When the engine is hot, the coolant system is under pressure (due to heat expansion). When the system is cold, there should be little or no pressure. If the coolant system is pressurised when cold, there must be something pressurising the system. The main source of pressure is from the engine through a faulty head gasket.
Or, water is escaping through a faulty headgasket and going out through the exhaust.
Or, or oil is entering the coolant system through a faulty head gasket.
Or, the engine is running roughly, overheating, through a combination of the above.
There are several tests that can be performed. Note. They may not always reveal a fault. Sometimes you have to go on gut instinct and putting 2+2 together to come up with a solution.
- Testing for hydrocarbons (exhaust gases) in the cooling system
- Pressure testing the cooling system.
- Pressure or compression testing the engine.
Safety is important - Follow the precautions contained in the vehicle handbook/repair manual. They are there for your safety.
The repair involves:
- Draining the coolant.
- Removing all the items that connect the head to other parts of the vehicle.
- Unbolting the head and lifting it off the block.
- Removing the old gasket.
- Inspecting for damage.
- Cleaning and inserting a new gasket.
Additional issues (that are less common) can be discovered, once the head is removed.
- A warped head (caused by continued overheating).
- Damage caused by corrosion / erosion.
- Cracked cylinder head or block.
Note: Items 2 & 3 are worst case scenarios.
- A warped Head - If the head has warped, but is still within machining tolerances; the repair will involve removing all the components from the head and getting a machine shop to ‘skim’ the head. This is common practice if a head is warped.
- Corrosion / Erosion – if not too bad, it is possible to do a repair using a number of methods (specialist filler, weld, or insert), but it may not be a ‘guaranteed’ fix.
- Cracked cylinder head or block – Normally a replacement has to be sought.
Head Gasket Cost
The charges for replacing the head gasket can be high due to the large amount of labour, typically 75% (if a large number of items have to be disconnected to remove the head).
The cost for the land rover was £70 for parts, but I was quoted £650 + vat as a starting price. Typical price researching on the internet was £800 to £1,000.
A more detailed description of repairing a Head Gasket (Land Rover TD5 - Discovery 2), including many more photographs, can be found here.