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Coolant loss/leak/i don't know

hi all I have a issue with the coolant on my Peugeot 307 1.6 hdi sport 2006.
I’m losing coolant at the rate of about a full resovour every 3hours :scream:
all the usual checks have been carried out, head gasket is fine, no oil in coolant tank and no coolant in oil, no traces from the exhaust, no visible leaks within the engine bay, the car recently had a new EGR cooler and coolant tank sensor fitted. I’m at a complete loss here, please help
forgot to mention I’m also getting both coolant level to low and risk of partical filter blockage on the dash display, I’ve replaced the air filter only a few days ago and cleared the error codes from the car

it has to be going somewhere… I’d bet on a leak that only starts when the engine is hot and the coolant system is pressurized. ie, when you are driving. It could be squirting somewhere on the highway and leaving no trace.

Did any of the checks include a pressure test of the system?

My second guess is a head gasket, and the previous test missed that somehow.


I haven’t done a pressure test yet, as I don’t have the equipment, looks like it’s a trip to the garage I guess, unless you know of a way to do it at home

The head gasket is fine it was replaced by Peugeot about 6months ago. I think your probably right with the leak under pressure, atleaat that would explain why I never seem to see any coolant anywhere and it only goes while driving

Just because the head gasket was replaced 6 months ago doesn’t mean it is fine or even that it needed replacing, it could have been a misdiagnosis and you have a cracked head or block. I am not familiar with Peugot engines, they have not been sold here in about 50 years but it could also coolant passages in the intake manifold leaking.

I think you are shooting yourself in the foot trying to diagnose it yourself given the car’s history.

tbh I’m at the point where a trip to the garage in drawing nearer, I’ve basic mechanic knowledge and I’m out of my depth here.
however the head gasket had definitely gone and the main Peugeot dealership carried out the needed checks, they verified that the head was fine and gasket need replacing I do have a 6month warranty so I’ll check the paper work And see if it’s still valid, I may be able to get a check free of charge