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Rover V8 cracking block and all the theories around


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#1 landybehr

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Posted 25 March 2008 - 10:48 AM

Hi,
The nice Rvoer V8 has that bad habit of cracking blocks. When I rebuilt my engine I tried to read as much as possible about that problem, trying to end up with a reliable engine. But I feel that this problem is not fully understood by me.. Many ideas but how can we know what really is behind ?
So if you like you´re welcomed for a discussion. Kind of brain storming ;)
Starting with tuning my Megasquirt I struggle to see some of the theories that are around compatible with tuning theories and my small experience.
I try to write down as much as I can about this problem for you to join in, correct me, add things and so on. Maybe I can get peace of mind with all this at last ? :)


Observations:
=========

- all the engines with 94mm bore have the problem with cracks developing behind the liner in the aluminium where it is thin, i.e. where the coolant runs close to the liner. So this counts for the 3.9, 4.0, 4.2, 4.6l engines. It is then possible for coolant to creep up behind the liner (not stopped by the head gasket which seals not ON but next to the liner) and into the combustion chamber. The same way exhaust gases can get into the coolant and put it on pressure and can let the engine boil.

- the cracks develop after a considerable mileage ? AFAIKS it takes 40.000-60.000 for it to happen.

- It is said that the block usually crack in the region where the head bolts are and that using studs may help.

- Rover deduced it to the thickness of the aluminium wall behind the liner and in the last years of production measured the aluminium wall and dumped blocks which, due to casting irregularities, had to small a wall.
- The 4.6l V8 with the Bosch engine management (Thor) has not a reputation for cracking. (But that my be because a) it is generally the youngest engine and we just have to wait, b ) there are too few engines of this kind around as it was the last series to be produced, c) it has a different fuel map, gerenally aimed for emissions).

- the 3.5 block does not crack. The 3.5 block has a problem with liners dropping, though. Is that true ? At least the 3.5l V8 has, due to it´s smaller bore, a thicker aluminium wall behind the liner.

- Replacing the standard liners with Top Flange Liners can repair a cracked block. It´s the only way to do that. And this liner gives a superior seal for the head gasket and water cannot leak


Theories about the cause:
=================

- Rover, as said, must have thought that the wall thickness of the aluminium behind the liner was the main cause.

- RPI have published an article in the LROi some years ago, saying, that Rover had leaned(*) the mixture on those parts of the fuel map where it was relevant for fuel consumption test runs. On those parts of the fuel map the engine would run lean. This in turn spells for a too hot mixture which damages the engine i.e. let´s it crack. They wrote that the 4.6l suffers more than the smaller engines because it is stronger and thus laboured more in a lower gear so kept longer in the “bad” regions of the fuel map. (I hope I summarize that article correctlly. But I think I do).


Some Other´s thoughts:
================

- some say that the best way to deal with a cracked block is to replace it with a new 4.6l. While that sure is a good thing to do that new block will have to be fuelled correctly to not suffer the same way. This can only be done with re-chipping.

- the same people could send me a picture of a block where a Top Flange Liner itself has cracked. The top flange disintegrated with the cylindrical part. To me it looked like that liner was overbored to gain a bigger displacement. I haven´t got an answer to that.

- Top Flange Liners are considered to be a reliable thing. I have once asked about them on pirate4x4.com BB and no one could report a problem of knew a top flange liner fail. So that said cracked Top FlangeLiners might be seen as an exception.



My thoughts (based on other´s knowledge):
=============================

- First there´s one thing about combustion I just have learned. The combustion temperatures are hottest when the mixture is a little richer than stochiometric. Which is below 14.0 (at about 13.7). That makes sense because heat is produced by fuel burning so much fuel equals to much heat. That way a lean mixture cannot be hotter. If on the other hand the mixture is too rich there will be incomplete combustion and temperature will be lower again (one effect is that with a too rich mixture more CO is produced. CO itself retains energy which would be freeded when the fuel would have had enough O2 to burn into CO2.
>> how does that fit to the theory that lean mixtures cause cracks.
It is true that lean mixtures can cause heat. Mainly if measured at the exhaust manifold. BUT this is mainly due to the following: a lean mixture burns slower (not primarily hotter) and if you do not advance the timing correctly the burn will still continue when the piston is well on the downstroke. It may be that even flames are there when the exhaust valves opens and this leads to a high measured temperature; not really reflecting the combustion temperature.
OTOH - we do know that the CO-content reduces continually as the mixture gets lean. And the leaner the more NO is produced (see: http://www.megamanua...FRemissions.gif ) until the mixture is leaner than what can realistically be had on the road – after which NO is reduced again. So how does this fit together ? NO is produced when heat is high – why doesn´t NO is highest when combustion temperatures are highest ?
For those with interest – I´ve started a thread on the MS-Forum here: http://www.msefi.com...5...asc&start=0

- damage to cylinder liners is, on other engines, mainly seen in the context of knock. When knock occurs the peak pressures are harmful. I can imagine that these pressures even deform the cyliner liner where it is less supported from the back i.e. where the coolant flows behind it. Usually head gaskets go first on other engines. Seems to depend on engine design.

- Des Hammill (“How to Power Tune The Rover V8”, kind of bible for our engine) wrote that Rover had to keep NO emissions within limits. To do that they had to make mixtures at cruise conditions richer than necessary. This is bad for fuel consumptions. So .. when Rover avoided too lean mixtures AND lean mixture=heat (which needn´t correlate as I wrote above) then how can be true what is in the mentioned article (* <= s.above)

- sometimes you read about a cracked blocked or the porous block syndrome. What is the difference between ?


(now .. I propably wouldn´t think about this that much if I wouldn´t try hard to tune my Megasquirt to give the V8 power on the relevant map bins and economy at other bins. I don´t want to follow false ideas by doing so and do especially the economy thing to a sensible maximum)

#2 FridgeFreezer

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Posted 25 March 2008 - 11:31 AM

I believe the Des Hammill book was written way before the later Thor EFI setups came along so the contradicatory articles are probably not talking about the same thing.

Also I would say that fitting MS and tuning it properly is both cheaper and more likely to produce a good (&safe) tune than chipping an ECU with a generic "tuned" map.

I've never heard that using head studs reduces the chances of liner problems, but since I have head studs fitted B) I won't complain :D

I think Ian (BBC) has some data on the fuel maps and has commented they run incredibly lean in some areas.
Yes*
*No

#3 paintman

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Posted 25 March 2008 - 08:54 PM

Porous block.
General term much bandied about in forums.
Seems to be an idea that water in the cylinder gets there by seeping through the block. Through a steel liner??????
What does happen is that the liner moves. It is held in the block as an interference fit. Small cracks in the block metal release the grip on the liner & that's game over. Coolant can now access up the side of the liner & enter the cylinder or via the head gasket.
Top hat liners (much disliked by RPi who sell blocks & engines, offered as uprated blocks by Turner Engineering who are specialist engine reconditioners) are clamped in place by the heads & can't go anywhere.

#4 Young bobtail Rhys

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Posted 25 March 2008 - 09:02 PM

This will not be your problem, however can the block develop cracks if it goes from one extreme temperature to another. e.g, running at high temperature and then driving through water at well below freezing.
Rhys

#5 FridgeFreezer

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Posted 25 March 2008 - 09:03 PM

I have to say RPi are very good at selling things, and often that coincides with offering good advice - but sometimes it doesn't.
Yes*
*No

#6 Bull Bar Cowboy

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Posted 25 March 2008 - 10:17 PM

Points worth noting…………..

I have never seen or heard of a 3.5 with a dropped liner………………………

Yes, the problem occurs on all engines with the thin liner 94mm bore.

Yes, it has happened to the very late engines……….see my next comment

The issue is often not the liners dropping, slipping, or the common misnomer that the liner is porous…………. It is due to the high pressure gases being forced between the liner wall and the block during the compression and the firing stroke. This is because the liner to block ‘seal’ sits within the ‘fire ring’ of the head gasket. This is the main method of failure……… I guess that in time it may well cause the liner to slip a little (only a few thou). Once the gases have been forced into the coolant system, then the system releases the excess pressure on the inlet and exhaust strokes by the same mechanism……….. this in turn creates a ‘path’ that gets worse over time…….. hot gases followed by coolant & stem many time per minute. Pressure release into the combustion chamber will also happen whilst the engine is at rest.


Yes, the liners are an interference fit ………. IIRC 3 thou………..

Top hat liners place the fire ring on the liner. The piston to liner seal is therefore well outside of the high pressure gas zone. This is known to be a permanent fix.



I am not so sure that more heat is produced by a slightly richer mixture…….. I will check up…………… but certainly an engine running lean will run very much hotter…….. this is a well known fact that applies to any engine, not only the V8’s.

I think there is some merit in the RPI diagnosis, as I believe that the failure is temperature related, however, the failure is always there and waiting to happen due to the poor design of the liner to block seal being within the compression area. I guess we must also look at manufacturing tolerances here……….. some block / liners will have a different tolerance fit………. I guess the tighter ones will last longer, and the loser fit combos will always fail first. The minute cracks are caused by normal metallic stress over a time period. Also I believe, some blocks came out of the factory with minute air holes in the castings, which give rise to the porous block syndrome. It is not the liner that is porous, it ids the area that is behind the liner that becomes porous, thus allowing forced high pressure gases into the coolant system.

Mine went on the No1 cylinder ………. when the old liner was carefully machined out, it was clear that the block was porous with a couple of minute pin holes. Yes, it appears that the area of concern is almost always near to the top of the liner. Some will have you believe that the holes and cracks are tig welded before being machined to the new liner size which is much larger than the original liner. However, it has been found that by CNC machining the new top hat liner size to a predetermined interference fit, and the fact that the seal is no longer sitting in the ‘fire ring’ (combustion zone), then the holes and cracks can be left as these are ‘sealed’ by the new fit.

LR had a run of castings produced by Cosworth to see if they could resolve the ‘pin hole’ syndrome and also produce a slightly thicker wall. These castings are still around and very sought after items…….. some folk confuse these with later blocks as this all happened at around the same time. However, the run was limited to a few hundred castings.

Any engine that has the original 94mm liners is at risk, regardless of what fuelling system is used, primarily due to he failure mechanism as described above……….



:)



Ian

#7 landybehr

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Posted 26 March 2008 - 09:04 AM

[quote name='Bull Bar Cowboy' date='Mar 25 2008, 11:17 PM' post='246972']


Cool, I´m making progress :)

Some things that came to my mind. May influence somewhat, because more heat => more stress to metal and more issue with different metal expansion of alu and steel

Combustion temperatures: my 3.9V8 was one of the first batch produced and has a 8.13:1 C/R. This together with it´s spark plugs (11?) indicates to generally low combustion temperatures. The 4.2V8 I have has 8.9:1 which is kind of compromise. Most engines will have 9.35:1
A friend who runs a garage remembers that the problem started when the HighCompression V8 was introduced to the RangeRover. Though this has to be based on statistic low number of cases as the V8´s weren´t too numberous around.

Block temperature: the 3.5V8 at least in Defender tune has a 82° thermostate. The RangeClassic EFIs usually 88°C and the P38, I think, way above 90°C. That´s why I chose an 82° themostate for my 4.2V8 which is easy to do with Megasquirt. 74° was considered but I think it´s too extreme and if wear would be accelerated with it that wouldn´t be tolerable.



BTW: Once it was aked, when you repair a cracked block with a Top Hat Liner, if coolant could be able to go down towards the sump as the way up is blocked by the liner. My answer would be "no way" because I think that the coolant might have done so already when the damaged engine had been in use AND the cracks are more at the top of the liner. There is still a lot of liner´s length below which is in an interference fit to the block.



Cheers,
Hendrik

#8 pugwash

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Posted 27 March 2008 - 06:26 PM

it would be fair to say that as BBC says most problems are liner related and as such calling the issue a porous block or similar is a bit of a misnomer.

Ideally when top hat linered blocks are fitted there should be a lip machined into the block at the bottom of the bore that the bottom of the liner meets, it only needs to be small. The liner is then slid into the block and machined flat- you then get a totally sealed Liner top and bottom, which gives a problem free block.

It really strikes me that the majority of the larger castings have problems- whereas none?!? of the 3.5s seem to- have landrover overbored the block? you'd have thought not seeing as it is dimensionally the same as the Buick 300! Perhaps the castings just aren't up to much- i have often heard of the so called "blue" castings that were supposedly better, but never seen any proof behind this. One thing i have often wondered about is the difference in expansion rates between steel and aluminium- usually you would say this was a good thing- Ali expands twice as fast as steel which should clamp the liner in place, but what happens on cooling- is there any possibility that the liner retracts slower and could feasibly creat a slight gap for any porosity to manifest- over time the cdonstant difference in expansion rates- especially where the ali block is quite thin and thus changes temp more rapidly as in the larger blocks- could create more issues. Its a theory of mine that i've never seen explorered but which i dont have the knowledge to validate.

both my most recent RV8s have been tophatterd , and i wouldn't buy another large RV8 without this issue.

Do the TVR boys suffer in the same way?
Jim

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#9 Bull Bar Cowboy

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Posted 27 March 2008 - 07:12 PM

Do the TVR boys suffer in the same way?



yes ...............



Ian

#10 landybehr

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Posted 27 March 2008 - 08:23 PM

Pugwash,

I think the same way about expansion of alu and steel and that the liner should be clamped more tightly at higher temps. On the other hand I heard about liners slipping. Not when the engine is cold, but when it is warmed up then the liner is said to lost it´s interference fit and could be moved up/down by the piston causing a knocking noise.

BTW: does the 3.5block generally have a better interference fit, or is better at defending the fit against combustion forces than the larger bores ? - As all the engines have the same design of head gasket seal and thus should offer the same weakness to the effect Ian described ?

#11 Hybrid_From_Hell

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Posted 27 March 2008 - 10:28 PM

3.5 blocks have more ali so as such can dump internal heat faster and not reach the critical levels the bigger CC V8 do, also the squish area is different. I have never ever heard of a 3.5 with liner issues carb or EFI

LR saw liner / block issues on boring out capaicities, less metal and bigger CC = heat = problems, coupled with silly lean engines for economy on cruise = the problem. Only some get this, basic servicing can help, or is that poor / lack makes worse, esp oil and water levels. The engines often have to have several 000s miles usage before the prob kicks in, some can never be affected if your lucky. The qulaity of the castings is 2Varaiable" shall we shall, as was / is LR Build qulaity and quialty control

LR didn't use THL as they add hugely to machining set up and costs.

THL do with quality machining solve the issue, as we have seen with Ians engine, the compnay I pointed him in the direction of does some of the finest machine work I have ever seen - they do a HUGE number of THL for V8s.

There is a huge amount of misimformation around re Porus blocks etc, much is myth .......
past on by people who have only heard it themselves etc etc etc,
you can't really tell if an engine is more likley to develop a prob vs another, its all internal, having it tested would cost more than a THL set :lol:

At the end of the day this could go on and on and on, bigger cap V8 can have problems THL sort this = end of ?

Nige

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#12 Bill Shurvinton

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Posted 27 March 2008 - 11:02 PM

I am not so sure that more heat is produced by a slightly richer mixture…….. I will check up…………… but certainly an engine running lean will run very much hotter…….. this is a well known fact that applies to any engine, not only the V8’s.


Peak EGTs are generally at slightly richer than stoich. Think of it this way. At or around 14.7:1 you will burn all the fuel and most of the 02.

At best power mix (usually around the 13:1 mark) you use up all the 02, but have excess fuel, which cools the combustion, giving lower EGTs

At less than stoich there is not enough fuel to consume all the 02 so EGTs are lower.

When people talk about running lean, they mean lean wrt best power mix. so at full throttle 14:1 is dangerous.

At light cruise it is no problem.

#13 HoggyN

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Posted 28 March 2008 - 12:09 AM

you can't really tell if an engine is more likley to develop a prob vs another, its all internal, having it tested would cost more than a THL set :lol:


Is there an easy (cheap) way of telling whether a engine has suffered from a 'porus block' problem? I mean by pulling it apart. I've got a 3.9 under the bench that I'm about to rebuild and I don't want to waste time and money on a duff block.

#14 Mo Murphy

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Posted 28 March 2008 - 02:04 AM

Oh dear, someone didn't read this thread before they posted did they ? :)

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#15 Hybrid_From_Hell

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Posted 28 March 2008 - 07:23 AM

Have a lookie at BBC V8 Thread.

If a engine HAS NOW a slipped liner as oppossed to may develop the problem, then often the head valve / combustion chamber and the top of the piston matching has a sparkly steam cleam look where as the others all have carbon and colour, the steamed clean look is a giveaway :lol:

Other than that look very very carefully at the top of the liner and the surface of the block, they should ALL be flush, if one looks lower and you can feel the difference in heights between block and liner look out, anything more than a fraction treat as very highly suspect :(

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#16 HoggyN

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Posted 28 March 2008 - 09:53 AM

Oh dear, someone didn't read this thread before they posted did they ? :)


Err.. Yes. Twice actually - it was all very interesting but none of it answered my question :unsure:

#17 Bull Bar Cowboy

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Posted 28 March 2008 - 10:30 AM

unless the engine has been run for a long time then the visual symptoms are not obvious and the visuals could also be that of a leaking head gasket.........here I am talking about steam cleaned plugs and piston crowns, white / grey smoke on a hot restart, pressurizing of the coolant system (particularly evident in the top hose), and a header tank that smells like an old exhaust pipe..............


Probably the safest check is a compression test............... if the compressions are more or less OK, then suspect a liner to block problem................ if the compression on one or two cylinders are way down then suspect a head gasket problem...............

See here (the last few pages)....................http://forums.lr4x4....opic=14006&st=0


:)


Ian




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