E36 Thermostat and/or thermostat housing replacement

Symptoms: Coolant leaks from the thermostat housing, car overheats, you want to do the "fan delete" mod, you have other cooling system problems, preventive maintenance, etc.

Goal: replace the thermostat and/or thermostat housing.

Performed on: '97 328is

Applies to: All E36 6-cylinder cars: 325i, 325is, 328i, 328is, 323i, 323is, M3 (I suppose)

Disclaimer: I do not assume any responsibility for the correctness of these instructions or their applicability to the specified models. Use at your own risk! Also, use commons sense, don't do stupid things.

Instructions

• Drain the coolant from the radiator:
  • Park the car on level surface and let it cool down for a while (I'd say at least 30min, probably more is better)
  • Lift either the left side or the front of the car, or all 4 sides (note: theoretically, you could do this job without lifting the car, if you have a container in which you can drain the coolant that can fit under the car when it's not lifted. I used a plastic box with relatively tall sides, so I had to lift the car.)
  • Look underneath the bumper (really stick your head under the car). You'll see a plastic hex-head screw (was blue on my car) closer to the driver's side. Put a bucket or other large container underneath it.
  • Pull your head away from the car and slowly start unscrewing that plastic screw. At some point it'll start dripping. I recommend using gloves although if some coolant gets on your fingers, it shouldn't be a big deal. Keep unscrewing until the screw comes off. It might start shooting out coolant more strongly as you unscrew it more. The first time I did this, the screw fell in the bucket of coolant, so pay attention. Remove the screw completely.
  • Untighten the coolant reservoir cap slowly. As you do that, more coolant will start shooting out from the draining hole at the bottom because air enters the reservoir and releases the vacuum that existed there and prevented all the coolant from coming out. Adjust the position of the bucket under the car if necessary.
  • When the coolant is done draining, remove the bucket (or whatever container you used)
  • Put the plastic cap/screw back on. Tighten it by hand and then a little more with a wrench, but keep in mind that it's plastic and can break easily.
  • Lower the car back to the ground if you want (you won't need to get under it anymore for this procedure)
• Remove the belt-driven plastic fan (the one between the radiator and the engine). If you don't know how to do it, look at my instructions on "Belt-driven (clutch) fan removal".
• (If you're not replacing the thermostat housing, you can skip this step.) Untighten the metal fasteners around the hoses that attach to the thermostat housing and pull off the two hoses from the thermostat housing. Make sure you don't lose the fasteners.
• Untighten the 13mm bolt above the thermostat housing, that holds the engine lift bracket. Don't remove that bolt, just untighten it with a few turns.
• Unscrew the 4 bolts that hold the thermostat housing to the engine block. 3 of them are 10mm bolts and 1 is a 13mm.
• The thermostat housing will fall off as you unscrew the last bolt.
• (If you're not replacing the thermostat, you may skip this step.) The thermostat itself (the round thing underneath the housing) will most likely stay attached to the engine block due to vacuum created in the engine. You'll need to pull it out somehow. Make sure you don't use the surface of the engine block as a fulcrum point to get the thermostat out because you may damage that surface, and it's very important that that surface stay flat and smooth (otherwise you'll have a coolant leak). As you remove the thermostat, remember which side was pointing inward and which was outward.
• Put the new thermostat in, if you're replacing it. Put the O-ring (circular gasket) around it.
• If you're reusing the old thermostat housing, I recommend that you replace the orange weird-shaped gasket ($5 at the dealer). I reused mine once and then got a coolant leak which made me buy a whole new thermostat housing ($45). Put the 4 screws back on and tighten them, but don't overtighten them. According to Bentley manual, tightening torque is 7.4 ft-lb for the 10mm screws and 17 ft-lb for the 13mm screw. I broke 1 bolt when trying to tighten it because i wasn't paying attention... luckily, some of it was still sticking out so I was able to unscrew it and pull it out.
• While holding the thermostat with one hand, put the new (or old) thermostat housing on over it with your other hand. This is a tricky step and it may be easier if you first put the thermostat onto the housing and then slap them both together onto the engine. But make sure you put in the thermostat with the correct orientation! If you put it inside-out, your engine will most likely overheat.
• Reattach the hoses and retighten the fasteners/clamps.
• Start the engine and let it idle. The antifreeze/coolant level in the reservoir should still be at the same level as it was before you drained it. And it will remain there until the engine warms up and the thermostat opens. That will happen approximately when the needle on the temperature gauge on the dashboard reaches the vertical position. At that point, the coolant level will start dropping quickly. Keep filling it as it gets sucked in. Monitor the temperature gauge on the dashboard and make sure it doesn't go much after the vertical position. If it does, shut off the engine and ask someone what to do... I suppose if it overheats, you'd need to let the engine cool off and start it again, making sure you keep the coolant level high. This step is easier if you have someone sitting in the car and monitoring the temp gauge for you as you pour coolant into the reservoir (otherwise you have to run back and forth between the two).
• Untighten the coolant bleed screw, which is next to the coolant reservoir cap.
• As you pour cold antifreeze, the temperature gauge will most likely go down and it may stop sucking in more coolant because the thermostat will close. Wait for the engine to heat up again and monitor the coolant level closely.
• When the coolant level stops going down, close the reservoir cap.
• Squeeze the two hoses that connect to the thermostat housing a few times to get any trapped air out.
• Watch the bleeder screw: when bubbles stop coming out of it as the engine is running, and instead, coolant starts coming out, all the air has been bled out. Retighten that bleeder screw.
• Throughout the whole procedure, keep monitoring the temperature gauge.
• Drive around the block a little, make sure it doesn't overheat. After a couple of minutes, shut off the engine and open the coolant cap (slowly). Check the coolant level and add more if necessary.
• Drive around more. After 10 miles or so, check the coolant level again and fill it up if necessary (let the engine cool off before you open the cap).
• Over the next day, keep monitoring the coolant level. It's not unlikely that your onboard computer will tell you to "Check coolant level", which means that your coolant is low. Note that the computer checks that only when you start the engine, i.e. it won't warn you if the coolant level drops while the car is running. So you need to check it frequently yourself, at least during the first day.

You're done!