C5 X7 heating-cooling issue

[Zarzorro]

Hello everyone.
My 2.2 hdi (204) C5s heating-cooling system has some really annoying issue. Wintertime there is no interior heating, even if the temp buttons set to max. If the fan blows mid temp air i am happy... However if the outside temperature is above 20 degrees the coolant and the engine oil is getting too hot. Above 20 degrees the radiator fans turn on almost immediately after I start the engine.
The coolant level is ok, there is no oil in it but its really dirty.
What could cause these problems? Is the cooling system clogged? I thought the previous owner poured some "leak stop" into the reservoir.
Should is flush the cooling system?
Thanks

[GiveMeABreak]

It’s not a good idea to put anything into the coolant system other than the correct coolant mixture, but appreciate you said this was done by the previous owner. That suggests there may of been a problem...

Don’t forget your radiator cooling fan will always switch on whenever the AC is active.

If you are not getting any heat I would first check that the air mixing motors are working. You can perform a diagnostic with Diagbox and test the motors are working. If you don’t have access to this, try listening carefully as you change the temperature from cold to hot and back again to see if you can here the motors engaging.

If these seem to be moving then maybe your heater matrix is blocked and therefore it could be stopping the coolant from circulating through the matrix. This would mean a flush for sure, but may need further work I’d the matrix is full of gunk.

[white exec]

If you can locate the two coolant hoses that leave the engine compartment and head off to the heater matrix (one is supply, the other return), with the engine warmed up try feeling these two hoses. Both should feel hot to the touch, maybe the return one a little cooler. If either feels cold/cool with a warmed up engine, then there is a blockage of (or no circulation through) the heater matrix.

How does the car's temp gauge read? If not showing properly up to temperature, then suspect the engine's thermostat. This shouldn't leave you with an absolutely cold cabin heater, though - just one not really hot enough - and an unstable engine temperature that is directly affected by how much fuel you are burning.

[Zarzorro]

Thank you both!
I'll check the heater matrix, unfortunately I dont have a diagbox.
The "leak stop" was just a guess I am not sure about it... if the matrix is cold which flush method and liquid can give the best result?

[Hell Razor5543]

If you do use a stop leak substance (and I am not saying you should) it is likely the best stuff you could use is Forté Stop Leak. If, for example, you had a Xantia V6 and the heater matrix failed, Valeo (who are an OEM) require you to use this specific make of Stop Leak for the warranty to be valid (as it will seal any micro leaks), and it is (AFAIK) the only one that does not block and then damage the gearbox heat exchanger.