It has come to the point where I have to replace my thermostat. I am planning on also having a cross over pipe on hand.

Anything else while I'm in there?

 

I suggest taking off the intake to have some space to do the pipe. Then clean the throttle body, put new gaskets on the intake and think about doing a walnut blast.

 

Not a bad idea to have a spare clip for the crossover pipe (the one that connects it to the thermostat body). They are easy to drop and hard to find.

 

I did this recently and here are my lessons from doing the job.

1. On your new crossover pipe, if you choose to use it, make sure you inspect the groove where the O ring sits. Mine had a manufacturing burr that was hidden beneath the O ring, and I didn't find out until I had completed the job, filled with coolant, then started it up....coolant leak from the water pump. Disassembled, found the O ring damaged, and used the old O ring (mistake) after filing the burr down. I kept the new, damaged O ring and overnight the crush damage fixed itself. And I had to use it the next day when I found out that after again assembling it and filling it with coolant, the old O ring had broken during installation. I wish I hadn't removed my crossover, as it was a beyayayatch to remove; and I noted it was not brittle at all and was serviceable - at least until I put so much effort into it that I broke it. Then it really had to come out. Left alone, it's probably alright....but that's a gamble I'm sure others will point out.

2. Don't use coolant when you first get it back together, just use tap water to test it for leaks. I waisted an entire gallon of undiluted coolant doing the job 3 times. You can easily flush and replace after testing.

3. Check and double check all hose clamps. I forgot to tighten one of mine and it blew about a mile from the house. It was a long walk home.

4. I found it was easier to do the job than to figure out how to remove the throttle body. I just could not for the life of me find the screw/nut/bolt that held it on.

5. Prior to the job I had a pinhole leak in the thermostat housing and the temp was running around 220 constant. Post repair, temp is typically 215 and sometimes momentarily hits 220. I did have an interesting issue a week ago where the temp dropped to @180; I switched on the heater and I guess that closed the thermostat and went back to normal temps.

Have a swear jar handy.
If I was going to do this all over again, I would probably buy OEM. I used recommended aftermarket brands from a vendor here and had the mentioned issue (they came through after I told them about the issues I had and sent me a new crossover pipe and gallon of coolant)

 

Replace the water pipe if it is more 5 years old, cheap insurance.

 

When I did my 100k service. I replaced the two main hoses from the radiator. The water pump an hose going from water pump to the thermostat an the other small hose from pump to block. A new coolant tank an ran resistor. Never had an issue. Temp on mine stay at 198 deg. An I live in north Florida. Justctake your time. Do not rush an double check every connection .

 

I just replaced the thermostat and some other parts (see below) while I was in there. N12 engine with about 60,000 miles on the clock.

Here are my learnings:

1. Do replace the crossover pipe if you're doing the thermostat (even if the pipe seems fine). You're already in this far, it's crazy not to replace that stupid plastic pipe also. Trust me. It will also make reinstalling the thermostat to pipe clip super easy.
2. Do remove the intake manifold to get access to the crossover pipe. There are some howto's that suggest you don't need to do this. Ignore them - again, trust me, it is MUCH easier with the intake off. There is a wiring harness attached to the middle of the pipe using a Christmas tree fastener. You'll never be able to get this off and back on properly without removing the intake first. (side note: BMW, what the hell were you thinking using the pipe as an attachment point!? sheesh!)
3. Do make sure you have a set of cable-type hose clamp pliers. It will make your life soooo much easier getting the hose clamps off/on the Jarvik 7 thermostat.
4. Do check the timing chain tensioner bolt to make sure it's torqued down. It's easily accessible at this point, so it can't hurt. You have read the threads about these sometimes backing out and destroying the engine, right?
5. Do tie fishing line onto the lower intake manifold bolt when you're reinstalling it. I didn't need it, but if you slip and drop the bolt you'll likely never see it again. Getting this bolt out and back in is the worst part of the job - you basically need to do it by feel. Once that bolt is back in, everything is downhill.
6. Do make sure you disconnect your battery before starting the job. I didn't do this initially, because I didn't feel like resetting the clock and stuff after. Well, beware - there is an exposed battery terminal on the starter motor within wrench distance of the lower intake manifold bolt. Ask me how I know.
7. Don't disconnect the entire warning harness from the ECU and remove it. I found that there is still plenty of room to work if you just disconnect all the engine connections and leave it flopping around (but maybe that's just on the N12).

I replaced the thermostat (small leak), crossover pipe (on spec - no leak), intake manifold gaskets (due to removal), vacuum pump (on spec - it was fine, but I had no desire to have it seize and cause the engine to self destruct), spark plugs (valve cover was off anyway).

 

I think a bunch of people have offered great advice and intake-track gaskets are one of them. Potentially the breather hose from your rear pcv if you haven't deleted or replaced it.

I would buy all new hoses, aftermarket or OEM. Definitely the water pump gasket, if the pump hasn't been changed in the passed 5 years, buy a new one of those and potentially the pulley if it's worn. While you're there inspect your harmonic balancer and I would inspect the belt or replace it.