1995 850T Overheating after checked fluids

[jjohnston1013]

So...
I let my mom's boyfriend check the fluids in my car the other day and then we drove it today and it spontaneously overheated and was spewing coolant. We were in a local dairy drive through and the guy there helped us put some water in and get it cooled down and he said he thought the coolant cap looked to him like it wasn't on correctly. The gauge never went below 3 pm for temp, though, and I had to have the heater on high constantly no matter what. Then on the way home it started knocking at one light, was still trying to over heat without the heat on high, and was flashing the oil light at me at every time I hit the brakes (and only when I hit the brakes and sat on them at idle). It made it the 10 or so miles home but as soon as I fully braked in the driveway all lights on dash lit and the car died. I haven't tried to restart it or read codes or check anything else yet.
This car has already needed a whole overhaul because of a broken timing chain (each valve replaced, etcetera) and I am just so overwhelmed as to even a starting point with this new problem.
Any help is appreciated because all I can think is that I should scrap the damned thing before more shit like this happens.

[tryingbe]

Knocking at light and oil light came on? Did you check the oil level? Sounds like the engine is toasted.

Reading codes is easy on a 850. Don't even need any tools.
https://www.matthewsvolvosite.com/read-codes-volvo-850

[jjohnston1013]

Ok...I came back to this after pulling codes and checking fluids and calming myself down so that I'm not just losing it emotionally and unable to really think about or see what's happening.
It's showing me an 02 sensor code (P0135).
It did start and ran actually seemingly fine. I did have to replace coolant as it was empty again even after filling in the previous scenario.
I cleared the code but don't really know what I should do next. From what I've read it's not a great idea to drive it with the sensor going.
My first question then is, could this have been caused by caps being put back on poorly causing a false code? (A slim hope I'm clinging to only so I don't have to try and fix this sensor...)
And my second question is, since I am very unskilled (though not entirely untalented mechanically), is this something I'm definitely going to have to take to a mechanic? And How soon? Immediately?

[cn90]

Are you sure the turbo coolant hoses didn't explode?

Check for leak under the driver's side firewall area.

Info in forum...

[RussB]

If you're losing that much coolant and overheating, DO NOT drive the car until you get it fixed

[abscate]

Your braking had little to,do with the oil light. When the revs fall to idle the oil pressure drops and in this case below the pressure sender threshold.

You need to do a compression test to find out how serious the engine damage is.

[jjohnston1013]

So, spoke with my father. (a veteran 35 year experienced master mechanic who unfortunately lives in another state but has worked on the car before) He said that the 02 error code was already in there previously and had nothing to do with this. He further said that my mom's boyfriend having apparently left the cap askew for a few days might have caused it to boil over and now what he'd be worried about is a cracked or blown head. He walked me through what to do to "burp" the system now that it's refilled and said to try driving it a bit tomorrow and see how the coolant goes or stays.
Says if I'm losing coolant like a bastard it's a cracked head.
Anyhow, he thought the turning off stuff was hopefully the car sort of "being smart enough" to try and turn itself off because it was way too hot.
I also am having absolutely no leakage tonight, have checked twice and it's dry as a bone under the car.
All hoses are fine and in tact from what I can see as well.

[Sommerfeldt]

If you're EXTREMELY, MASSIVELY, UNBELIEVABLY lucky, then it MIGHT have been just the cap, and fluid boiling out that way. Your dad's right - bleeding the air (or "burping" it, which is pretty apt) is often overlooked but important when enough coolant has been lost.

Your lack of leakage might be just lack of pressure - you could still be spewing coolant the moment it gets a little hot.

BEFORE you drive it or even start it, you HAVE TO see if there has been coolant coming out other places than the reservoir cap. Check ALL your hoses, INCLUDING the ones you can't see from the top, i.e. turbo coolant hoses, etc. Check your block CAREFULLY with a bright flashlight, and look for coolant leaks or fluid of any kind. Check ALL connections, check fluid levels again (YOURSELF!) including oil and tranny and coolant. Be safe, not sorry.

- S

[jjohnston1013]

I did all of that, ran it to warm and checked fluids again... seemed to be fine/normal (temp sensor at 3 pm no noises or smells that I could detect). So I drove once around the block and nothing until I turned the AC on. Then it seemed to be getting hot quickly. I only started the AC near my house. Stopped in driveway but left the car running and when I got out and popped the hood the damned cap seemed off it's threads a bit again but it was not showing signs of boiling over. Also, the main fan behind the radiator never came on even with the temp almost to red and AC blasting. I didn't let the temp get into red, turned car off and am now debating what to do. I don't want to mess with it too much more and cause more damage than what might already be done...

[Sommerfeldt]

The cap on the expansion tank is an integral part of the system. If you suspect that there's something wrong with it, get a new one ASAP. It's part #9445462 if I'm not mistaken (and if I am, someone will surely correct me forthwith ).

- S