I just replaced my crossover pipe and thermostat housing and now my fan is staying in for a lot longer after I turn off the car. Coolant dropped from full to low position in coolant reservoir but I don’t see a leak underneath.

Before replacement the car would reach 190° on a drive, now it barely goes over 160°. So how does it stay on longer after when it’s running cooler?

What did I goof?

Not sure on the temps or fan, but the N18 engine is known for having large trapped air pockets after draining/refilling coolant. Answers your large drop in coolant without a leak. I've read in other spots here that the procedure is to (with a COLD engine) sstart the car with the coolant expansion tank cap OFF, run the car until it gets up to temp. Shut off car and let it cool down, check coolant, add as needed. Do this 3-4 times. Hope the air pocket works its way out. If you never add any coolant during this, something else is the culprit, it didn't work, or you have bad luck. I've also read there is an air bleed screw for adding coolant in the top of the block, transmission side, accessible from top, but under all the hoses and cables. I think the preferred method for adding coolant is a vac-u-fill, but I don't know who pays for the equipment for that.

I did run it for a while before putting the cap back on, but didn't realize it would take 3-4 times. The only difference from this process to when I did a coolant flush last summer was that i didn't slightly jack up the side to help remove air bubbles. (read about it in a thread, but couldn't remember which side it was so I didn't do it lol).

I'll give the bleeder valve another go, but there was a weird improvement in temperature this morning that leads me to believe there is an air pocket or a bad thermostat part.
I've had a P0128 Code show up as of yesterday. Nothing showing up on my dash as far as a check engine light, but I still ran a code search to see why I couldn't get above 160° F. Typically I would be running 147°-158° F, but this morning I hit 182° and the fan did not come on.

So... Maybe It got up to temp enough to pull that air pocket out, or I'm completely ruining my car. IDFK.

Regardless, I would still love and appreciate any input or thoughts anyone has.

After redoing a head gasket on a Camry I had, It was overheating and had an air pocket somewhere I assumed. Not that I suggest it, but after I saw it start to get hot after a trip around the block, I took the best safety precautions I could and in the driveway, and popped the radiator cap off and that blew out the bubble and some hot coolant of course.... Was good to go from there. I'm sure there are much better and safe ways to bleed the air out as have been mentioned.

Makes sense to me. Hot and under pressure, remove cap and shock it, bubble comes out. I buy it. If anyone tries this, as he said, take precautions. Good clothing, cover the cap and spray area around it with good cloth, maybe wear leather gloves.

EDIT: Not trying to imply you don't know to be safe. But if someone who doesn't stumbles onto it from a google search...coolant can burn you.

If you are chasing an air bubble, you could just drain coolant and refill it using vacuum filler. If you have a small conpressor at home, just get the venturi filler on Amazon. It works well. Quick refill and no bleeding.