Though six-cylinder, inline engines are not as popular today as they were just 25 years ago, their sound and layout still translates well into high performance, durability and smoothness. The six-cylinder M50 series was originally used in 5 series cars in the early 1990s, and is now the basis for the 3 and 5 series cars in displacements from 2.5L to 3.2L. All of these engines share basic design characteristics, with different forms of intake, exhaust and valve control determining specific power and use parameters.
This series of engines is smooth running, powerful and still very frugal with fuel. Originally designed with dual, chain-driven, overhead camshafts, this design has evolved into a world-class expression of technological progress. The engine now has variable camshaft timing on both camshafts and can easily approach 2 hp per cubic inch.
Unfortunately, BMW has found it necessary to move to extended service intervals, involving condition-based oil changes and excessively separated maintenance inspections. Oil sludging, overheating problems and emissions faults are the direct result of the lack of reasonable maintenance. We’ve found an unusually large amount of cooling system failures on these cars just after the initial no-service, free-service period ends.
The BMW M50 engine series is a very durable powerplant. The frequency of actual failure is very low considering the number of these cars that we service. Even with repeated bouts of overheating, they seem to survive well. The least common cooling system failure is the head gasket, or at least leakage in the head-to-block interface. In this article, I’ll cover the diagnosis, repair and prevention of these failures, but it will be up to you to convince your customers that regular maintenance and inspection is the only way to prevent expensive failures.
The events leading up to a head gasket failure can almost be plotted on a graph of typical events. The customer complains of overheating, but the coolant level doesn’t drop. The fan, belt and hoses all appear OK, and thermostat function tests fine with a non-contact temp probe. The engine temperature spikes at times, but then is normal. The engine does or doesn’t overheat at idle, but the temperature climbs quickly at higher engine speeds.
On most M50 engines, the water pump impeller installed at the factory is plastic. For whatever reason, the impeller separates from the shaft or disintegrates and the water pump no longer pumps. In the first case, with the impeller loose, especially with the engine at operating temperature, diagnosis can be especially difficult. Just when you think there is no flow, some coolant starts to move and everything tests OK, temporarily.
Another problem along this same line is that the auxiliary coolant pump for the heater system (See Photo 1) can circulate enough coolant to provide cooling at idle, but not at higher engine speeds.
Then the problem increases in frequency and intensity, coolant is lost, a leak starts at the plastic thermostat housing or, in a worst case scenario, the upper radiator hose blows off, due to failure of the plastic outlet neck (sometimes caused by over-tightening). Then, a series of overheating events culminate with a severe loss of coolant, a misfire and a customer with a very red face.
At this point you can’t pressure-test the system, a block test may or may not have been conclusive and there are still the questions about the water pump, fan clutch, auxiliary fan and switch and a stuck thermostat. It’s usually only after all of these other possible overheat sources are eliminated that a head gasket failure is suspected or indicated.
Trying to convince a customer that all of these things can cause an overheat situation, individually or in tandem, is one of the service manager’s most difficult tasks. Having a complete service history on a car is a good starting point for any diagnosis, but, of course, when a car changes hands, that service history is sometimes lost or incomplete.
At the first sign of overheating on our regular customers’ vehicles, we try to sell a water pump removal, inspection and, if needed, replacement with a brand that has a metal impeller (See Photo 2). That eliminates the most common cause of cooling system failure. If the pump has already been replaced, then a little additional investigation is necessary.
One of the most common leak points on the M50 engines is the factory-installed plastic thermostat housing. Cylinder head temperature or overheating warps the housing and causes a crack and the leak. We strongly recommend a metal replacement part when there is one available, along with a new thermostat. One thing to watch on thermostat replacement, on models where the stat is separate from the housing, is that the arrow on the outside edge should point up when installed.
Other less common but possible sources of overheating are the fan clutch and operation of the auxiliary cooling fan. Depending on the model year, the auxiliary cooling fan will be controlled by the ECM or a temperature switch in the radiator (See Photo 3) and will operate in slow or fast modes depending on A/C operation and coolant temperature.
On models where the fan is controlled by the switch on the radiator, a simple test of operation is possible with a test light connected to ground and touched to the harness side of the switch connector. Both fan speeds can be tested in this way. Fan clutch testing is like any other.
With the engine off at operating temperature, there should be some tension in the fan clutch and, with the engine running, the fan should obviously change speed with engine speed. A plugged radiator can also cause these symptoms.
If you’ve been able to eliminate other possible causes for overheating, the testing for a head gasket failure should come next. If there are hoses blown off, or other leaks, a pressure test is out of the question. A block test (See Photo 4) for exhaust gases in the cooling system will only work if the cooling system is intact. Do not use your exhaust gas analyzer to test for exhaust gases in the cooling system. A visual inspection (pulling the spark plugs) may be the only way to verify a gasket failure, but that only works if there is actually a leak into a cylinder (a crack into an exhaust port may not leave any evidence of coolant in a cylinder).
An obvious indicator of a head gasket failure on this engine is a misfire code that cannot be traced to a fuel injection or ignition cause. A slow loss of coolant, with no obvious leaks, is a fairly common symptom. A failed oxygen sensor can also be an indicator, especially if it has failed prematurely, or repeatedly. Coolant odor from the exhaust or excessive cooling system pressure (hard to test for with the radiator blown up) would also be common symptoms.
So what will be the most definitive test for the failure of a head gasket? The presence of coolant in a cylinder, in the crankcase or in the cooling system is obvious evidence of a blown head gasket. Coolant in the exhaust, but nowhere else, is probably a cracked cylinder head.
In either case, a teardown is warranted. A caution to give to customers at this point is that head gasket failure or severe overheating can be caused by, or a cause of, the head warping. Teardown can also bring to light other problems.
Due to the design and operational differences over the lifespan of this powerplant, I recommend attempting this repair only with up-to-date and complete information for the particular model you’re working on. You will need some basic special tools (See Photo 5) to prevent damage during disassembly and reassembly, as these are interference motors and the valves can add up real quickly when the proper reassembly process is not followed.
I’m going to walk through the teardown, indicating where special attention is needed. The standard rule of removing the battery cable before starting applies.
Depending on vehicle year and model, there may be a clearance problem with the body and cylinder head removal (See Photos 6A and 6B). On all models, the rear of the cylinder head is very close to the firewall. You may need to remove part of the heater plenum, the bulkhead wiring harness and more to have enough room just to get the covers off the engine. This part of the job may not even be mentioned in the service manual. What needs to come off will be pretty obvious, but some of the fasteners may be difficult to locate and remove (See Photo 7).
Getting things out of the way should include removing the radiator, fan and belts. Draining the cooling system can be accomplished by removing the block plug at the rear of the lower or right side of the engine (See Photo 8). With some models, removing the secondary air pump and hose will provide additional working space.
Once the engine covers, ignition coils and valve cover are removed, pull the plugs and then turn the engine over by hand to install the camshaft-locking bridge and the crankshaft-locking pin. The crankshaft is locked through a hole in the block near the starter.
This is where having the complete set of disassembly instructions is important. With everything lined up, the cam drive chain and sprockets need to be marked, the tensioner loosened and locked, and then the variable valve timing mechanism can be dealt with. Everything should be marked or separated in a logical manner to make reassembly easier. Note how each of the pulleys comes apart, the differences in their attachment and the rotation necessary to remove the various pieces.
Tying the cam chain to the sprockets to maintain spacing is a good way to ensure ease of reassembly (we use cable ties to hold things together). The tensioner is spring-loaded on the main chain, so be careful not to let it slip out while you’re removing it. Of course, everything in the cam drive should be inspected for excess wear, though failure of these components is very rare.
On many of these cars, there is enough space to leave the manifolds in the car, tied back on either side (See Photo 9). This can save a lot of time during teardown and reassembly as most of the wiring and hoses can stay where they are. Since the intake manifold is plastic (See Photo 10), care should be taken to not abuse the mounting surfaces or hose connections.
With a correct thin-wall socket, the headbolts can be removed without pulling the camshafts. Removing the camshafts for any reason will require a special reinstallation procedure. On engines with the aluminum crankcase, there have been reports of the headbolts pulling the threads out of the block when attempting to remove them. There are a number of ways to address this — with an impact fitting for an air hammer or the tension-and-snap method.
My first inclination is always to slightly tighten before attempting to loosen a steel bolt screwed into aluminum. This almost always seems to work, as long as the bolt head doesn’t strip due to a worn socket. There is nothing magical about this, just patience and experience. The head bolts should always be replaced after an overheat situation; it’s just cheap insurance that a bolt won’t snap when attempting to reinstall.
Inspection of the head gasket may or may not reveal the location of the leak. If there are no obvious damaged sections of the head gasket, a crack may be the culprit. In either case, even if the head looks and checks OK with a straightedge, the head should go to a reputable machine shop for pressure testing and minimal resurfacing.
Even on heads that seem OK to us, oftentimes, due to the length and width of these heads, there will be a “twist” that was not detectable with standard checking. If the camshafts are not removed, that may be all that is holding the head straight. Removing the camshafts and lifters will require extra patience during reinstallation. Failure to follow the guidelines for reinstalling the lifters and camshafts can lead to expensive contact between pistons and valves.
Once the head is determined to be suitable for reinstallation, and everything is cleaned, take the time to clean, chase and blow any liquid out of the head bolt holes in the block.
A final, close inspection of the engine before reassembly can save a lot of heartache. Since the engine really can’t be turned over with the head removed (due to the chain being slack), the condition of the visible cylinder walls will determine expected remaining engine life. These engines routinely go 300,000-plus miles when properly maintained, but overheating can lead to a shortened lifespan.
Once the engine is back together and ready to refill with coolant, a few extra precautions are recommended. On any model with an auxiliary water pump, the key should be on and the heater control set to the HI position, to ensure that the heater core will be filled.
We use a vacuum-fill system to allow both a system integrity test and full refill (See Photo 11).
Road-test the car to bring the engine to full operating temperature, then let it cool. Recheck and top up the system to avoid any chance of an air pocket before returning the car to the customer.
I consider the six-cylinder BMW cars to be some of the best in the world. The combination of performance, durability and economy is hard to match. Every manufacturer has its standard bearer; for BMW, the M50 series engine is the holder of that title.