Tips to Maintaining Your BMW’s Coolant System


e36 coolant flush

One of the most important aspects of maintaining a BMW is the coolant system. BMW owners should be particularly attentive to their coolant system. BMWs typically have reduced heat dispersion due to the high amount of heat that is produced by performance engines and the fact that BMW really likes to stuff their motors into the engine bay. This leads to a heightened risk of major engine damage if the coolant system fails.

There are a variety of different things that a BMW owner can do to maintain their vehicle’s coolant system in tip-top shape. Flushing the coolant system, replacing coolant components preemptively, and monitoring your vehicle for coolant leaks from time to time are all important factors in the maintenance of your BMW’s coolant system.

In this article, I will discuss why it is important that BMW coolant systems receive proper maintenance, symptoms of future problems, and what kind of maintenance you should be doing on your BMW coolant system.

I have also made two videos on bleeding a BMW coolant system and diagnosing BMW coolant system problems you can check out.

Why do BMW coolant systems need more maintenance?

There are several reasons why BMW owners must pay more attention to their coolant system than other vehicle owners.

If you own an inline-six BMW you have probably taken a look at the engine bay and been amazed at the number of engine components that BMW stuffs under the hood. BMW tends to over-stuff their engine bays which results in an inability to dissipate heat quickly.

I have seen the temperature gauge on my 328i still above the cold line, even several hours after I last drove it. My Ford Ranger, on the other hand, dissipates heat extremely quickly and will need to warm up again within thirty minutes of driving it. more than five hours after I last drove it. The poor heat dissipation tends to put added stress on BMW’s coolant systems.

Another strain on BMW coolant systems is the fact that BMW uses so many plastic parts. Coolant hoses with metal clamps have been replaced with quick-connect plastic retainers. Sure, they are easier to install and remove, but they also fail prematurely due to the high heat.

The radiators and reservoirs are largely made of plastic. I have had to prematurely replace several reservoirs because the plastic shell cracked. The radiators have plastic necks where the hoses connect and the neck often cracks.

Finally, BMW tends to install coolant hoses in weird places, like underneath the intake manifold. This isn’t a problem until it is one, and then a mechanic is charging you a thousand bucks to replace a coolant hose. 

For all of these reasons I strongly advise that you give your BMW’s coolant system the maintenance it needs. Otherwise, it will end up failing prematurely. You will end up paying for the maintenance on the back end, and it will be significantly more expensive.

Important maintenance on a BMW coolant system

Flush your coolant

Coolant system flushes are one of the most neglected parts of a car. Most owners only have the coolant flushed when it coincides with a major repair such as a water pump or radiator replacement. The problem with this is that the coolant system typically only needs to be rebuilt every 80,000 miles. Luckily BMW coolant flushes are relatively simple to do, and well within the reach of most DIYers. Coolant needs to be flushed, because over time it loses its rust-inhibiting abilities, becomes more acidic, and builds up contaminants such as dirt.

tips to bmw coolant system maintenance

See the white residue around the connector above the radiator hose? That white residue appears as a result of coolant system neglect. This picture was taken during a coolant system rebuild, and the system had this white residue throughout the hoses and mechanical components. Basically old coolant starts breaking down and leaving a white pasty substance, especially if the car sits at idle. This substance can gum up a heater core, and decrease the coolants ability to travel throughout the system. This is just another reason why it’s so important to keep up the maintenance from time to time with these vehicles.

For most model BMWs it’s important to bleed the coolant system of air after flushing the coolant. Air pockets in the system cause the coolant to flow improperly. Depending on the model BMW you have, you may have a self- bleeding system or a manually bled system. The self bleeding systems will usually have a separated reservoir tank and will sit above the radiator. The electronic water pump systems are different from other BMW’s, as you can bleed these without even having to start the car!

I flush my vehicle’s coolant every two years or so, check your service manual to confirm what interval is recommended for your vehicle.

Check out this video for Bleeding a BMW Coolant System

Use Distilled Water for Flushes Not Tap Water

e36 coolant flush

Tap water quality in the U.S varies between municipalities and states, however, it all has minerals in it. This is a problem because tap water with high mineral content can cause sediment buildup in our coolant systems. Our cars, unfortunately, don’t have a way of filtering out the sediment in a coolant system so it simply gets stuck in there. The best way to avoid introducing minerals into your coolant system is to resist the urge to fill it up from the hose and go buy a couple gallons of distilled water.

Although plenty of BMW owners, I dare say most simply top off the coolant system with water straight from the tap, distilled water is such a cheap way to ensure that you get the full life out of your coolant system. Additionally, I have seen in older models where coolant passageways have actually become partially blocked due to sediment buildup overtime.

Check and test your coolant periodically 

coolant tester

Every so often you should be checking your coolant level and testing the quality of the coolant. I personally would suggest if you live in an area like North Carolina that has wild variations in temperature, perform a check & test once before the high heat of the summer and once before the cold winter rolls in. Checking the coolant is as simple as opening the coolant reservoir cap (when cold) and checking if it’s at the proper level. Testers can be bought at a local autoparts store for a few bucks. If the coolant tests poorly, it would be best practice to perform an e36 coolant flush.

Check the serpentine belt periodically

Most BMW water pumps are driven by a serpentine belt system. The belt should be inspected every time you or the mechanic is performing an oil change and/or other regular maintenance. When a belt develops hairline cracks it is time to replace it. Why is belt maintenance important to your coolant system? If the belt snaps while your on the highway your water pump will immediately stop pumping water through the system and the engine will promptly overheat. This also emphasizes the importance of maintaining the integrity of your car’s pulley system. A frozen idler/tensioner pulley or a failed tensioner can cause the belt to snap as well.

Checking the belt is not as important to maintaining newer model BMW coolant systems, as most feature an electric water pump that runs regardless of the belt. Be wary about electric water pumps however, typically they will throw a code when they begin to fail and it’s best to replace them ASAP when they do.

Replace coolant components preemptively

failed bmw radiator
cracked radiator neck

This picture is of my brother’s cracked radiator. Automotive radiators in general have an aluminum core, and plastic housing on the sides. The plastic cracks overtime along the filler neck and a leak ensues. Even the aluminum thermostat housing unit was cracked on this vehicle and required replacement!

BMW coolant system components are something that you want to replace preventively, instead of simply waiting until your car overheats. The reason for this is simple, a lot of heat gets trapped in these vehicles because of how tightly BMW packs their motors, and the engine heads are made out of aluminum. When these cars overheat (especially inline 6s), they overheat very quickly and you may not notice in time. If you don’t notice in time you will have a blown head gasket and/or a cracked head (usually both). This is why I rebuild the entire coolant system on my E36’s every 80,000 miles.

Before I moved to Washington D.C I ignorantly ignored my own advice. I replaced the water pump, thermostat and radiator on my E36 but I did not replace the coolant overflow tank. Why not? The stunod parts store sold me the wrong reservoir and I did not have the time to wait a few days for another one. Well lucky me, I get a call a week later from my brother informing me that the reservoir blew on the highway and the head gasket was toast.

Symptoms of Failing Coolant Components

If you have a coolant system issue there are several things that can help narrow down the problem, and get your car back on the road faster!

  1. There are quite a few things that can go wrong with the coolant system and cause overheating. If your car is overheating the first thing to do is wait for the vehicle to cool down and identify if you have a coolant leak. There is a Kalt (Cold) line on the side of the expansion tank and the coolant level should be at or close to the line. If it is well below, you likely have a coolant leak. 
  2. If you have a coolant leak your next mission will be to find the source. Most frequently I find the radiator hoses, expansion tank, or the coolant hose underneath the manifold to be the sources of leaks. A mechanic will have a machine that pressurizes the coolant system and finding a coolant leak is a piece of cake with that. Unfortunately, you more than likely do not have that expensive piece of equipment so you will need to do a combination of good ol’ eyeballing, placing paper towels beneath the area in question, and potentially using coolant dye. 

Fan Clutch Failure

You will not notice a failed fan clutch driving on the highway or even going 35-40 mph. You will notice a failed fan clutch in inner-city stop and go traffic. The fan clutch engages when the engine warms up and helps cool the motor down during slow speeds and at idle. One of the most common fan clutch failures is that the internal clutch fails and the fan will fail to engage. At that point the fan essentially freewheels, causing the engine to overheat at slow speeds. 

The easiest way to test the fan clutch is with the “rolled newspaper test”. Wait for the car to heat up to operating temperature, and roll up the newspaper. Don’t jam it in the fan, but brush the fan with it, if it stops and even starts turning the other way you have a bad fan clutch! Don’t worry, replacement is fairly easy but you will need a fan clutch removal tool.

Thermostat Failure

Is your car heating up slower than usual? Does it rapidly overheat after reaching optimal temperature? Thermostats can fail in either the closed or open position. You are more likely to notice failure in the open position during the winter because you will not have any heat. Failure in the closed position is deadly and the vehicle can not be driven until the thermostat is changed.

If you replace the thermostat I strongly suggest swapping the plastic housing unit with the more durable aluminum unit.

Coolant Reservoir

If I had to put a finger to my least favorite part on BMW E36s and E46s it would probably be the coolant reservoir. Why? They suck plain and simple. They are made out of hard plastic which will develop a crack over time if not replaced every 50-80k miles. Even worse is that I have seen them develop a crack far earlier than their scheduled replacement time. They frequently develop hairline cracks in the area right around where the overflow hose connects to the top of the reservoir.

If you are replacing the coolant reservoir I would strongly suggest replacing it with an aluminum reservoir from Mishimoto. Yes I know it’s more expensive but trust me it’s worth every penny!

Water Pump Tip: If your planning on changing the water pump check the propeller! In all of BMW’s wisdom, they put a plastic propeller on the water pumps on E36s! The metal propeller units can be found at the dealership or one of my favorite brands is Beck Arnley. If you’re going to spend the money on a new unit do it right and get the metal propeller pump!

Conclusion

That’s it for these tips on helping you maintain your BMW coolant system, remember to check your service manual if uncertain what your vehicle’s recommended service interval is. 

Maintaining your BMW’s coolant system not only helps you enjoy a better ride, but it also will help ensure the longevity of the car as a whole!

Stephen Metellus

I am a BMW enthusiast and owner of abetterbmw.com! I have been repairing, flipping, and parting out BMWs for nearly ten years. I love these vehicles and I hope you will find my articles and YouTube channel helpful for whatever BMW project you have in store!

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