| 15 How to keep cool under pressure - V8 cooling system Here Colin Leisk (Damask 1729) from Warwickshire provides a comprehensive and seasonal article on maintaining the V8 pressurised cooling system so your V8 does not overheat. (May 79) Cooling system leaks appear to be a persistent problem with the V8 engine after a few years service, thanks largely to the massive system pressure of 15psi - about one atmosphere! Since I replaced the radiator cap on my car some 7,000 miles ago, as it was not maintaining pressure, I have suffered two burst hoses, a leaking heater valve, a cracked expansion tank and finally (I hope!!) a radiator leak. Many of these problems are not confined to the V8 and I hope this article may also be of interest to owners of other MGs. Why does the V8 have such a high pressure? Perhaps the obvious solution is to swap the radiator cap for the 10psi cap fitted to the later MGBs but this is one I would not recommend. We all know that increasing the pressure raises the boiling point of a liquid - in fact water will boil at around 110oC under 15psi pressure. Normally the temperature on the gauge will not rise anywhere near this point, and will only reach "normal" in certain conditions such as town driving when the cooling fans will limit the temperature. This is when the danger arises from either fitting a lower pressure 10psi cap or a faulty 15psi cap. Although the temperature at the thermostat may be registering normal, the local temperature of the water around the hot spots such as exhaust valve seats and ports may be considerably higher, with a risk of boiling if the pressure is not sufficiently high, and consequent damage to the engine. The first symptom the driver will discover is the loss of water through a cracked cylinder head! If localised boiling occurs, heat transfer from the hot cylinder head walls is greatly reduced hence the casting may overheat, warp or crack. On older MG engines there is the possibility of an exhaust valve may start to burn out due to overheating. Needless to say, these risks will be higher with a "tuned" engine, or if the mixture is set to weak or the ignition too far retarded. The engine in the Triumph Stag with its long and narrow water passages and lower system pressure is an example. Long before the temperature gauge registers anything amiss, the damage is done. The incidence of warped heads on the Stag is quite remarkable, largely due to driving in traffic in a high gear which the V8 engine is quite capable of doing. So beware! Let's look at some of the components of the cooling system and the problems which may arise: Coolant Tthe V8 Drivers Handbook (AKD8423) specifically recommends the use of Blucol antifreeze at all times. As well as protecting the engine from frost damage, a glycol based antifreeze performs two other important functions: Avoiding electrolytic corrosion. It reduces the electrical conductivity of the water, so reducing corrosion. This is the opposite effect to salt! Without Blucol, the water will start to pick up iron compounds from the steel components of the system, which will then accelerate corrosion of the aluminium parts - the block, head and pump housing - by electrolytic action. Moreover the deposits will tend to block small waterways, reducing the efficiency of heat transfer and the flow of coolant. My engine needed a thorough flushing with Holts Radflush to remove all the sludge built up due to a lack of corrosion inhibitor in the antifreeze. Raised boiling point. It raises the boiling point of the coolant water, so reducing the risk of local boiling near hot spots. Note the methanol based antifreezes sold under some well know brands are not recommended. It pays to set aside the makers' claims and read the small print on the tins. It also seems reasonable to conjecture that the useful life of a methanol based antifreeze will be reduced in the V8, since the vapour of methanol exceeds 15psi at 80oC - about the normal operating temperature of the V8 engine - and will evaporate. However glycol based antifreeze may be safely left in a cooling system for at least a year. How can one test the strength of an antifreeze solution? I found that a hydrometer for measuring the specific gravity of a battery electrolyte will give a satisfactory indication - flush any residual battery acid out first! As an example a 25% solution of Blucol has an SG of about 1.05 which is just discernible on the hydrometer scale. It is then simply a matter of adding neat antifreeze until this reading is obtained. For topping up, the best plan is to make up a bottle of coolant with the correct proportion of Blucol glycol based antifreeze and keep it handy in the garage. Filler cap It pays to have the cap pressure tested every year or replaced. A faulty cap may be spotted when a hot engine is tuned off. As the temperature rises slightly, and hence the pressure in the coolant system, strange gurgling noises may be heard in the vicinity of the cap as the steam and hot air escapes. The cap also has a vacuum relief valve in it to prevent damage as the engine cools and the water contracts. This may be tested by sealing your lips round the rubber seal on the cap and sucking! Air should flow in quite readily. (Editor: I think I would prefer to get my lips round a pint of Hook Norton and buy a new cap!) Thermostat It restricts the coolant flow until the coolant reaches the correct operating temperature when the thermostat opens thereby allowing the coolant to pass for cooling in the radiator. This allows rapid engine warm up and prevents the engine from running too cold. It should have a small hole in the frame, normally blocked by a plastic bead. If the thermostat jams closed, then the |
plastic bead should
melt allowing the water to flow through the hole. Copyright
reserved by the V8 Register
|