worf928

I know what you're expecting.

I did the t-belt on this 928 in 2005. I drained the block and radiator and filled it up with 50/50 coolant and distilled water as part of the work. (Incidentally this was the first 928 on which I ever saw a 'milled' volute surface on a block from a failed-rebuild metal water pump impeller.)

The owner moved all'round The States, never did much of anything else to it (except oil changes and a dead-LH swap) for the next 14 years; swears that coolant was never changed. It was driven 7545 miles in those 14 years.

This is a shot of the worst part of the deck surface after soda. My 'head guy' was kinda cross 'cause there wasn't anything hard to do here. Note, I've not seen the heads yet with my own eyes. These will only get a minimal surfacing of the deck. I have one other picture - a closeup - that shows very light pitting, and not a lot of it, in the discolored areas.

I have a theory about why the heads aren't 'swiss cheese.'

Thoughts?

Rob Edwards

Porsche/Audi/VW coolant? The pink stuff?

77tony

Evans waterless coolant ?

worf928

Prestone Long Life Coolant (the 5 year / 150k-mile (advertised) stuff.)

But, other than being silicate and phosphate free, and apparent confirmation that I didn't make a bad decision 14 years ago, that doesn't factor into my theory.

EDIT: Of *COURSE* I didn't expect that coolant to stay in this 928 for 14 years when I put it in there...

Rob Edwards

Does your theory have anything to do with 5R heads?

worf928

Nope. Think of the dynamics of chemical reactants and reactions.

FredR

An interesting question and something I have researched given all we have heard and read about. About 15 years ago I saw several engines out of our relatively small fleet that had experienced corrosion issues and the problem was invariably located at the heads and/or the top of the cylinders the latter effectively writing off the engine in at least one case.

When I lost my late 90 S4 a year or so later we harvested the motor from it along with a shed load of other parts and upgrades. After I acquired my GTS chassis the main agents initially recommended that I put the motor back in more or less "as was". I advised them that I wanted to pull the heads and the sump to inspect the motor and duly asked them to perform a compression test that revealed two bent inlet valves on No7 cylinder that further investigation revealed had been caused by a cracked cam sprocket and a timing belt with a few teeth missing. Off came the heads and the sump.

Inspection of the heads and cylinders proved them to be in perfect condition and very careful inspection failed to produce any evidence of structural damage [thank goodness]. We did find a small localised patch of block corrosion on the inner face of the outer wall of the water jacket that we treated with a layer of Belzona to hopefully arrest further development and nothing untoward so far. This further intrigued me and caused me to want to determine why my motor was seemingly spotless when others had failed so miserably.

In my professional capacity I have had a lot of exposure to oil industry corrosion and the key to understanding corrosion is to identify the corrosion mechanism in play and therein lies "the interesting bit". A clue to the problem lies in the fact that the glycol can degrade to form acids and they in turn like to nibble away at aluminium alloys- stop acid forming and corrosion will not take place and as I understand the chemical inhibition package is designed to stop this from happening. Thus test the coolant for pH and as long as it is alkali corrosion should not occur. I did a thread about this not long ago and how I found a simple way to determine such using fish tank water test strips. Like yourself I use Prestone because it is the only recognised make I could find here other than Porsche coolant.

Now herein lies a logic problem- if coolant degrades to form acids then corrosion engineers would tell us that we should expect a general weight loss corrosion mechanism- i.e. corrosion takes place evenly on every wetted surface until such time as the corrosive media is exhausted - has anyone ever seen across the board corrosion happen in a 928 motor - I sure haven't? So what is actually going on? The original; GTS heads on the original GTS motor that my 928 was supplied with failed due to corrosion on the head surfaces shortly after the original owner sold it on to some poor unfortunate soul. The main agents told him the heads were paper weights and that it would cost around US$12,000 for new heads- he did not pay a whole lot more for the entire car and duly crapped himself. Unfortunately he did not about me and he took the car to backstreet mechanic and then all kinds of crap happened but basically they abandoned the GTS heads and put some early S4 heads on that did not have the matching cam clamps.....! When I acquired the car with the original heads [and bearing caps] as spares I inspected the heads carefully and the corrosion took place inboard of the head gasket- a similar pattern to what I had seen on the earlier failures I had seen previously. However, inspection clearly showed that the surfaces of the heads exposed directly to coolant were pristine- so how could coolant corrode so viciously mm's away from the same surface that had no corrosion whatsoever?

Needless to say I believe there is a very easy answer to this "conundrum". The one thing that I knew about the GTS prior to my ownership was that not only had it done a low mileage, I knew that it sat in a garage for long periods of time and therein lies the root cause of the problem. The coolant is an aqueous solution and the cylinder head sits perched on a gasket that seals the coolant volute from both the outside of the motor and the combustion chamber. Invariably there are going to small localised voids at the extremities of the gasket and more so around the fire ring that is sealed by the bolting forces pressure compressing the head, gasket and block in a sandwich. At that location it is the fire ring that is relatively rigid and the material that seals the water jacket quite conceivably may not be "water tight" as for sure the fire ring is the critical seal and the body of the gasket acts as a carrier for the fire ring. Now, as the car heats up and cools down the stresses at the gasket face change- the coefficient of thermal expansion of the steel bolts is about half that of the casing and head material so logic tells us that in the cold condition there has to be less stress in the gasket material and that of course is not a problem as long as the gasket material remains elastic. However, as the gasket material is not the main sealing mechanism at that location this suggests that to some extent it is quite feasible that coolant migrates into the paper fabric of the gasket. Those who follow Greg Brown as I do will know how many times he has waxed lyrical about the state of the head gaskets after 20 years or so of service and how they "seemingly" dissolve. Herein lies the real clue as to what is probably happening.

Corrosion engineers know only too well about a phenomena known as "Crevice Corrosion". I have first hand experience of this and it is lethal. When it happens it can chew through steel in otherwise corrosion free environments. When ionic solutions are trapped in a crevice and are in locations where there are no free flowing media to wash the acids away, they eventually form concentrated acids and that concentration does for the metal exposed. In addition to that one has the fire ring [steel?], aluminium and the aqeous solution with some potential for galvanic corrosion and if that happens the aluminium will go preferentially. What I suspect happens is that cars that sit still for long periods of time get exposed to these mechanisms and in all probability it is the crevice corrosion that does the real damage as the galvanic bit has a small amount of steel relative to a large amount of aluminium alloy. Cars that are used regularly do not need a high mileage to flush the system they just need to be run occasionally and no corrosion issue will occur.

So, the engine in question has a low mileage, it has a good coolant rated for 150k miles, the coolant is in there for 14 years and no corrosion takes place. If the system is well sealed the coolant will not pick up oxygen and f the pH is stable it should not corrode. My best guess is that if Dave were to ask the owner about his user profile he will find the engine has been run regularly if not for great distances. Note this is my theory of what happens- it is not a proven fact - but were it to be correct it would trash the notion that coolant needs to be replaced every couple of years. If Dave had a sample he could have checked the pH but [sadly] I suspect the stuff has long gone.

Apologies for the longish post but hopefully Dave and others will find it "interesting" irrespective of whether my theory is correct or not. At the end of the day there is always a reason for everything - it is just a question of whether we understand it [or not].

Regards

Fred

GT6ixer

I would guess that if you are not going to change the coolant in 14 years, then it is best to also never drive it, i.e. ~500 miles per year. That way you are not constantly attacking the head gaskets with pressurized and heated "acidic" coolant.

worf928

This —^ is my theory. (Not running the engine doesn’t circulate new reactants between the deck and gaskets. And some assumptions about the reaction.)

More later.

IcemanG17

I'm no chemist or corrosion expert, but I think quality long life coolant and distilled water helped.... i bought a used 4.7L USA engine that had nasty rusty cooloant...and when the heads were pulled there was extra holes rotted through the heads...but we got over 200 hours on track with that engine....no head gasket left except for fire rings..... The race cars typically run only distilled water and maybe an additive like water wetter...but they still get plenty of rust in the water

worf928

Things we know to be true:

- The heads and cylinder block are different aluminum alloys
- Coolant gets trapped between the head gasket and head deck surface.
- Coolant and aluminum react.
- Additive packages in coolant inhibit the reaction until they are depleted.
- In general a chemical reaction ceases when there are no more reactants.

That last is *generally* true. But, there are reactions that are self-sustaining because the product of the reaction is more reactants. In those cases the question becomes does the reaction decay or not?

I think it is safe to say that the precise nature of the reaction of coolant in our 928s will be dependent upon the make of coolant used. However if we assume that:

- the general nature of the reaction is similar regardless of coolant make and
- the reaction decays as reactants are depleted

then,

- when coolant is trapped between the gasket and head there is a reaction
- in this localized between-deck-and-gasket volume, as long as the coolant's additive package isn't depleted corrosion is inhibited
- however, once the additive is depleted corrosion occurs
- but, if the engine's not running and coolant circulation stops, this reaction proceeds to its end and stops(*)
- until the engine is started and coolant with 'fresh' reactants is circulated to the between-deck-and-gasket volume, when once again the reaction will proceed until reactants are depleted.

(* in reality Brownian motion will introduce new reactants but the effect will be miniscule)

With respect to crevasse corrosion, this is an explanation for the 'worm holes' in 'swiss cheese' heads. However, even in crevasses a reaction decays until reactants are depleted. We should also note that once crevasse corrosion becomes dominant at a particular location on the deck, corrosion damage goes non-linear in that location.

Thus, the theory:

1) 928s heads sitting in additive-depleted coolant will corrode but as long as the engine isn't run the overall effect on the deck surface isn't guaranteed to be 'swiss cheese'

2) However, every time you operate the engine with additive-depleted coolant you re-start the corrosion process on the head decks. More operation means more corrosion.

Theory in short: sitting quietly, for years, in old coolant is far far far better than running the engine with additive-depleted coolant every month so as to 'circulate vital fluids' (or whatever.)

Obviously, the correct process is to change the coolant before the additive package is depleted.

I have four(-ish) data points to support this theory.
- the long-time-sitting heads pictured above, with pretty-good deck surfaces
- a very-low mileage 928 with documented long periods between coolant changes with pretty-good deck surfaces
- one mid-mileage 928 with a 10+ year documented period of no running and no coolant change with pretty-good deck surfaces
- a mid-high mileage 928 with strong 'suspicion' of several tens-of-thousands of miles with no coolant change and 'swiss cheese' heads.

GT6ixer

So if the depletion of the additives is a key factor in allowing the reactants to take over, then is it possible to restore the additives in old coolant with products like this?

https://www.dober.com/corrosion-tech...y-duty-coolant

chart928s4

Those heads are something. FYI, unlike these, my heads gave the 'head guy' aluminum wizard plenty to do. The theory rings true here also: Per the PO, my car was started every week for 5 years (assume old coolant) So reactants were replaced and left to corrode over and over again. They are now perfect, btw.

^^THIS sounds pretty cool. I never heard of this stuff but just watched the Jay Leno video. Seems like it's pure propylene glycol, so not poisonous and not flammable. The downside: At $40/gallon you would have to be able to reuse it, since the radiator has to be drained for just about every 928 job. The upside: No need to throw away coolant every time you do a job!

worf928

FYI --^ @chart928s4's heads were the Swiss cheese data point listed above.
Right. And this brings up another point: Low mileage is not a guarantee of 'just sitting.' No mileage is accumulated if the engine's running and the car's not moving.

Should I post the pictures Charlie?

chart928s4

Sure, post away. Do you have an 'after' pic also?