Overview: I believed my coolant to be mixing with my oil and after conducting a compression test and having negative results, I suspected my heat exchanger. Replacing it is quite easy and requires very little tools and knowledge. Do this while the car isn’t too hot to avoid burning yourself with hot coolant or oil. If you suspect your coolant and oil are mixing, I highly recommend changing both as water makes a terrible lubricant and oil can clog up your cooling system.

Time required: 1 – 1.5hrs

Tools required:
Jack/jack stands
17mm w/ breaker bar to remove wheels
T-30 socket
Ratchet w/ 10”extension
Locking pliers or 90% bent pliers
Large flat blade screwdriver
Oil/coolant catch pan

Optional:
Telescoping magnetic pickup tool

Parts required:
Heat exchanger
(2) new gaskets
Coolant to top off
Oil to top off (<1/2 quart)

Procedure:

1. Jack the right side of the car up and place it on jack stands. Remove the front right wheel.
2. Pop the bonnet and open the coolant expansion tank slowly with a rag over top of it to release pressure in the system. As always, use caution and don’t do this with a hot engine. Tighten the cap back up.
3. With a drain pan below to catch coolant, slide back the spring clamps using your pliers and remove the two hoses that attach to the heat exchanger and drain as much coolant from them as possible.
4. With a drain pan below to catch oil, remove the (4) torx screws holding the heat exchanger to the oil filter housing. These screws are difficult to get to but it can be done with a bit of patience and the correct angle (you may need to turn your steering knuckle one way or the other to reach better). Be careful to push your socket in hard as you turn your ratchet as to not strip out your fasteners as this will surely ruin your day if you do. I would start with the two screws furthest right as they can be a little more difficult. The magnetic pickup tool can help you avoid dropping screws in your oil pan.
5. Remove the old oil seals and install the new ones being VERY careful to seat them perfectly flat. Make sure the new seals get wet with a little bit of oil to help seal better.
6. At this point, remove your extension from your ratchet and have a screw standing by on the tip of your torx bit. The new heat exchanger mates up to alignment pins on its top and right sides with the oil filter housing. Do your best to try to place it on in one shot and hold it in place with one hand while you start the screws using only your extension with the other hand. The goal is to move the exchanger as little as possible, particularly until you have one screw tightened down, so your new seals don’t shift or fall unknowingly. Ratchet them all down -- I like to use a cross pattern here.
7. Reattach the two coolant hoses and slide down the two spring clamps.
8. Add an equivalent amount of new oil to your engine as that which was in the oil catch pan.
9. Add coolant to the max line on your coolant expansion tank.
10. Place your coolant catch pan under the coolant bleeder screw. With your flat bladed screwdriver, open the bleeder screw a few turns.
11. Start the engine and check for leaks. Let the engine run a minute or two and look for any dripping at the bottom edge of the exchanger. I like to bring the RPMs to 2000-2500 for a few seconds to build oil pressure. Keep an eye on the engine temps at this point too and don’t let them get to critical levels. You’ll also need to watch for air/coolant coming from the bleeder screw. Once you’re sure no more air is trapped in the system, close your bleeder.
12. Here, I turn off the engine, reattach the wheel, get the car back on the ground, clean up and then carefully check and top off the coolant.
13. Take it for a test drive. Take a rag, some coolant, safety glasses and a flat bladed screwdriver with you. Monitor your engine temps until you’re satisfied all of the air is out of the system.
14. Check your oil and coolant levels over the next few days and check for any dripping where you’ve parked.

Notes:

• My car had 105k when I changed my oil cooler and the seals were toast. Not only were they hard like plastic, they were so compressed that they would NOT reseal on reinstallation of the new unit even though I had no prior leakage.
  
• You may want to consider ordering an extra oil gasket or two as although I was careful, I still managed to crush and damage one of the seals. Fortunately a MINI dealer 45 minutes away had only one in stock left. However if you’re careful seating it and not moving it while installing the new exchanger (easier said than done with one set of hands, but possible), you’ll be fine.
  
• If you want to monitor your actual coolant temperatures, follow the below instructions.

1. With the key in the ignition, but in the off position, press and hold down the odometer reset button with one hand, while holding the button down, switch the key in the ignition to position 1 (first click) with the other hand.
2. The screen will have a number and the word “tESt”.
3. Scroll through the numbers by pressing the odometer rest button, through to 19 and wait a moment. (Note: the number order is: 1,2,10,19)
4. The message will say 19 "L i-off", flash to "L i-on", and back to "L i-off" again. When "log i-off" appears, press the trip reset button again.
5. Scroll through to 7 and wait a moment. Your actual coolant temperature in Celsius will now be displayed where your clock is normally displayed. You may now start your car and motor around.