BMW E34 head gasket failure and cylinder head removal
How do I recognise an E34 head gasket failure?
Signs are pressure and coolant loss in the cooling system, a heater that blows cold and bubbling in the expansion tank immediately on a cold start. Hard hoses when hot are normal. Confirm the fault with a compression test or by measuring exhaust gas in the coolant, preferably with a cold engine.
What are the E34's compression reference values and what does a leak-down test show?
In a healthy engine the pressure is 10 bar or more; for the M30 the reference value is 10-11 bar. Under 10 bar is too little, and the cylinders should be within about 2 bar of each other. In a leak-down test, a leakage of over 15-20 % is a pressure leak. A head gasket leak bubbles in the radiator, an exhaust valve hisses in the exhaust pipe and piston rings blow out of the oil filler.
What is replaced when changing the E34's head gasket?
The head is surface-ground at a machine shop, and the head bolts are stretch bolts that are always replaced. Tightening is done according to the gasket set instructions, and no thread locker is used. It is worth replacing the water pump, thermostat and timing belt at the same time if it has been a while since the last replacement.
What is the tightening torque of the aluminium head bolts?
The bolts are tightened from the centre outwards to 30 Nm (40 Nm on some), and then a further 2 x 90 degrees is added to each. The bolts are single-use. Always check your own engine's values from the gasket set instructions.