Aluminum/Iron engines, coolants and head gaskets

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Originally Posted by JHZR2

So I know upfront that this design took some liberties, to say the least. And you hit the point exactly - Al is going to expand quite a bit more, shearing or otherwise affecting the HG somehow...

The 3.5L "rod bending" set of theories is an interesting set of discussions to read.

Essentially lots of folks started to observe high oil consumption, then found out of round bores, especially in cylinder 1, due to bent rods. Of course this is a bored out 3.0L, so there's less material, less gasket, etc.

One main theory is that the HG gets breached, starts to allow liquid into cylinder 1, which then causes a "mini hydro lock" which in time causes the rod to bend, causing an oblong stroke, causing an ovoid bore and consumption, which around 1qt/400 miles starts to affect drivability.


Another theory that I heard 30 years ago (not for this specific issue, but for engines that were originally all iron and retrofitted with Al heads) was that the differential expansion makes the engine into a "bimetallic strip", some of the engine builders claiming that they'd measures the 6 cylinders flexing like one when hot and loaded.

I can't vouch for the veracity of this claim, as I've never tried even simple beam calcs to get a feel.

But the early engines were all flat deck, and flat bearing mating and sump surface (give or take), rather than deep skirted and cross bolted like a lot of stuff is now.

I've not discounted it as a theory...

I've had great success fitting the thermostat on the lower hose (inlet) side of the pump...much more stable operation, and warmup is incredibly even...freezing days, my V-8s wouldn't even warm the radiator, they lost enough heat simply circulating it around themselves.

As opposed to what eljefino has hinted at, providing the radiator with a constant temperature fluid source, batchwise, while throwing a slug of relatively cold water at number one cylinder in particular...my Holdens would always have a major ridge on #1, that progressively lessened with cylinder count.
 
Originally Posted by JHZR2
^ thanks. I am on there.


To be clear to all, my HG never blew. I observed a slight amount of oil in the coolant overflow and knew it was time. UOA didn't indicate that there was coolant or water in oil.

But the time was right, IMO.



I'm not sure how related the diesel engines are to the petrol, but in MB petrol engines around the '90's, the headgasket would get eaten away around the oil gallery, and oil would pump into the coolant....no headgasket symptoms, just oil in the coolant.
 
Originally Posted by clinebarger
Originally Posted by Char Baby
I've had alum heads w/iron blocks that blew HG all day long. However, these engines were know for doing so(Chrys. 4cyl 2.2/2.5, GM 2.8/3.1 V6). And I've had alum heads w/iron blocks that never blew a HG in 300K miles(Honda). So, it depends on the mfg.


I can't think of a Honda engine with an Iron Block?


Obviously not anymore.
However in the late 80s-early 90s, there were many Honda's with this setup. But the OP has a '91 MB w/ Al/Fe setup so, we're talking some ol' stuff here. Similarly, the Chrys/GM engines that I mentioned were older too. So yeah, it's been a while.
laugh.gif
 
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That lower hose thermostat setup is OE on many Euro cars, it does improve cooling system and heater performance but makes them difficult to bleed without a vacuum bleeder.
 
Originally Posted by Silk
Originally Posted by JHZR2
^ thanks. I am on there.


To be clear to all, my HG never blew. I observed a slight amount of oil in the coolant overflow and knew it was time. UOA didn't indicate that there was coolant or water in oil.

But the time was right, IMO.



I'm not sure how related the diesel engines are to the petrol, but in MB petrol engines around the '90's, the headgasket would get eaten away around the oil gallery, and oil would pump into the coolant....no headgasket symptoms, just oil in the coolant.


That's the situation here. Just got oil in coolant, seemingly not anywhere else. I'd suspect that if I left this it could seep further and then cause the mini-hydrolock resulting in bent rods. I think/hope I caught it at the right point. Time will tell...
 
Originally Posted by Shannow


I've had great success fitting the thermostat on the lower hose (inlet) side of the pump...much more stable operation, and warmup is incredibly even...freezing days, my V-8s wouldn't even warm the radiator, they lost enough heat simply circulating it around themselves.

As opposed to what eljefino has hinted at, providing the radiator with a constant temperature fluid source, batchwise, while throwing a slug of relatively cold water at number one cylinder in particular...my Holdens would always have a major ridge on #1, that progressively lessened with cylinder count.


I'm certainly no expert in cooling system design. My understanding is that the 603 engine is set up with a thermostat that blocks one of two flow paths. Something like a "balanced flow" system. One path is constantly circulating coolant within the head, the other path is through the radiator. Either can be fully blocked depending upon thermostat position. The same rate of flow occurs one way or another, and the path is biased based upon the thermostat position.

Essentially the thermostat has two disks instead of one, and the location of them relative to the flow paths decides how much of the flow goes which way. It's an important nuance because apparently flushing with no thermostat can be a problem, in that it facilitates a localized flow loop that can cause an overheat despite the whole system being full and flow being fully open...

It seems that this would ensure a fairly smooth, gradual changeover, it's not just a single opening and rush of different temperature water. Of course, that's assuming that the thermostat works properly.

Again, no expert... maybe they're all this way, or not...
 
Originally Posted by Char Baby
Originally Posted by clinebarger
Originally Posted by Char Baby
I've had alum heads w/iron blocks that blew HG all day long. However, these engines were know for doing so(Chrys. 4cyl 2.2/2.5, GM 2.8/3.1 V6). And I've had alum heads w/iron blocks that never blew a HG in 300K miles(Honda). So, it depends on the mfg.


I can't think of a Honda engine with an Iron Block?


Obviously not anymore.
However in the late 80s-early 90s, there were many Honda's with this setup. But the OP has a '91 MB w/ Al/Fe setup so, we're talking some ol' stuff here. Similarly, the Chrys/GM engines that I mentioned were older too. So yeah, it's been a while.
laugh.gif



I just couldn't remember any........I've seen my fair share of Honda head gasket failures with their Open Deck aluminum block design.
 
Seems like my CVCC had an iron block and aluminum head.

It was a dreadful and terrible engine, but it never blew a head gasket. Probably the lack of compression from hard parts lower down saved the head gasket ...

I've lost two head gaskets, both on all aluminum engines - one side of a Jaguar V12, and another AJ6 inline.
 
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