Heater Matrix/Rad - No hot air.

Bit of a long one, but i'll try and shorten while keeping all the details necessary.

I've spent the last couple of days sorting, or at least trying to, out an issue with a MK I Mondeo. It's a 96 1.8 tdsi. With the colder days my sister (owner of the car) noticed the heater would not clear the front windscreen. It only blew cold air. It got to the point where she would be driving, the windscreen would mist up and nothing would clear it meaning she was essentially blind. A hazard for driving.

I checked the usual suspects and the fan, controls, etc are all working. The only issue is no heat. As i went to get under the dash i noticed the floor was soaking wet. I mean soaking. To me that signified a leak in the system and as it was inside i immediately thought of the heater matrix/rad. I stripped out the glovebox, centre console, and side coverings. It gave me access to the matrix without having to remove the entire dash. A few scraps, cuts, and some cursing later and i have the matrix out. I have before this disconnected the battery and tied the pipes from the header tank into a loop to cut off the matrix.

I poured water into the matrix and it emptied through a hole almost immediately. So i ordered one online and a new thermostat (as it was cheap and an easy fix to resolve a further possible issue). I fitted the matrix and thermostat, but noticed the header tank was only down a small bit from the max level, but assumed it would drop in level when i ran the engine to check if the matrix was now working. It only dropped about half a pint. However after 25 - 30 minutes the car was still blowing cold air. It was late so i finished for the night.

This morning i took the pipes off and connected up the water hose. I blew the system out until the water ran clear (there was a lot of dirty/rusted colour water). I then done it in reverse (water in the out pipe) and again ran it till the water ran clear. I then ran it through the pipes from the header tank. All was good so i filled the tank with the proper coolant/water mix/solution. Started the car and ran it for while.

I now have warm air. Warm being the key word. It's barely lukewarm which while an improvement is not fully working. The air should be hot. I checked the pipes and everything that should be hot is. So hot you cannot keep your hand on the pipes for too long. So the coolant/water into the matrix from the engine bay must be hot, but that heat is not getting into the car in the form of hot air.

I done all this with the header tank cap on. I'm wondering is the system somehow airlocked. Not enough to stop it functioning, but not clear enough to let the matrix heat up sufficiently to blow hot air. I intend to run the car in the morning with the cap off, massage the pipes to work any air through, but i'm wondering if anyone else has done this and if so are there any "speedbumps" i have missed or should be aware off.

Lastly, thank God says ye, the thermostat i got has an opening temp of 87. The original one had a temp of 92. I checked both of these with hot water, measured with a digital thermometer. Would this be enough to cause the difference between hot and warm air? Would the system need flushed again? Have i missed anything?

Any help, advice, guidance would be appreciated to save me having to do the same task multiple times needlessly/repeatedly.


Thanks in advance.