FredR

As part of my current ongoing annual programme of maintenance i have about reached the bottom of the pit as it were in terms of removing and inspecting things. Yesterday was the day I dreaded most- removing the coolant. No matte how hard I try I invariably end up spilling some no matter how hard or how many times I do this. The best approach I have come across is to have a kiddies paddling pool under the car whilst doing this but given one is working under axle stands to do so one would have to be in the pool at the same time! That being said underneath the car I did rather well yesterday and spilt very little. I make a baffle to stuff up the opening in front of the drain ports- the idea being that as the coolant pees out of the drain port it hits the baffle [a piece of cardboard box] and that allows it to drain uniformly into the catch bowl [the cat's litter box] beneath- see pic.

Splash guard made of cardboard


I did manage to kick over a 4 litre tub full of the recovered coolant outside the garage losing about a litre- grrgh!

First thing I do with this task is wear industrial eye protection [googles]- one does not want coolant getting into one's eyes! Upon recovering the coolant I then used one of my test strips to measure pH. The coolant has been in the engine for 4 years and the pH was a healthy 8.4 [see pic below].


pH value is the red strip next to bottom




Why is this important?- the answer is simple. Ethylene glycol makes an excellent anti freeze. On its own the glycol is poor compared to water in terms of heat capacity and its natural freezing point is about minus 10C but mix it 50/50 with water and something "magical" happens- its forms what is known as a eutectic mixture and the freezing point drops to around minus 40C- just what I need in the Middle East!! The not so good is that the glycol breaks down at about 50C to form organic acids- not good- and once the pH drops below about pH6 it will start to eat aluminium alloy for breakfast, lunch and dinner as it loses its passivity. The anti corrosion package contained in coolants is primarily intended to stop this from happening. The coolant originally specified for the 928 was the stock IAT [inorganic acid technology] commonly presented with green dye. They used green dye to make it easier to spot rust formation in the days of cast iron blocks and heads. In terms of corrosion protection to this day I suspect there is still no better coolant and that is what I use. In the late 80's there was a demand for longer life coolants and chemists came up with OAT [Organic acid technology] type coolants but the problem was they were not very good so they compromised and stuck silicates back in the blend [no phosphates in this brew- not needed in alloy blocks/heads]. Rather than admit they cocked up they called it HOAT [hybrid organic acid technology] and this lead to the coolants currently recommended by Porsche that are labelled as G- 48 coolants. Ironically G-05 is also a HOAT coolant but it also contains something the G48 does not to protect against corrosion in wet liners but that is another matter and not really needed in the 928 as it does not have wet liners [not in a stock 928 that is].

So Porsche originally recommended IAT coolants for our 928's as that is what was available back then. The later variants did not come to market until about 1994. Porsche also states in its supporting booklet that the recommended coolant change interval is 2 years and we often see this quoted. I have just removed Prestone green coolant that has a pH of about 8.4 so it is still viable. A quick look inside the radiator and the inside of the thermostat nozzle and they are both in perfect condition- so what gives? I suspect that coolant life is controlled by the depletion rate of the chemical package and if the coolant is below 50C then the chemicals are not being molested as it were- this suggests to me that coolant life is run hours oriented not time based. I suspect that coolant manufacturers worked on the premise that an average mileage was 15k miles per year so after two years 30k miles were covered and you can bet that is very conservative. However get the punters in every two years and it is a win win for the dealers and coolant manufacturers- not so good for green peace though!!.

Now take a look at the pics below:


Thermostat nozzle corrosion

Tee piece corrosion

As you can see from the above pics some corrosion has been taking place but note in particular the thermostat nozzle- look inside the nozzle and one can clearly see it is in immaculate condition. What is not apparent is that both these pieces appear to be in exactly the same condition I found them in 4 years ago. At the last inspection

The problem is not internal corrosion it is external corrosion- dors this look familiar to anyone? I'll bet it is a very common problem. The problem is not uniform - far from it- it is scattered with corrosion pits. On the thermostat nozzle they are not too deep but on the steel tee piece the pits actually perforated the raised surface in a few places. This is a classic crevice corrosion attack- not a theory- a fact of corrosion engineering. Those raised surfaces are the problem- they create a small gap between the pipe and the rubber that the coolant can seep into. Over time the coolant is not replenished, the heat degrades the coolant into the likes of formic acid and then with an electrode and two different fluids in close proximity the crevice corrosion attack mechanism promulgates. It is not a rapid attack rather it slowly eats away at whatever is in its way. The steel Y piece being much thinner wall perforates quicker. Once perforated the attack tends to cease at that locality because the tapped fluid is replenished by the main stream. I took action to save the pieces. In the case of the branch on the tee it was so bad i had no choice but to cut off the raised section. To save the day I cleaned up the surfaces for about an inch in length with a wire brush. I then buttered up the cleaned surfaces with a smear of silicon RTV, butted up the hoses and left the assembly overnight to cure. The following day I installed the hose clips. As I can tell no further corrosion has taken place. I suspect the main cause are those raised rolled sections and then clamping too far away from them. Had the hose been tightened at the correct location close to the rolled section I suspect non of that would have happened.

A couple of years ago I introduced the notion that this problem was causing most if not all the cylinder head corrosion we see on some models- again this is not a theory it is a fact- the evidence is there in plain sight - the theory part comes when trying to explain why some models see that particular problem and some do not. Based on what i have seen, if the water bath and the flow ports on the cylinder head are not corroded but the cylinder head is corroded between the open areas of the gasket and the head deck the most likely cause of that is due to said vehicles being sat for weeks or months on end not being used. What do many owners in colder climes do in winter time- hibernate for 3 or 4 months. Do they start them up and let them warm up every couple of weeks or so? On the ones that have head corrosion it is a fair bet the answer will be "no". I first experienced this problem 16 years ago when transplanting my S4 motor into the GTS. I feared corrosion may have taken place- my motor was immaculate- it was used almost daily for the first 16 years of its life. The original cylinder heads from the native GTS motor were given to me by my PO- they had this problem. At the same time two other examples were in the workshop both with severe head corrosion both fully Porsche serviced cars- both sat for months on end whilst their owners went to Europe for the summer.

For the Y piece I intend to make something different0 probably use more hose and less pipe- still have to work that one!

StratfordShark

Thanks for the baffle idea Fred! I always make a dreadful mess when opening the block drains, and it's a job I always do in the garage on a lift which makes the clear up worse. I've considered fitting those neat taps in place of the plugs, but I'd always be worried about some kind of failure or a tap being broken off so end up taking a coolant bath every few years.

FredR

I have one of those taps on my sump and they are very reliable. they do have a problem in that the invert level is a bit higher than a straight plug so there is more captive fluid in the sump being drained- not a huge amount and in the context of the water bath probably irrelevant. When servicing the 928 the car is on my home brew "lift bars" - for oil drainage I lower the left hand side three notches on my jack stands to create a gradient with about 4 inches of differential height- I do not get under the car again until it is back on a level playing field!