BMW V12 Engine Repair Manual: M70 and M73 (5.0L and 5.4L). A comprehensive technical guide to servicing and repairing the flagship BMW V12 engines found in the 7 Series (E32, E38) and 8 Series (E31) models. This is an indispensable resource for working with complex systems:
- Dual engine management (two DMEs) and system synchronization.
- Step-by-step instructions for overhauls and oil leak repairs (especially critical for the V12).
- Complex fuel injection and ignition systems.
- Complete electrical diagrams and DME pinouts for diagnosing electronic faults.
- Servicing components with limited access in the cramped engine compartment.
- Timing chain and cooling system maintenance guide.
BMW 840i (E31) M70 and M73
| Password For Document: bimmer-service.com |
| Engine M70 and M73 |
| Engine Electrical System |
| Fuel System, Fuel Supply M70/M73 |
| Radiator |
| Exhaust System |
BMW M73 Typical Problems for this Engine
The BMW M73 engine (5.4L V12, found in the E38 750i and E31 850Ci/CSi) is an evolution of the M70 and offers improved reliability, but being a complex V12, it still has a number of expensive and time-consuming problems associated with its packaging and electronic controls.
The M73 is considered more reliable than the M70, thanks to updated fuel injection and the use of the robust Alusil block coating (eliminating the Nikasil issue). However, its primary drawbacks remain the complexity of the electronics (dual DME) and the poor component accessibility, making any repair extremely expensive.
1. Electronic Control System Issues
| Problem | Cause and Symptoms |
| Dual DME System | Like the M70, the M73 uses two DME (Bosch Motronic M5.2) control units, one for each cylinder bank. Failure in one does not shut down the engine completely but causes critical running issues. |
| Electronic Throttle (E-Gas) Failure | The M73 uses electronic throttle control. Position sensors or the actuator motor itself can fail. |
| Wiring Harness Degradation (WOT) | Breakdown of wiring insulation in the engine bay (though less severe than on the M70) causes short circuits and false signals. |
2. Heat Aging and Leaks
| Problem | Cause and Symptoms |
| Oil Leaks (Gaskets & Seals) | Due to high operating temperatures and tight packaging, gaskets for the valve covers, oil pans, and critically, the Oil Filter Housing (OFH) gaskets harden and begin to leak profusely. |
| Cooling System Failures | Complex cooling system with numerous hoses, plastic connectors, and thermostats. |
| Intake Manifold Gaskets | Gaskets can develop vacuum leaks. |
3. Mechanical and Serviceability
| Problem | Cause and Symptoms |
| Service Labor Costs | Extremely high maintenance cost due to the severely restricted access to components within the V12 engine bay. |
| Hydraulic Valve Lifters (HLA) | Wear or contamination of the hydraulic valve adjusters. |
| Ignition Coils and Plugs | Individual ignition coils are prone to failure with age. |