Why does the cooling system need to be replaced every 75-100k miles?
The E36 BMW is by many accounts a very reliable car. They can last for hundreds of thousands of miles with little more than routine maintenance if a few key steps are followed. By a wide margin, the biggest weakness of the E36 BMW in terms of reliability is the cooling system. As these cars age, the plastic used in the cooling system components becomes very brittle, and by around 75-100k miles, they are at high risk of a catastrophic failure, which can produce the following outcomes:
- Cracked/sheered upper or lower radiator hose flange
- Cracked radiator tank(s)
- Radiator leaks at tank/core seams
- Leaking radiator cap
- Cracked/exploding expansion tank
- Cracked bleeder screw
- Cracked/sheered thermostat housing
- Ruptured upper/lower radiator hoses
- Water pump impeller failure
- Water pump bearing failure
- Mechanical/clutch fan failure/explosion
Failure of any of the above items can and will cause the engine to overheat. When most people acquire a new-to-them E36, they will inspect the cooling system visually, and if there are no leaks and the engine is not overheating, they assume that everything is fine and will continue driving the car as-is. The reason why this is a risky scenario is because the failure-prone cooling system components will look and function perfectly until they fail suddenly and without warning, thus leading to an overheating incident. In other words, visual inspection is not a sufficient means of determining the lifespan of cooling system components. Something can look brand new today, and then break tomorrow.
What are the consequences of a cooling system failure?
As mentioned earlier, the primary motivation for replacing all cooling system components every 75-100k miles is to avoid overheating the engine in the first place - this is preventive maintenance. The reason why avoiding an overheating scenario is so important with these cars is due to the fact that they utilize aluminum cylinder heads, which are extremely susceptible to warping and cracking from overheating. Warped heads and blown head gaskets are very common occurrences with the E36, but fortunately, this problem can largely be avoided if you overhaul the cooling system before something breaks. Generally speaking, the cost to repair a blown head gasket and/or warped (or cracked) cylinder head is in the ballpark of $1200 (sometimes more, sometimes less), and is not a job that is within the skill set of most of DIY'ers. On the other hand, the cost of a comprehensive cooling system overhaul is in the ballpark of $350-450 (parts only), and is within the skill set of most average DIY'ers. So, it's easier and less expensive to replace the cooling system before you have a problem on your hands.
Many people take the risk and figure that they will simply drive the car until something breaks, and when it does break, they assume they will notice that the temperature gauge has started to rise and will be able to shut the engine off before it has truly overheated. There are a few reasons why this is not a wise strategy:
- When driving, one spends most of the time looking through the windshield at the road ahead (thankfully), rather than constantly looking at the instrument cluster. Because of this, you may very likely not notice that the temp gauge has risen until it is too late and damage has occurred. In most cases, people won't notice until the temp gauge is buried deep in the red and the warning light illuminates, at which point your engine has been thoroughly cooked.
- The temp gauge is not accurate, or in more specific terms, the temp gauge has a built-in buffer/delay. This means that in some cases (such as in the event of an overheating), the actual coolant temperature is considerably higher than what the temp gauge is indicating. Many people figure that as long as the temp needle does not reach the red zone, they are safe. This is not true, because by the time the temp gauge has risen beyond the normal 12 o'clock position, the actual coolant temp can already be dangerously high, despite the fact that the needle hasn't yet reached the red zone. This is due to the buffer/delay.
So, now that we have established how the cooling system fails (brittle plastic), and why it is so important to avoid overheating the engine (high risk of blown head gasket and/or warped head), we are brought to our next topic of discussion: