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DIY: Do It Yourself
Post here to share or improve your wrench turning skills! All BMW E46 DIY tips, tales, and projects discussed inside. Learn to work on your car and know the right BMW parts you will need!

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Old 10-09-2011, 04:17 PM   #1
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Cool DIY Guide: Coolant System! Bleeding, Leaks, Hoses & Expansion Tank Explained!



I just completed my coolant overhaul successfully following the infamous 330ci.net DIY.


His DIY was on point and should be followed by those attempting to do this easy project.

There was however a few exceptions I came across that should be addressed for sake of clarity.

Also everyone that attempts this project comes across the same post-DIY issues regarding bleeding and leakage. I will clarify this below.

PART ONE: BASIC E46 M54 COOLANT OVERHAUL - Replacing Expansion Tank, Upper and Lower Hoses, Sensors, And what you need to know before attempting to do so.

The word BLEED is used so many times it gets confusing. When you begin the DIY you will DRAIN OUT coolant from one or two drain sites. These are the blue Radiator/Expansion Tank drain screw(s) or the Engine Block drain plug bolt. When you drain coolant from one site or both air WILL enter the system.

When you refill coolant you DO NOT want air to still be in the system so you will have to BLEED OUT Air (force it out by flooding the cooling system with coolant and letting gravity do the work as air is lighter and rise up will bubble out) as you REFILL the coolant. This technique will be discussed later on. But it is important to understand that whenever you drain out coolant (even if it is a little spoonful) from one or both of the drain sites you MUST BLEED OUT air from the system as you refill the coolant.

2. You do not have to drain the entire coolant system to change the Upper and Lower (U/L) hoses, Expansion Tank (ET), Thermostat, Water Pump (WP). For the general Coolant Overhaul DIY you can only drain the radiator/ET blue drain screw(s). This will evacuate coolant out of most areas that you are overhauling.

That said, when you are done with the overhaul and are refilling coolant into the system you STILL NEED to properly bleed the air out of system the same way as if you had drained the entire system.

3. There is a huge structural difference between the Coolant System of the E46 M54 Automatic/Steptronic and Manual/Standard cars.

-Manual cars have two blue color drain screws below the radiator/ET area whereas automatic cars have only one which is attached to the Radiator.

-There is the obvious distinction between the fans but I will reiterate.

Manual cars have one electric fan. For the Coolant system overhaul this fan needs to be removed. It sits inside the engine bay but is not attached to the engine block. It plugs into a wire harness connector and attaches to the inside brace of the radiator support.

Automatic cars have two fans: one small auxiliary-electric fan that is outside the engine bay behind the bumper-grill area and one large mechanical fan that sits on the front of the engine block inside the engine bay. This mechanical fan is the one that needs to be removed via the special fan tools in order to access the belts, water pump, pulleys, thermostat, and to create space that will allow you to remove the hoses and expansion tank.

4. The removal of the Upper hose is easy and should be done before attempting to remove the Expansion Tank (ET). With the Radiator drained and ET drained on a cold engine the U/L hoses are empty. The only hose that will have a good amount of still warm/hot coolant is the smaller heater hose that runs into the lower half of the ET. Even with the ET/Radiator drained this heater hose will spill out some coolant.

5. Removal of the Expansion Tank is not hard at all. It is only tricky. The 330ci.net write up says to hammer it out from underneath, someone else said to pull up from above. Either way it takes some force to remove it but the thing is the ET is attached to the radiator via a relatively frail plastic bracket that most likely did not come with your overhaul kit and would need to be ordered seperately if you break it.

What worked for me: I took a simple portable Sears Hydraulic Jack (even the OEM jack will work) and placed a socket extension (or a sturdy socket screw driver will work) between the jack's pad and the bottom of the ET and slowly jacked it up. This took a few seconds and worked because the force from the jack is only directed and centered on the coolant tank.

Surprisingly there was still a lot of coolant in the ET when it popped out of it's radiator support socket so I was glad to not be under there hammering the thing up from below.

6. ALWAYS REPLACE YOUR EXPANSION TANK'S CAP! The old Expansion Tank cap may still look new but it is not! On the neck of cap are two hard to see rubber seals that seal in and regulate pressure inside the tank. I believe the seals get imprinted onto the neck of the old tank overtime and deform to work only with that tank. When the tank is replaced and the old cap is used on a new tank it will not work properly since the seals on the old cap are, for lack of a better word, used up already.

When I applied the old cap onto the new tank it felt "loose" and "gritty" on the tank as it screwed down.

7. Removal of the Lower Hose at the Radiator junction was indeed the toughest part. I found that with the sensor removed and using a long narrow sturdy steel pipe I was able to fit the tip of the pipe in thru the sensor's hole and leverage the hose off the radiator connection using the top metal edge of the radiator support frame to pry it lose. This helped a lot. When you are replacing the lower hose make sure you replace the sensor. It doesn't come with a lot of the general overhaul kits so be sure to buy one.

The green seal ring wears out even if it doesn't look like it.

In the picture below you can see how the old sensor's green seal ring was worn flat and wasn't going to be working anymore!

8. When replacing the coolant it is important to BLEED air out of the system. I'm not an expert but I will write here what worked for me.


a. Our driveway, like most American driveways, at my parent's house is on a slight incline. I just parked the car with the front on the upper most part of the incline so it was elevated. This allowed me to not fuss with a jack to jack up the front of the car. You can choose to jack up your car but 5 inches max for the front tires to be off the ground is all you need.

b. I turned on the car (not the engine just the car) and set the TEMP to 91 with fan speed low.

c. I returned to the ET, removed the cap and screw open the upper hose's black colored bleeder screw midway(it may also be brass or metal).

d. I proceeded to pour a 50/50 mixture of BMW Coolant/Distilled water into the ET until it filled up to the brim and kept pouring until it was overflowing and stopped. When the coolant level went down inside the ET I kept pouring some more coolant mixture in pausing each time it began overflowing.

At some point as the ET repeatedly gets filled to the brim a trickle coolant will begin to exit from the bleeder screw.

e. When that happens stop pouring coolant and close the bleeder screw. Depending on how much air was in the system before you began bleeding it out and the current amount of coolant in the system the coolant level in the ET may or may not subside down to with in the factory's specification level range. Every case will be different here and this is where you have to stop reading what I am telling you determine what your situation is and use your common sense.

- If the level indicator is high and the coolant level in the ET hasn't subsided then you can use a turkey baster and suck some coolant out until it is in with in the normal level. Drive the car around until engine is warm and check the level again

- If the coolant level in the ET subsides below the spec level then add some more coolant in to get it level. Drive the car around and check the level again.

d. If you had drained the entire coolant system at both the radiator and engine block drain sites then after you bleed the system during the refill procedure it may become obvious that you did not pour back in as much coolant as you drained out the system yet your ET is still filled with coolant.

You need to close the ET cap and bleeder screw, start the engine (and/or drive around some) until the engine is warm. Then park the car, turn the engine off, let the engine cool and open the ET cap to see where the level is.

It most likely may be low so keep adding coolant until you have replaced all the original coolant that was initially drained from the system.

That is it.

It is a simple procedure that is easy to get wrong because each car and each project will have it's own unique situation concerning the amount or air that was in the system before bleeding versus the amount of coolant in the system after bleeding. The value of these two variables are tricky to pinpoint without fancy lab equipment so bleeding becomes sort of a guessing game that is based on obersvation (keep looking at the bleeder screw) and using common sense to understand what to do in your unique situation.

There are lots of other ways to refill coolant whiles bleeding air out but this was the most simple way that worked 100% for me.

If you drive immediately after the overhaul and find that -

A. The Climate Control Heat is not hot or working - There is still air in the system and the system needs to be re-bled to evacuate air. Also check to see that you reconnected the heater hose to the ET correctly.

B. The temp needle fluctuates wildly between the normal temp and the red -
There is air in the system. The funny thing here is the engine is actually not overheating but as pockets of hot air passes thru the lower hose and across the surface area of the temp sensor as coolant flows from the hose into the radiator it creates the impression on the sensor that the engine is running hot and the needle goes towards the red. Also having coolant leaking at the sensor location due to reusing an old sensor with worn out seals may cause this issue.

C. Coolant is Leaking at the Lower Hose/Radiator Junction - Most likely the sensor needs to be replaced.

D. Coolant Leaking at ET - Coolant Level Sensor needs to be replace.

E. Coolant Leaking at Center of Car Directly under engine or towards driver's side of engine bay - One of the other least popular and hard to get to coolant hoses are leaking. There get old and can leak although rarely. Also check the Engine Drain Plug to make sure you replace the crush washer and bolted it back on correctly.

More information will be added soon...

Last edited by delmarco; 10-09-2011 at 09:18 PM.
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