The BMW M50 engine is the successor to the M20, and predecessor to the M52 engine. The M50 was used solely in the E34 520i and 525i, and the E36 320i and 325i. It is a straight six, or inline-6, coming in at either 2.0L (M50B20) or 2.5L (M50B25).
In September of 1992, the engine received a technical updated (dubbed TU on the engine code) which added a single Vanos system. The single vanos system adjusts the timing of the intake camshaft, and was the first introduction of the BMW Vanos system.
BMW M50 Reliability and Engine Life
Much like its successor, the M52, the M50 engine is considered to be bulletproof. With very strong internals, the engine blocks are able to deliver outstanding longevity with proper maintenance and care. Still to this day, the M50 is known as one of the most reliable BMW engines ever built.
However, while the engine itself might be strong, the M50 suffered common problems with various engine-support systems, such as the cooling system. The cooling system tends to be the common weakness on all M50 engines, with frequent failures across the radiator, water pump, thermostat, and coolant expansion tank.
Outside of the cooling system issues, most other common problems are relatively contained and easy/inexpensive to fix. The suspension, drivetrain, and electronics all hold-up very well and do not frequently cause issues.
Overall, these engines are very reliable, but maintenance is key. With some M50's pushing 30-years of age, you are going to run into your fair share of age-related problems. While the suspension might not be an area of concern due to its reliability, on a 30-year old car with high mileage, you are going to have wear and tear replacement needs there too.
BMW M50 Common Engine Problems
- M50 Cooling System Failure: Radiator & Thermostat
- Cracked Expansion Tank & Coolant Leaks
- M50 Water Pump Failure
- Bad Ignition Coils and Spark Plugs
- Rough Idle from Faulty Idle Control Valve or TPS
1. M50 Cooling System Failure
The engine cooling system is the crux of the M50 engine. Theoretically, there is nothing wrong with the design or make-up of the cooling system that causes it to fail, these symptoms simply fail over time due to age and normal wear and tear. The one exception to this is the water pump, which does have a common failure point.
On the M50, common failure points are the radiator itself, radiator hoses, the water pump, and the thermostat. Additionally, the coolant expansion tank cracks frequently. Due to the frequency of failed water pumps, and cracked expansion tanks, we are going to cover those issues separately.
Cooling system failures need to be repaired immediately, otherwise the excess engine head can warp the engine head and lead to total engine failure.
Symptoms of a Failed Radiator or Thermostat
- Engine overheating at idle and during driving
- Coolant leaking from the radiator or thermostat housing
- Way low temperature reading after driving for long periods (caused by thermostat being stuck open)
2. M50 Cracked Expansion Tank
The radiator expansion tank is the tank where you pour engine coolant used in your radiator. The tank is made of plastic and it is pressurized. The pressure, combined with the normal wear and tear on the plastic from prolonged heat exposure, can result in cracks in the tank.
Cracks are usually small, hairline cracks. But the problems it can cause are usually far from small. A leaking tank will drain engine coolant quickly and result in engine overheating, which is bad for all internal and external parts of an engine. Additionally, a tank with a small crack could also pop, or explode while driving, resulting in the need for a tow.
M50 expansion tanks tend to cause problems around the 100k mile mark, and should be replaced as a form of preventative maintenance if there appears to be any wear and tear, broken nipples, mounting damage, etc.
Symptoms of a Cracked M50 Expansion Tank
- Coolant leaking around the expansion tank
- Low coolant levels, or having to replace coolant frequently
- Engine overheating due to lack of coolant
Replacement options: the OEM tank is made of plastic, which is one of the biggest faults and reasons for cracking. There are some aftermarket expansion tanks out on the market which are made out of aluminum. We recommend replacing your tank with an aluminum expansion tank to prevent any future issues from arising. However, the cost of an aluminum tank is about 10x that of an OEM-spec M50 replacement expasion tank - so it depends whether you are okay with replacing the tank again in the future or not.
3. M50 Water Pump Failure
Unfortunately, we haven't gotten past the cooling system problems of the M50 yet! Fortunately, this is the last one.
Most early model BMW's came with water pumps with a metal impeller on the inside. In more recent years, BMW finally transitioned to a metal impeller. But unfortunately, the M50 along with its successor M52 still had plastic impellers.
Over time through natural wear and tear, these impellers go bad which prevents the water pump from being able to pump coolant through the system. Therefore, the engine is unable to effectively be cooled down which results in overheating and you being stuck on the highway. Water pumps on the M50 tend to go bad in the 60k-80k range. If you are around this mileage on your current water pump, we highly recommend replacing it as preventative maintenance. A bad water pump will leave you stranded on the side of the road!
Symptoms of a Failed M50 Water Pump
- Overheating after driving for awhile, or during high-speed highway driving
- Whining noise coming from the engine bay
- Check engine light and car entering limp mode
Replacement options: aftermarket manufacturers of M50 water pumps have picked up on the plastic impeller issues and created water pumps with metal impellers. These pumps are cheap, ~$50, so get yourself a M50 water pump with a metal impeller and change it out before it becomes a problem.
4. Bad Ignition Coils and Spark Plugs
Ignition coils have advanced significantly over the past century. Up until right around 1990, BMW was using "canister-style" ignition coils which used wires to connect the canister to each spark plug. The M50 engine was one of the first BMW engines to use "coil pack" ignition coils, which uses an ignition coil for each spark plug.
The benefit of coil packs over canister coils is more ignition control, improved gas mileage, and overall better performance. Despite the new style coil packs offering numerous advantages, they are common failure points on the M50 engine.
The coil packs, along with the spark plugs tend to go bad on the M50 every 30,000-50,000 miles. We recommend replacing them as preventative maintenance somewhere in the middle of that interval range. You can determine when these parts are going bad by these common symptoms below. The coils are reasonably expensive, so feel free to try replacing the spark plugs only before buying the coils as well if you are looking to save money. But note, if the plugs are bad, the coils are likely soon to follow.
M50 Bad Ignition Coils and Spark Plugs Symptoms
- Rough or poor idling
- Jerky or sluggish acceleration
- Cylinder misfires
- Overall poor performance
- Slow starts, especially in cold weather
- Engine cranking but not turning over
5. Rough Idle from Idle Control Valve or Throttle Position Sensor
While rough idling can also be caused by bad ignition coils or spark plugs, if rough idling is the only symptom you are experiencing, you are probably facing a bad idle control valve or throttle position sensor.
Idle Control Valve
The idle control valve is located near the throttle body and intake manifold. Over time the valve collects dust, dirt, grime, etc. and can become faulty. A faulty valve will send incorrect readings back to the car's engine control module, resulting in variable and sporadic idle speeds. Fortunately enough, the idle control valve can be removed and cleaned which commonly fixes this issues.
You can read an ICV cleaning guide here: https://www.bimmerfest.com/forums/showthread.php?t=759718
Throttle Position Sensor
The throttle position sensor sends readings of the throttle positioning back to the car's ECM. With a faulty sensor, bad data is sent to the ECM which then incorrectly tells the engine how quickly it should be revolving. This can be very dangerous if you are braking or in heavy traffic and the sensor tells the engine's computer that the throttle is wide open.
Common symptoms of a bad TPS are poor idling, but also surging, jerking, hesitation, etc. from the engine while driving or at idle. Additionally, you will receive an engine malfunction light on the dash, and will notice your RPM's acting sporadically.