Toyota Prius head gasket repair

A head gasket seals the cylinder head to the engine block, keeping combustion, coolant and oil in their own passages. When it fails on a Prius it usually leaks coolant into a cylinder rather than onto the ground, which is why the early signs are easy to miss:

• The coolant reservoir keeps dropping, but there's no puddle and no wet hose.
• The first start of the day is rough, shaky or rattly, then smooths out after a minute. Coolant that seeped into a cylinder overnight is being burned off.
• A check engine light with misfire codes — P0300 through P0304. A flashing check engine light means an active misfire.
• White smoke or steam from the tailpipe with a sweet smell, especially at start-up.
• Bubbles in the coolant reservoir, or coolant hoses that feel hard with pressure soon after starting.
• The heater blowing cold, or the engine temperature climbing on a hot afternoon.

It comes up most often on the 1.8-litre engine (Toyota's 2ZR-FXE) at high mileage. The same engine is in several cars, so this page applies to all of them:

The Prius recirculates a little exhaust back into the engine to keep combustion temperatures down. That exhaust passes through an EGR valve, an EGR cooler and small ports in the intake manifold — and over a couple of hundred thousand miles, all of it slowly packs with carbon.

As the flow chokes off, combustion runs hotter and less evenly, cylinders start to misfire and knock, and the head gasket takes the strain. So the gasket is usually the victim, not the cause. Replace it without cleaning the EGR system and the new one is under the same stress as the old one.

1. 1Scan the engine and hybrid modules, including the freeze-frame data showing what the engine was doing when each misfire set.
2. 2Pressure-test the cooling system to see whether it holds and where it goes if it doesn't.
3. 3Combustion-gas test — a chemical check for exhaust gases in the coolant, which a healthy head gasket keeps out.
4. 4Borescope each cylinder. Coolant steam-cleans a piston top, so the leaking cylinder often looks visibly different from the others.
5. 5Check the EGR cooler, so we don't sell you a head gasket for a cooler leak.
6. 6Test the hybrid battery. It doesn't change what's wrong with the engine, but it changes whether fixing the engine is the right call.

• Cylinder head removed and checked for flatness and cracks, and resurfaced by a machine shop if it needs it.
• New head gasket and new head bolts.
• The whole EGR path cleaned — valve, cooler, pipe and the intake manifold ports — or the cooler replaced if it's leaking or too far gone.
• New PCV valve, since an engine breathing oil vapour feeds carbon straight back into the EGR system.
• New gaskets for everything that comes apart on the way in: intake, exhaust and valve cover.
• Toyota-spec coolant, with the cooling system bled properly. Trapped air is a common reason a fresh repair overheats.
• Codes cleared, road test, and the coolant level rechecked after the engine has been through a few heat cycles.

You get a written quote after testing and before anything is taken apart. What moves the number: whether the head needs machining, whether the EGR cooler can be cleaned or has to be replaced, and whether coolant has already damaged a cylinder — at which point an engine swap may be the better repair.

Then comes the question most shops don't ask. A Prius old enough to need a head gasket is often still on its original hybrid battery, and that battery deserves a test too. Putting an engine repair into a car whose battery needs replacing next year is how people end up spending more than the car is worth. We test both, show you both results, and tell you straight whether to repair, swap the engine, replace the battery as well — or sell.

• Check the coolant reservoir with the engine cold each morning and mark the level with tape. A steady drop with no leak is useful evidence.
• Take a phone video of a cold start, including the tailpipe. Text it to 623-323-0737.
• If the check engine light is flashing, stop driving. An active misfire can damage the catalytic converter quickly.
• Don't keep topping up and driving for weeks. If a cylinder fills with coolant while the car is parked, starting it can damage the engine past a head gasket repair.
• Don't pour in a head gasket sealer. It rarely holds on this engine and can clog the heater core and the small coolant passages in the EGR cooler, making the proper repair harder.
• Never open the coolant cap on a hot engine.