coolant leak, and pressure in the expansion tank

[SeanOB]

I could use some help with my ole e30 please; m20b25, facelift.

I have been chasing a coolant leak at the front of the engine lately which only seems to appear after running the car.

So it does not visibly leak when on, does not overheat and no sign of steam. No mayo anywhere.

When I park her up (facing slightly downhill) I get a decent puddle of coolant in the engine under tray over night. I thought it was just a loose hose clamp, but tightening this one up has not got rid of the leak.

Seems that this brings the coolant expansion tank level down below the low mark, but not any lower than this. I did see an old post here about how the level marker on the expansion tank seems a bit too high, so people do find that filling it past the mark results in an overflow.

I wish I could rule out my leak on that bit of info, but I just want to check whether it is normal for there to be pressure in the expansion tank, as I get a rush of air out of it when I uncap to refill.

[mark_i]

are you able to see from where the coolant leaks ?
also the coolant is pressurised to 160 so the tank cap will hiss when system is hot

[SeanOB]

are you able to see from where the coolant leaks ?
also the coolant is pressurised to 160 so the tank cap will hiss when system is hot

I can't spot the leak, annoyingly. Spent a while looking around the hoses, water pump, radiator etc but no drips. The only drip I ever found was from the one hose I mentioned, which is gone after a tighten up.

Hiss when hot makes sense... how about if I leave it overnight then crack it in the morning? Would it be normal to still hiss ? Just wandered if it could be a head gasket leak that was pressurising the coolant system.

[Brianmoooore]

I've always found that an M20 will eject coolant until the level is around 10mm below the seam in the tank. This is still plenty for the cooling system to operate - it's still OK right down and past where the coolant light comes on.
The hiss in the morning is more likely to be air going into the tank, rather than out, and is normal.
Connecting up a pressure testing kit to the cooling system on any car is usually quite educational. I've found that very few cars are entirely watertight when pumped up to working pressure while cold.
The most obscure coolant leak I've ever traced turned out to be a leaking water pump seal, that only leaked (very slowly) when the engine was running, and dried almost instantly after the engine was stopped.

[SeanOB]

I've always found that an M20 will eject coolant until the level is around 10mm below the seam in the tank. This is still plenty for the cooling system to operate - it's still OK right down and past where the coolant light comes on.
The hiss in the morning is more likely to be air going into the tank, rather than out, and is normal.
Connecting up a pressure testing kit to the cooling system on any car is usually quite educational. I've found that very few cars are entirely watertight when pumped up to working pressure while cold.
The most obscure coolant leak I've ever traced turned out to be a leaking water pump seal, that only leaked (very slowly) when the engine was running, and dried almost instantly after the engine was stopped.

Thanks Brian, sounds like mine is just doing normal e30 things then! I will resist topping back up and will keep a mark of where the level is now, then see if it still drops more after a hard run.