The information here is mainly aimed at Rover 200/400 but much of it also applies to other models.
- Coolant flows from outlet on front of head to heater control valve, then through heater matrix and back to the thermostat where its temperature is used to control the stat opening. When the heater control valve is closed, coolant by-passes the matrix and flows through the rubber block between the pipes directly back to the stat.
MGF Cooling Circuit with Standard Thermostat
MGFT Cooling Circuit with PRT type Thermostat
(Diagram by Titanium)
First off – does the heater fan work?
If it does not work on some speeds and this is the problem then refer to this
thread. If the heater fan does not work on any of the speeds then the problem is more likely to be a fuse or the fan motor itself. If the heater fan works, but the air is always cold then the coolant has stopped flowing through it, the coolant is cold or the hot air control flaps are not working correctly.
Lack of flow through heater matrix can be due to:
– thaw out and make sure that coolant is up to strength and changed at regular intervals. Freezing can also cause much more serious damage to the engine and other components.
Heater coolant valve seized / Cable Problems
– the control valve in the heater feed pipe under the bonnet should open and close as the temperature knob is turned. On Rover 200/400 models it is situated near the bulkhead where the heater pipes go through and is operated by a cable from the heater. If it does not move fully, try to work it free with some WD40 or similar. If the knob feels sticky to turn then also read this
thread. Or if the knob spins freely it may just be loose on its spindle and need replacing. MGF valve operation is described here
, and MGTF heater cable adjustment is here
. Another bent control cable thread is here
. And another faulty TF valve investigated here
Typical Heater Valve and Mechanism.
Heater matrix blocked
– this can be caused by mayo deposits following a head gasket failure or corrosion debris due to poor system maintenance. Flush through both ways with a hose pipe then run the system with degreaser or descaler in it before refilling with antifreeze mix (coolant). It is often difficult to clean the matrix completely because as soon as a clear path through it exists, the flush will take this route and the remainder will stay blocked. Also, system cleaners will not work where there is no flow at all.
– perhaps the most common and usually caused by allowing the coolant level to get too low or by incorrect filling from empty. This usually (but not always) results in the engine overheating because it can stop flow through both the heater and the heater by-pass and this will stop coolant from reaching the thermostat in order to open it. System should be topped up with antifreeze mix and if the heater still blows cold the system should be bled to get rid of the air locks. Links to bleeding are –
How to bleed coolant?
coolant system bleed screw
MGF only - TF 160 overheating/ HELP guys!
Note that mayo deposits left in the cooling system, especially if they block vent pipes, can make this process very difficult!
- similar to an air lock, but caused by steam from localised boiling within the engine. The difference here is that when things cool down it will get better by itself as the steam condenses. This is usually caused by a leaking filler cap allowing pressure to escape from the system. The caps are known to be unreliable - buy a new one.
SAAB (Ranco) valve malfunction
– late cars had their heater by-pass controlled by a black plastic valve
instead of just a restrictor. This looks like a T piece and is in the heater pipes near the coolant control valve. If this valve falls to pieces, it can block off the heater and allow all the coolant to take a short cut back to the engine. If this happens, the heater may still get hot when the engine is revved up. It may be possible for the valve to be replaced with a 4mm restrictor, but I am not aware of anyone trying this yet. Perhaps it would be easier to replace the entire system with the heater hoses from the earlier car.
Blocked Oil Cooler
- High performance versions of the MGTF and 75 have a water fed oil cooler fitted. This forms part of the heater/by-pass circuit and can easily become blocked by mayo following head gasket failure.
Water Pump Impeller Failed
- If the water pump impeller has corroded away or become loose the flow may not be sufficient to warm up the heater matrix, but things may improve as the engine speed is raised.
Coolant passing through heater too cold caused by:
– thermostat failed partly open, thermostat has been removed or someone has drilled a hole through it. Beware! If you find that there appears to be no thermostat fitted, check to see if the vehicle is fitted with a PRT type stat in the pipework. These cars still keep the old thermostat housing, but there is no thermostat inside it, just a carrier for the gasket.