The Land Rover Discovery is a vehicle which is typically beloved by its owners, offering European luxury and legendary off-road capability at an economical price. Unfortunately, the Land Rover 3.9L/4.0L/4.6L V8 gasoline engines are prone to head gasket coolant seepage, especially as the vehicles approach the 100,000-mile mark. If this issue is to be repaired, it is important that certain steps are taken to prevent repeat failure, and to address other potential problems while the engine is disassembled for head gasket replacement. These include installation of ARP Head Studs, careful cylinder head inspection, and replacement of additional gaskets and seals that may lead to repeat failures if left unaddressed. It is also important to consider several factors when making the decision to repair or replace your Land Rover.
The issues
Head Bolt Stretch
The Land Rover V8 is prone to coolant seepage from the head gasket to the outside of the engine. This occurs because the clamping force between the cylinder head and the engine block is reduced as the head bolts stretch over time. Head bolt stretch is a result of the heating and cooling that the head bolts are subjected to as the engine heats and cools. The cyinder head and block have coolant ports that feed the cyinder heads at the extreme front and rear of the heads, with only a very narrow sealing area between the coolant port and thefront of the engine. When the clamping force on this portion of the head gasket is reduced below the force caused by the pressure in the cooling system, coolant seeps from the head gasket to the outside of the engine. Often this will only occur when the engine is hot, and the cooling system may pass a pressure test when cool.
The factory cylinder head bolts are tightened with a small amount of torque, then turned a certain angle, in two stages, to finish out the cylinder head tightening process. This turning causes the bolt to thread further into the engine block. The head gasket, and the cylinder head compress as the bolt turns, and the bolt itself stretches as well. Turning the bolts a certain angle, rather than turning them to a certain torque, provides a more consistent clamping force across the ten bolts in each cylinder head, since torque is actually only measuring the frictional force which is resisting the turning of the threads and the turning of the bolt head on the cylinder head. This would be problematic since friction between these parts can vary greatly. Turning the bolt a specific angle, in contrast, provides a predictable amount of tension on the bolt, since the bolt threads are all the same pitch and will move a predictable distance as they are turned into the engine block.
Localized Overheating
While you may never have seen the temperature gauge rise, or any warning lights indicate an engine overheat, you may have experienced localized engine overheating without knowing it. This phenomenon typically occurs due to low coolant levels as a result of a leak, such as a water pump/gasket, timing cover gasket, or throttle body heater gasket leak. A low coolant level can cause localized overheating, particularly in the highest parts of the cooling system such as the cylinder heads. This situation can not only increase the likelihood of head bolt stretch, but can also weaken the head gasket itself, or even warp the cylinder heads and accelerate the occurance of a head gasket leak. The best way to prevent localized overheating is to check your coolant level frequently, particularly if you smell coolant. The Dex-Cool coolant in your Land Rover has a very distinct sweet smell which can be easily identified once you're familiar with it. Also, if a gurgling sound is noted from the right-hand dash area when accelerating the engine, this can be a sign of low coolant and should prompt an inspection of the coolant level.
The Answers
ARP Head Studs
As described above, the factory cylinder head bolts stretch as they are tightened, and continue to stretch over time due to heating and cooling, which frequently leads to head gasket leakage. All Land Rover V8 head gasket replacements performed at Fleet Service Northwest include installation of ARP Head Studs. These head studs are much stronger than stock head bolts, and do not stretch as far when they are tightened or as they are heated and cooled over time. The reduced stretching causes a higher initial clamping force between the cylinder head and the engine block, and allows this clamping force to be better maintaned over time, drastically reducing the possibility for head gasket leakage.
Cylinder Head Inspection
A critical part of head gasket replacement is inspction of the cylinder head surface, as well as the valves and valve seats. The cylinder head must be flat, and free of scratches or other defects if it is to seal properly against the head gasket. Also, valves are prone to carbon buildup which can cause them to leak, leading to reduced engine performance and engine misfires. It is very important to inspect and address valve issues while the cylinder heads are removed, since any future valve problems will require repeated removal of the cylinder heads.
Timing Cover/Water Pump Gaskets
Most Land Rover V8's which have leaking head gaskets also have coolant and oil leaks from the timing cover and water pump gaskets on the front of the engine. It is critical to repair all coolant leaks and to replace aging water pumps when performing head gasket replacement, since coolant leaks can lead to engine overheating, such as the localized overheating described above, and possible repeat cylinder head gasket failure. It is also very desireable to correct the oil leaks which are likely present, and adding timing cover gasket replacement means there will be very few engine oil and coolant gaskets and seals left unreplaced after head gasket replacement. This will also allow for inspection of the timing chain and gears, which may develop excessive slack on higher mileage engines.
The Decision
When considering major repairs for your Land Rover or any other vehicle, one question is always raised: "Is it worth it?" Finding an answer to this question requires considering multiple factors, including repair costs, resale value of the vehicle while it is in need of repair, and the value that the vehicle holds with you, the owner. While major repairs can often seem to approach the value of a vehicle, and may seem to be not worth the expense, it is important to look carefully at these other factors. The resale value of a Land Rover Discovery is relatively very low compared with the purchase price of a comparable new vehicle, and this value can be reduced further if needed repairs are known to the potential buyer.
While the low resale is often looked at as equivalent with "vehicle worth," it is important to distinguish the two. A vehicle's worth, to the owner, should take into account the cost of replacing it with a vehicle which is unlikely to need repairs which may approach the current vehicle's repair costs. This typically rules out replacing your Land Rover with another Discovery, since any Discovery which has not had ARP Head Studs installed is likely to soon need such a repair. The best way to determine if a vehicle is worth repairing is to add a realistic resale value, in its current condition, to the estimated repair cost. Vehicles in this price range should then be considered as potentially economical replacements, if they satisfy your needs and are unlikely to need repairs in the near future. Unfortunately, many used vehicles are sold for the same reason you might be considering selling your Land Rover: repairs are needed and the owner had decided they would rather sell the vehicle. For this reason, it is rare that a Discovery with leaking head gaskets would not e worth repairing.