SUMMARY (This section is again split into PRIMARY and SECONDARY considerations)
PRIMARY CONSIDERATIONS: (In no particular order, except for point 0)
0. Temp gauge operational
1. Incorrect physical FORMAT of thermostats;
2. Thermostats do not open far enough to regulate flow via the bypass circuit;
3. Temperature rating of thermostats.
4. Thermostats do not open(or close) at the temp they are supposed to;
5. Fans don't come on, or if there are two, only 1 comes on;
6. Fans don't turn in the correct direction, or 1 of them don't;
7. Fan(s) don't run as hard as they should;
8. Radiator blocked with leaves/grass/plastic bag other in front;
9. Radiator cores blocked/partially blocked;
10. Inefficient water pump;
11. Incorrect temp rating of the otter/fan switch.
12. Incorrect ignition timing;
13. Excessively rich/lean fueling;
FURTHER EXPOUNDING on points Above
0. Temperature gauge: Perhaps this ought to be top of the list??? An entire book can be written on this topic alone. Suffice it to say that the temp gauge, temp sender, wiring, connectors, voltage "stabiliser", fuses, ignition switch all has to be good. I intend finding the Ohm value that should have the gauge bang smack in the middle as a quick way to check everything except the sender.
1. Thermostat Format: This means that the thermostats fitted to the car do not have the bypass valve "footplate" (an extra disc on the bottom of the thermostat); Here is the correct FORMAT for the V12: ;
2. Thermostat physical movement range:
First some comments on the merits of a bypass route for a cooling system such as fitted to the V12 (and 4.2's): As the name suggest, the bypass route, allows coolant to bypass the radiator by flowing out the engine, directly back to the intake side of the waterpump. This concept has various benefits: 1) It allows coolant to flow at all times, even when the thermostats are fully closed. By allowing this flow even when the thermostats are closed, it 2) prevents excessive coolant pressure resulting from the water pump "dead heading" against the thermostats (and potentially blowing out the freeze plugs from the block - which I have actually experienced on a Rover V8 engine) and 3) it results in the heat absorbed into the coolant being distributed more evenly throughout the engine resulting in the whole engine having a more uniform temperature.
Thermostat movement range refers to the physical distance the bypass valve moves towards the bypass port. It has been found that some (particularly new thermostats) do not move enough (or the same amount) as they open to physically close the bypass port when the thermostat reaches is "100% open" position. Stopping short of the bypass port by as little as 1mm allows a meaningful amount of coolant to still bypass the radiator (via the bypass circuit) rendering cooling system less efficient. The above pic shows one method (bolting a 2-3mm disc to the bypass valve) of solving the "does not open far enough" problem. Ideally you want to find a thermostat that moves at least 1mm more than the required range of 42mm (IIRC) to be sure that the bypass port will be fully closed if the thermostat is fully open.
3. Thermostat temperature rating: The temp rating of a working thermostat is highly unlikely to CAUSE overheating. At best it may reduce the cooling margin of the cooling system. Eg, if you are running 88ºC thermostats, there is only 12ºC of "margin" in the cooling system before water starts boiling. With 74ºC thermostats, the "overheating margin" would be 26ºC. IMHO: there is no need to run thermostats higher than 82ºC. Although further expounding is offered below on the otter/fan switch, it should be mentioned that the temperature rating of the fan switch is MUCH more important than the temperature rating of the thermostat. MarekH points out as follows:
4. Every thermostat MUST be tested before installation. A "new" thermostat is no guarantee that it works correctly. Ask me how I know.... (BTW, an IR gauge testing against brushed alu or brushed stainlees steel gives false readings. I painted my kettle matt black where I measure). I have seen stats that do not open, stats that stay open, stats that do not open nearly as far as they should (this was a NEW one!!). Bottom line, make no assumptions. Since there are TWO thermostats, BOTH should be tested every time the cooling system acts up. The S3 is only fitted with one temp sensor on the RH bank. Things could thus be going wrong on the LH bank and the driver may not know it.MarekH wrote:A contributing factor to NOT overheating is having enough time (EDIT: margin as jagwit called it above) to do something about the cooling system if it is not performing (or alternatively shutting down the heat generation system if you like).
In that respect, an 82'c thermostat gives you more warning time than an 88'c thermostat by the time it takes to heat the engine by 6'c. You have more headroom for the system to misbehave before it becomes critical.
I would thus include the thermostat temperature rating as a contributing factor to protecting yourself from overheating, but it's actual rating number doesn't tell you anything about overheating.
7. Fans: The original fans ARE good enough!! Stronger fans most likely only serve to treat a symptom, not a cause and may even CREATE charging issues. Fans may not run as hard as they should due to damaged/worn brushes, bad electrical contacts (including bad earths), insufficient voltage from alternator, sticky bearings etc;
9. Radiator blocked either for coolant flow or air flow: This results mainly due to neglect and/or ignorance. A cooling system should only have distilled water and reputable anti-freeze. Tap water should only be used in emergency. Brackish water and sea water should be completely avoided. The latter causes calcification and eventual blocking of the radiator core. Poor quality or no anti-freeze results in excessive rust of all the metal parts of the cooling system (That red powdery stuff you so often see in coolant).
11. There is a lot of misunderstanding around the purpose of the fan switch and how it interacts with the thermostats.MarekH wrote:Most of this solidifies and settles in the engine block around the base of the cylinder liners and this does nothing to help cooling. Some of it, if churned up, will circulate and settle at the bottom of the radiator and the engine bottom hose.
A thermostat is NOT something which is either open or closed. It is meant to always run at a point between fully closed and fully open when the engine is at operating temperature. At this "in between" point, they are able to regulate the engine temperature around its rated temp, BUT it can only do so IF the coolant entering the engine is sufficiently cool. On cars with a bypass circuit, it should be noted that the bypass circuit also remains in play for as long as the thermostat is not fully open (when the bypass circuit should be fully closed. As the thermostat starts closing (implying that the engine needs to run warmer) from its fully open position, the bypass circuit immediately starts coming into play, as Colin pointed out below to maintain coolant flow and to avoid excessive temperature variation the block.
The purpose of the fan switch is to ensure that the coolant entering the engine from the radiator is sufficiently cool such that the thermostats can regulate the engine temperature. Eg, if you have 82ºC thermostats but the fan switch is rated 92º-87ºC (meaning the fans will switch on at 92 and off at 87), the thermostats can no longer regulate the engine temp because 1) They are already 100% open and 2) the coolant that passed through the radiator is already at 87-92ºC even before it enters the engine!!! The otter/fan switch should have a rating where the "ON" temp should be at least 4ºC below the thermostat rating.
Eg for 82ºC thermostats, the fan switch should have a rating of typically 78ºC/74ºC. This way the coolant that enters the engine block can effectively cool the engine as determined by the thermostats that can now regulate the temp by allowing coolant to flow faster or slower as required.
12. Ignition timing could also have a substantial effect on the cooling system. This is mainly caused by timing too far retarded or too much advanced. Retarded timing can result from incorrect distributor adjustment or the centrifugal advance mechanism not operating correctly. Many more modern engines deliberately retard ignition timing during engine warm-up, to render the engine INefficient, in order to achieve faster warmup of the engine (and CATs). There is a lot more info here: viewtopic.php?f=3&t=12885&hilit=ported
13. If fuelling is set too rich, it would also result in excessive heat being produced by the engine. Modern cars deliberately use rich mixtures to achieve faster warm-up of the engine and CATs.