Hey guys! It's been a while since i've been on this forum.
But my car has a very strange issue, I cannot seem to solve.
So my car is an 523i, with a 5hp18 5-spd automatic transmission, prior to this, my car was a manual 520i, we've swapped the engine, tranny etc.
Note: It's the M52 based E39, not the M52TUB or M54.
I don't have a secondairy waterpump on my radiator shroud, although I do have one under the heater control valves.
So now the car itself is driving okay, sometimes the automatic shifts when it shouldn't (shifts at exitting roundabouts)
But the problem is with my cooling system, it keeps getting air inside, and there is very little coolant flow.
So the following things have been chaged/replaced:
Meyle HD waterpump (metal impeller)
Gates thermostat, and the metal M50 thermostat housing.
All new hoses (BMW mixed with gates)
New radiator: Not OEM hella, or Behr, i took the Nissens one, those are (at least in our country) the better ones.
The issue: My car will randomly overheat, when idling, or just slow driving, but when flooring it, it's fine, it has very little waterflow in the expansion tank.
Also, every morning when i check the fluids, i notice that my cooling system hisses when I remove the radiator cap, also, I lose coolant, but no vapour from the exhaust, or water inside the cabin / footwells, the windows don't fog up.
Also no water traces on the ouside of the car, or in the engine bay, it's all dry.
We did a compression check with each individual cylinder, no leaks with a cold engine.
Now I think, the issue is the head gasket..... Any thoughts?