After developing a slight coolant leak late last year and having topped it up with 50/50 mix for a few months 
I finally got around to fitting a new Thermostat. Here's how.
The tools you will need,
On my car the first thing is to drive the front wheel onto a 2" block of wood so I can get my "low" jack under the jacking point. After raising one side I can then get my other jack under the other side of the car. Once I'm up to working height, (so I can get under to remove the under tray) and lean over the front to access the Thermostat I place stands under the cross brace at each end.
Next remove the cover from the landing panel, 8 plastic fasteners,
and two steel Torx 30 screws. One at each end.
Then remove the engine under tray. I use and electric screw driver as it's much quicker that way.
On the bottom of the radiator and the expansion tank there is a bright green drain plug which you need to remove to empty the system(use a big flat blade screwdriver to avoid damaging the slots in the plugs). The one in the expansion tank doesn't come all the way out.
Place a large enough tray (the system holds 10.2 lts.) under the car and remove the two plugs, then you can remove the bleed screw from the top hose and the filler cap. This aids draining. I left mine over night but you don't need to do this.
Once the system is empty you need to remove the hose connectors from the T/stat housing, to do this you need to pull out the spring clips which maintain the seal. A smaller flat blade screwdriver is all you need. Pull the clips as far back as you can then wiggle the connectors until they come free of the housing. Then tuck them away from the area. Next disconnect the 2 electrical connections, 1 on the housing and 1 one on the bottom hose connector. Disconnecting both allows you to get the wiring out of the way when undoing the housing screws. By the way contrary to what you might think it is better to position the wiring below the T/stat rather than above.
The housing is secured to the Cyl. head by 4 screws, 1 x 13mm and 3 x 10mm. The 13mm. one also secures the engine lifting front bracket which is also connected by an 11mm. nut on a stud at the Vanos housing.
You need to remove the oil feed pipe to the Vanos by removing the 19mm. Banjo bolt and be careful to collect the 2 alloy sealing washers as you do.
Remove the 13mm. screw and the 3 remaining 10mm. screws and the housing is free from the head. You may need to administer a small tap to dislodge it.
Once it was off it was easy to see why it was leaking, but bear in mid it has been there for almost 17 years. The rubber seal is totally flattened and was leaking past the screw at the lowest point of the housing.
As you can see the seal is well and truly knackered.
You can see here that it was leaking down the front of the motor behind the drive gear.
Before fitting the new unit you need to clean the sealing face. I used a razor blade type scraper and then some 1200 wet/dry paper wetted with brake cleaner.
To start fitting the new one I very lightly greased the rubber seal so that it slips into place. When in place firstly fit the 13mm. screw and tighten up to snug fit. Then fit the 3 x 10mm. screws and then you can remove the 13mm. one and replace the lifting bracket making sure you get it on the stud at the Vanos housing and finally refit the 13mm. screw. Evenly tighten all 4 screws then the 11mm. nut on the stud. You can now reconnect the oil pressure feed pipe to the Vanos solenoid, but remember to clean the Banjo bolt and 2 alloy washers and tighten the Banjo bolt.
Next wipe out the connectors which attach the hoses to the T/stat housing and lightly lubricate the rubber sealing rings inside them and relocate the spring clips into their locking position. The top one will go on easily because you have good access, for the lower one you will have to lay under the car and push it up from underneath (it goes on easily from here). If you try to do it from the top you will only scrape and bruise the back of your hand Trust me on that.
Next reconnect the 2 electrical connectors one on the bottom hose and one on the T/stat housing. Finally refit the two drain plugs in the Radiator and expansion tank. Car still up on stands. Ready for filling the system, I used a funnel which just happened to be a good fit in the top of the tank so no spillage as it backed up. I filled up with 5 ltrs of antifreeze and while that gurgled through the system i cleaned the under tray so I would be able to spot any possible leaks.
After the antifreeze I started filling with water and as it filled I left the bleed screw out and as the water pushed the antifreeze round the system it flush the remaining coolant out through the bleed hole, As soon as the new blue stuff started coming out I replaced the bleed screw.
Next start the engine, heater on HOT and blower on full. Took a few minutes to get up to temp to cut the fan in but then left it to cool over lunch.
I was going to change the water pump at the same time but it's not that easy on the Z4. There is a cross body brace in front of the engine about 1" from the Water pump shaft as you can see,
However the water pump has 2 1/2" inches of it inside the block.
So, you either have to remove the complete front of the car or lift the engine off it's mounts, at only 48,000 I decided to do neither and put the pump back in the cupboard. Right decision? We shall see. Down off the stands next, recheck level and road test.
After a 6 mile road test, back in the Shed when I opened the bonnet there was no whiff of antifreeze as there had been and nothing on the under tray, even after standing for a couple of hours. Final check for leaks, including at the oil feed pipe in the morning but I'm quietly confident.
Hope this helps anyone doing this job. Any questions please feel free to ask.
Tony.
I finally got around to fitting a new Thermostat. Here's how.The tools you will need,
On my car the first thing is to drive the front wheel onto a 2" block of wood so I can get my "low" jack under the jacking point. After raising one side I can then get my other jack under the other side of the car. Once I'm up to working height, (so I can get under to remove the under tray) and lean over the front to access the Thermostat I place stands under the cross brace at each end.
Next remove the cover from the landing panel, 8 plastic fasteners,
and two steel Torx 30 screws. One at each end.
Then remove the engine under tray. I use and electric screw driver as it's much quicker that way.
On the bottom of the radiator and the expansion tank there is a bright green drain plug which you need to remove to empty the system(use a big flat blade screwdriver to avoid damaging the slots in the plugs). The one in the expansion tank doesn't come all the way out.
Place a large enough tray (the system holds 10.2 lts.) under the car and remove the two plugs, then you can remove the bleed screw from the top hose and the filler cap. This aids draining. I left mine over night but you don't need to do this.
Once the system is empty you need to remove the hose connectors from the T/stat housing, to do this you need to pull out the spring clips which maintain the seal. A smaller flat blade screwdriver is all you need. Pull the clips as far back as you can then wiggle the connectors until they come free of the housing. Then tuck them away from the area. Next disconnect the 2 electrical connections, 1 on the housing and 1 one on the bottom hose connector. Disconnecting both allows you to get the wiring out of the way when undoing the housing screws. By the way contrary to what you might think it is better to position the wiring below the T/stat rather than above.
The housing is secured to the Cyl. head by 4 screws, 1 x 13mm and 3 x 10mm. The 13mm. one also secures the engine lifting front bracket which is also connected by an 11mm. nut on a stud at the Vanos housing.
You need to remove the oil feed pipe to the Vanos by removing the 19mm. Banjo bolt and be careful to collect the 2 alloy sealing washers as you do.
Remove the 13mm. screw and the 3 remaining 10mm. screws and the housing is free from the head. You may need to administer a small tap to dislodge it.
Once it was off it was easy to see why it was leaking, but bear in mid it has been there for almost 17 years. The rubber seal is totally flattened and was leaking past the screw at the lowest point of the housing.
As you can see the seal is well and truly knackered.
You can see here that it was leaking down the front of the motor behind the drive gear.
Before fitting the new unit you need to clean the sealing face. I used a razor blade type scraper and then some 1200 wet/dry paper wetted with brake cleaner.
To start fitting the new one I very lightly greased the rubber seal so that it slips into place. When in place firstly fit the 13mm. screw and tighten up to snug fit. Then fit the 3 x 10mm. screws and then you can remove the 13mm. one and replace the lifting bracket making sure you get it on the stud at the Vanos housing and finally refit the 13mm. screw. Evenly tighten all 4 screws then the 11mm. nut on the stud. You can now reconnect the oil pressure feed pipe to the Vanos solenoid, but remember to clean the Banjo bolt and 2 alloy washers and tighten the Banjo bolt.
Next wipe out the connectors which attach the hoses to the T/stat housing and lightly lubricate the rubber sealing rings inside them and relocate the spring clips into their locking position. The top one will go on easily because you have good access, for the lower one you will have to lay under the car and push it up from underneath (it goes on easily from here). If you try to do it from the top you will only scrape and bruise the back of your hand
Trust me on that.Next reconnect the 2 electrical connectors one on the bottom hose and one on the T/stat housing. Finally refit the two drain plugs in the Radiator and expansion tank. Car still up on stands. Ready for filling the system, I used a funnel which just happened to be a good fit in the top of the tank so no spillage as it backed up. I filled up with 5 ltrs of antifreeze and while that gurgled through the system i cleaned the under tray so I would be able to spot any possible leaks.
After the antifreeze I started filling with water and as it filled I left the bleed screw out and as the water pushed the antifreeze round the system it flush the remaining coolant out through the bleed hole, As soon as the new blue stuff started coming out I replaced the bleed screw.
Next start the engine, heater on HOT and blower on full. Took a few minutes to get up to temp to cut the fan in but then left it to cool over lunch.
I was going to change the water pump at the same time but it's not that easy on the Z4. There is a cross body brace in front of the engine about 1" from the Water pump shaft as you can see,
However the water pump has 2 1/2" inches of it inside the block.
So, you either have to remove the complete front of the car or lift the engine off it's mounts, at only 48,000 I decided to do neither and put the pump back in the cupboard. Right decision? We shall see.
Down off the stands next, recheck level and road test.After a 6 mile road test, back in the Shed when I opened the bonnet there was no whiff of antifreeze as there had been and nothing on the under tray, even after standing for a couple of hours. Final check for leaks, including at the oil feed pipe in the morning but I'm quietly confident.
Hope this helps anyone doing this job. Any questions please feel free to ask.
Tony.
