928GT Net Tech Articles No.1

Why change my coolant ?

Something that is very often neglected in any cars’ service schedule is the regular change (and flush) of the engine coolant. The Porsche 928 maintenance schedule calls for this to be carried out every two years but it often gets left for several years and seldom gets properly flushed out !

Why bother—it’s only coolant and it’s working fine !?


Engine coolant is the stuff that pulls the heat from your engine and then dissipates that heat as it passes through the radiator. The engine cooling system is made up of a number of different components made from different materials.  A cooling system includes components made with Aluminum, Silicon (Alusil) Copper, Brass, Steel, Rubber and Plastic.  As a result, the coolant for any particular cooling system must be compatible with all of those different components – a good reason you should use only the type of coolant the manufacturer recommends. One other good thing coolant does is raise the boiling point above the 100C boiling point of straight water.

As coolant circulates through your engine cooling system, it also performs some other very important functions.  For example, it impedes corrosion and provides lubrication for the water pump seal. But, as it ages, coolant picks up corrosion and different deposits created in the engine and, as a result, it can become acidic.

That acidic fluid along with the dissimilar metals used in the cooling system can actually create a sort of crude battery. The resulting electrical charge in that acidic fluid can then accelerate corrosion of the different component parts of the system. When you change the coolant it prevents this acidic condition and thus helps prevent corrosion and expensive repairs.

In some cases, engine coolant can be in such poor condition that a coolant flush becomes necessary. A coolant flush ensures that new coolant is not immediately contaminated and does not become acidic as a result. A proper flush helps remove corrosion and other deposits that were created in the engine and radiator and often sit in the water jacket of the engine block. This is why the engine block drain plugs must also be removed (two 13mm stubby bolts located either side of the block).

Recommendation for a flush (and the recommendation of any reputable service provider) is based on a careful analysis of the old coolant and the condition of the cooling system.  It is not an “every time” kind of thing. If you have no record of when this was last done then do it now !

To contact me

E-Mail: paul@928gt.net

Mobile: 07 928 298 928

Typical damage (sadly) to an S4 cylinder head from coolant corrosion action

So what happens to my engine ?

The design of the Porsche V8 engine is such that the block (main part) of the engine is made of a specific type of aluminium (Alusil) whilst the cylinder heads are a “plain” aluminium. The seal between these is made by the head gasket. Already we have several dissimilar materials !

The head gasket effectively seals the combustion chambers from the coolant and oil passages that exist in the block and cylinder head. When a head gasket “goes” there is the potential for these three areas to mix, combustion pressure, coolant, and oil. This is terminal and requires a total engine rebuild. This will be £1500 in parts alone + the engine has to come out of the car, total cost can easily reach four times this figure.


The design of the engine means that parts of the head gasket are directly exposed to the engine coolant. Not only this but it’s at the hottest possible internal areas of the engine, right where the fuel mixture is ignited. So you can imagine boiling acid at hundreds of degrees eating into these exposed areas—it’s a bad environment.

A 928 engine block — the yellow areas are the coolant filled jacket surrounding the cylinders and providing the cooling required for the engine to run efficiently at a regulated level:

A 928 engine block — here the head gasket is fitted and you can see where it is exposed to the coolant (compare to above picture) :

What happens when the coolant gets through the head gasket ?

The coolant will then start to “eat” the cylinder head causing a failure which is often quite gradual and by the time it is noticed extensive damage may have been done:

I have seen this type of damage many many times and it is not related to mileage.

The coolant can eat all the way through and then replacement heads are required. Lesser damage can be repaired but it’s a specialist job to fill these areas with aluminium weld and then skim the head flat again. There is also risk that this welding generated local heat can distort the head and make it unusable.


It is difficult to show in these photos but the picture below shows the depth of this particular area of damage !

! Don’t neglect your coolant !