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BMW Technical Article

Flushing The Cooling System On BMW E36
Jared Fenton
Wayne R. Dempsey

Difficulty Level: 2
Difficulty scale: Adding air to your tires is level one
Rebuilding a BMW Motor is level ten

     In this tech article I will go over the steps involved with flushing out the cooling system on the BMW E36 3 Series models from 1992-99. Keep in mind that this article is specific to the E36, however the procedures and principles here apply to nearly every water-cooled car on the road. (So if you own an air-cooled car, you are in the wrong place.)

     So why should you flush the cooling system? Basically, to ensure that all the small passages and ports inside your vehicleís engine are clear of things like rust or corrosion. In some case, I have seen dirt and tar in there as well. When you flush out the system, it removes all this corrosion, and keeps coolant flowing freely through the engine. If the corrosion gets too bad, it can cause all sorts of problems from overheating, to blown head gaskets and even cracked engine blocks.

     Iíve torn down a lot of engines before and seen some really interesting things. In most cases, the corrosion was minimal and would easily be removed during a good flush. On the most extreme was my friendís Triumph TR-7 (quite possibly the worst car ever made) The corrosion had grown so bad, that nearly all the cooling passages to the head were completely clogged, causing the head to overheat and warp. (As to why the engineers put an aluminum head on a cast iron block is still beyond meÖ)  While you most likely wonít have a failure as extreme as my friendís TR-7, this is a good example of a worst-case scenario.

     So what causes the corrosion in there? The most common is failure to put any coolant/anti-freeze in there. You probably know this story, driving to Vegas or some other ridiculously hot area. The car starts to overheat, so you pull into a gas station and add water without adding the correct amount of coolant too. And what happens to metal when you leave it immersed in water for too long? The nemesis of all Gearheads worldwide, RUST! When you add coolant to water, it acts as a protectant for the metal. And this rust doesnít just stay still in the engine, it circulates all over, getting caught in cooling passages clogging, up the radiator, until eventually you have a nice iron oxide fortified smoothie running through your engine. Other culprits include leaks or rips in cooling hoses, faulty water pumps or even leaking head gaskets. One other thing to keep in mind is that whenever possible, use distilled water to fill up your radiator. Distilled water is water that has been boiled and re-condensed to remove any minerals or calcitants. (BAD STUFF Ė CAUSES RUST)

     So with these words out of the way, letís begin.  With the engine cool, begin by crawling under your car and remove the plastic panel that covers the radiator. There are two 10mm screws on the side that hold the panel on and 2 8mm screws at the front. Once you remove the 8mm screws, you will be able to pull the brake ducts off. Unhook the electrical connection on the driverís side brake duct and remove the ducts.

     Get a hold of some empty coffee cans, buckets or any other large container. We will need these to catch the overflow from the radiator. Now locate the drain plug on the bottom of the radiator. You should be able to remove this with a flathead screwdriver. Keep in mind to wear goggles when you are under there. You donít want this stuff spilling into your eyes while you are under the car.

     Now let the radiator drain completely. It may help to remove the coolant overflow cap. This will help release the vacuum inside the system and allow more coolant to flow out. Once the radiator is completely drained, we can move on the draining the engine block. Look under the exhaust manifold on the passenger side, you will see a 19mm screw. Move the drain tray under the screw and remove it. Now the block will drain out completely.  Keep a hand on the drain tray to move it as the coolant drains out.

     For the next step, itís a good idea to have a particularly large drain tray. Something along the lines of a childrenís wading pool might suffice. (I highly recommend that everyone out there try to be at least somewhat ecologically conscious)  Stick a garden hose in the coolant overflow tank and turn it on. Now watch the water draining out the bottom of the engine. You will want to let the hose run until the water running out is clear. Basically, when the water is clear, you have flushed out the system. Dispose all the old coolant in a safe way. Obviously, donít let your kids swim in it or let animals drink the water.

     Climb back under the car and re-install the 19mm plug on the engine block and close the radiator drain plug. Itís a good idea to use a new rubber sealing ring every time you do this. It will help prevent against any further leaking.

     Now we need to re-fill the system. The first step is to locate the coolant bleeder screw on top of the thermostat housing. This small screw is used to let any air in the cooling system bleed out.  Use a 8mm open-end wrench to get it open. 

     Next, you will want to determine the correct water to coolant mixture for your car. This varies from climate to climate, so be sure to check your ownerís manual. I usually pour a gallon of coolant in the coolant reservoir then pour in a gallon of water. Once you have this small amount of coolant in the motor, start the car; turn the heater vents to full on, and let it warm up to operating temperature. This will let the thermostat open and allow the coolant to circulate throughout the engine. Once warmed up, top up the coolant reservoir tank as needed.

     Now look at the bleeder screw.  Because of physics, air will naturally go to the highest point possible. In this case, the bleeder screw. With the screw open, you will see bubbles pouring out the side. Keep watching the screw until you see a steady stream of coolant coming out. Once you see this, all the air has been bled out and you can close the bleeder screw. Lastly, re-install the brake ducts and lower plastic cover, and thatís it, youíre done!

     Well, there you have it - it's really not too difficult at all.  If you would like to see more technical articles like this one, please continue to support Pelican Parts with all your parts needs.  If you like what you see here, then please visit our online BMW catalog and help support the collection and creating of new and informative technical articles like this one.  Your continued support directly affects the expansion and existence of this site and technical articles like this one.  As always, if you have any questions or comments about this helpful article, please drop us a line.

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