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For the last six months, I have been losing about a 1/2 pint of coolant a month. Did not take long to figure out that it was the upper fitting (water pump side) on the return hose (lower hose) leaking by.

Today it was 60' outside so it was time to fix this issue.

There are two ways to do this.

One: Do everything from the top of the car. It's harder, but you don't have to jack the car up.
Two: Jackup the car and remove the large plastic aerodynamic bottom cover. The go back to the top of the car.

First thing is to remove the radiator cover. If you can't figure this out, have someone else do the repair...
I should not have to say this, but wait until the car has cooled down before you start...........

Remove the electrical connector to your fan.


Now remove the torx screw that holds the fan in place. It is just to the left of the fan electrical connector. There is only ONE screw.


Now go over to the right side of the fan and pull out the plastic push on grommet.


Now just pull the fan unit out of the car.


Here is a picture of the lower (return) hose. This is what I have to replace.


In the previous picture, notice the small square box on the upper portion of the hose. This is the water temperature sensor connector.

Here is a better picture.


Now PUSH the silver spring clip and remove the connector.


Using a picture of the new hose as an example, pull up the hold down clip on each end of the coolant hose. With the clip flush against the hose, the hose is locked in place.
With the clip pulled up, like in the picture, it is now able to be removed from the engine and radiator.


Now pull the upper hose end off of the water pump.
Now the harder part, pull the lower hose end off. This is why it is easier from the bottom, but you can get it off by working from the top. About a quart of coolant will spill out.

Here is a picture of the new hose.


Now with the old hose out of the car, press the two tabs on the sensor and remove it from the old hose. Clean it up a bit, then reinstall in the new hose.


Install the new hose. It is keyed so it will not go on wrong. After each end is back on the fittings, push the clips flush with the hose and it is locked in place.
One thing I did to help with this was to put a light coat of silicone grease on the internal O-ring of the new hose that connects to the radiator.
Because of the placement of the connector on the radiator, I could not get enough purchase on that end from the top of the car. So it would not seat properly until I did this.


Reconnect the temp sensor connector.
Slide the fan back into the car. The fan unit should be snug up against the radiator and should not be loose on the bottom.
If it is, then you missed one of the clips that the fans slides into.

Put the torx screw back in but do not torque it down yet. Now put the push clip back in.
You may have to slightly slide the fan unit right or left to line up the hole. Now tighten down the torx screw.
Reinstall the fan electrical connector.

Now remove the coolant fill cap, and loosen the bleed screw just to the right of the fill cap. It is on the other coolant hose.

Start the car, and turn ON the heater...

When to engine gets up to the proper temp, the thermostat will open and allow coolant flow to the radiator.
Add coolant to the reservoir until coolant, not air, is flowing from the bleed screw.
Now add coolant to the proper fill line and put the cap back on.
Put the radiator cover back on. Your done.

You might have to add more coolant after you drive the car again, since the thrashing you give the car will force any leftover air that is trapped in the system to move to the expansion tank and it will then pull more coolant from the tank.

Total time was about 1 hour from start to engine start up.

The new hose was much firmer and not as soft as the old hose, so it was time to replace it. I will replace the upper hose later this summer.

Now, on the new cars that do NOT have an expansion tank, I am not sure how to add coolant. I will figure that out later.

Another great DIY!!! I always enjoy looking at pictures of parts and cars taken apart... Thank you!!!

A couple of suggestions I would add:
1) I've drained the coolant on the Z3 several time for a reason or another and I found the bleeding is much easier if you drive the front of the car up on the ramps: this way the reservoir and the bleeding screw are even higher compared to rest of the cooling system making the air less likely to get trapped somewhere... And just before someone gets an unwanted shower, don't step on the gas while the bleeding screw is removed (you can try to ask me how I know this, but it should be clear... ehehe).
2) I would recommend you carefully clean the coolant you might spill and putt some old newspapers under the car to catch it: it has a sweet taste but is poisonous, so animals could lick it end end up feeling sick or even worse...

Yep,

I did this on my driveway with the car on an up-angle. The M54 engine and radiator does not use very much coolant... Very surprised...

I probably could have just replaced the internal O-Ring on the upper hose end, but coolant hoses are a failure part, and fail at the worst times...

Ship
I was having a hard time trying to figure out a coolant leak on my 2003 2.5i Long story short I followed your directions and when I went to disconnect the electrical connection from the temperature sensor, the movement made coolant immediately flow. I went downtown and my friend who is a mechanic put a new O ring on the sensor. I reassembled the fan and Rad cover after checking to make sure there was NO LONGER A COOLANT LEAK. This is just a short note to thank you very much for posting your instructions as I was not sure as to whether to approach the job from above or underneath. I am very pleased and you can sleep "the sleep of the just" tonight
Robbietoo

Once I got my hose off, I saw the o-ring, but I did not have one that size, since it is metric... I was ok with a new hose.... better safe than sorry.

Great write up Shipkiller.

My hose has just gone......leaking somewhere below the sensor. Could be the crimped hose clip. Hard to see where it's dripping from but it's below/behind the sensor and definitely not the large internal O-Ring on the main hose connector.
Gonna order a hose and get it fitted. Looked like it was gonna be a PITA getting to the bottom hose clip until I saw your write up.

Thanks Again.

jimbo1958 wrote:Great write up Shipkiller.

My hose has just gone......leaking somewhere below the sensor. Could be the crimped hose clip. Hard to see where it's dripping from but it's below/behind the sensor and definitely not the large internal O-Ring on the main hose connector.
Gonna order a hose and get it fitted. Looked like it was gonna be a PITA getting to the bottom hose clip until I saw your write up.

Thanks Again.

*** Update - Replaced my hose following the guide a couple of weeks ago. Really simple job if you follow this guide. Thanks Again.

Edit.

Hi Shipkiller,

This is a great DIY!!! I enjoy looking at those clear pictures... Thank you!!!

What's the year of your Z4 in this post? Can you confirm your part number of Radiator hose?

Thanks
Satcraft

Shipkiller

do you still have this Howto with pics anywhere?

Tommac69

According to Shipkillers profile, he last visited the site in January 2014, so doubt you will get a reply

cheers swiftly

i got a really good diy for soft top fix from shipkiller i saved to harddrive a while back. i'll keep looking

Great write up, just a shame the pictures aren`t showing now
Does anyone have the O ring part No. details as i think that is what has gone on mine.