a scorching engine block.
Perform the Curing Idle Cycle
Start the engine and turn vehicle cabin heater and blower fan to maximum heat. Allow the vehicle to idle smoothly for the precise time designated by the sealer manufacturer (typically 50 minutes). Keep a steady eye on the dashboard temperature gauge to ensure standard operating heat without overheating.
- • Engine running smoothly at idle speed.
- • Cabin heater blowing hot air.
- • Temperature gauge stabilized at normal midpoint.
- • Revving the engine excessively instead of maintaining a steady idle.
- • Leaving cabin heater off, which traps cold air in the heater core loop.
Install New Thermostat and Fresh Coolant
Turn off the engine and let it cool completely down to ambient temperature. Drain the sealer fluid if manufacturer specifies (or leave in if permanent style), remove the housing, insert a brand-new thermostat with fresh gasket, and re-torque bolts. Fill the cooling system with proper 50/50 antifreeze mix and bleed all trapped air.
- • New thermostat seated flat in housing recess with fresh gasket.
- • Coolant level steady at the full line with no air bubbles surfacing in funnel.
- • Installing the new thermostat backwards (spring side must face down toward engine block).
- • Leaving air pockets in the cooling system after refill.
Credit: inspired by an original tutorial by Phillips Vision — watch the original on YouTube.
Don’t have everything?
- No Spill-free radiator funnel? Standard automotive funnel with a tight-fitting rag wrapped around base to catch splashes
- No Pliers for hose clamps? Channel-lock adjustable pliers or flathead screwdriver depending on clamp style