Question: what would cause the overflow tank to completely overfill after a drive, when it was at the mid-range of the tank when the engine was cold?
Hoping this is not timing belt/ water pump time.
Overfill or Overflow?
Overfill but not overflow may be normal expansion until the engine/cooling system cools off.
If Overflowing...
Any other work done on the cooling system like changing out a hose or thermostat?
Maybe it still has some air pockets in the cooling system?
This happened to my son's P5 when he first got it (and was overheating) so in order to purge the air out of the system, I had to elevate the car's front end up on an incline (air seeks the highest point to escape) and then I topped off the coolant in the radiator, cap off (while engine running), with heater on high and blower fan on, (so it would circulate through the heater core as well) and then tried to burp the air out of the system by squeezing the upper radiator hose best possible repeating the process and keeping the coolant topped off.
Then after that, I "overfilled" the coolant expansion tank (intentionally) and after running/driving it for a little while (with heater/blower fan on so you can purge the air and feel the heat from the heater core), it overflowed the tank pushing excess coolant out.
(NOTE: Always have to remember if you suspect the engine is overheating, TURN ON the AC AND BLOWER FAN so that at least the A/C fan will run all the time to assist the other fan to help keep the engine cool. You can also turn the temperature up to high on the heat while running the AC to dissipate cooling system heat if needed.)
Once the air is purged out, the engine/cooling system cools off, the coolant should settle back down near to the mid-point at the middle line of the tank.
I've noticed that on occasion I can hear the water in the overflow tank rumbling and checking it (just in case) the level goes down once it cools and has been staying around the mid-line.
Once it gets to a "normal" fill level, I recommend using a Sharpi to mark the level on the tank for a quick referenece line to quickly detect any abnormal coolant loss (like a system or head gasket leak).
The middle of the tank is too high when its cold. The low line is like 2 or 3 fingers from the bottom. Once mine puked out the overfill tube on the overflow. I opened the rad cap and filled it then drained the overflow down to the cold mark. It never did it again. I think it had too much in the tank and when i drove it it just overfilled when it warmed up. Try that and see if it works.
Agree.
I had a honda long ago that also did that but the cause of that was a bad head gasket. The temp shot up and fluid came gushing out. Hopefully this is not your issue
White steam/smoke from the tailpipe/coolant level loss might also be an indicator of a head gasket water leak.
Milky oil on the radiator cap might be an indicator as well but not all the time as the gasket may not have burned through the oil portions to mix into the gasket's water passages.
Stuck or proper type of thermostat.
The P5 needs to have a thermostat with a "jiggle valve" and postioned in the housing in the right orientation so that it lets some coolant/cavitated air bubbles pass so as to not starve and overheat the head.
In the "old days" the water pump/thermostat was located high on the engine. Newer days, the thermostat is at mid-engine level on the engine making it harder to bleed the air out of the system.
I believe I had this issue last year when my radiator cap failed. Parked my car after a drive and heard boiling over at the expansion tank, dumping coolant out of it. Replaced it and all has been well since
Yup - radiator cap valve can go bad for sure and let coolant blow right on through to the coolant tank.
That would possible explaing the boiling water sound I've noticed on occasion after a drive.
When I changed the radiator out on my son's P5, (cracked upper hose inlet), I got an afermarket radiator but the original radiator cap would not screw on as the cap mounting surface was too large (production flaw).
Rather than sending it back, I sanded evenly around the radiator cap's mounting surface neck until the cap would screw on.
It's still a little snug to get on/off for the new cap I bought and wonder if some of the coolant that expands into the expansion tank is just bypassed coolant under pressure?
That would explain why when I measured the temperature of the "boiling" sounding coolant in the tank one day ith a cooking thermometer, I got 165* where the thermostat temperature is 185*.