IMS data collection update - 132 cars (Page 3)

What I find interesting is that the data shows that the most valuable models (C4S) are the most failure prone. That suggests to me that the market is permeated by fear and irrationality.

More data would enable more robust conclusions.

The fact that these cars have fragile engines is already built into the price.
Interestingly an engine rebuild appears to add very little value. I'd say that the majority of potential buyers wouldn't have a clue about IMS or any of the other potential pitfalls for that matter. If you actually take a look at the number of cars for sale with well north of 100k on the clock it shows that you are more likely not to need a rebuild as a result of IMS failure.

Our experience is that some original bearings were fitted tighter than others into the IMS housing limits and fits always make this possible) and these ground away more "running in" surface imperfections early on when the bearing was filled with grease and this turned some into a type of grinding paste (metallic particles and grease).


The outside of the early bearings have a very thin wall thickness and can easily distort out of round inside their housing and this can alter how nicely they run and how much running in metal they rub off early on.


The seal is relatively ineffective - but gradually the heat made the grease thin (like oil) and it leaked out. The IMS bearing is at a level that the sump oil bath is at and anyway there is a lot of oil being thrown in all directions by the chains and sprockets - so oil was always being thrown at the bearing seal.


Failures seemed depend on fate and whether there was too much grit retained before it escaped out of the seal and before enough oil was forced back in to lubricate it.


Therefore some failed quite early but if they lasted beyond that the seal became more worn and allowed more oil in and that washed out any remaining grit and the bearing could last a very long time.


The first bearings were double row but spec wise not much different to the single row that replaced it (because the double row had less recess area for the balls to run in compared to a single row).


When the bearings are replaced the hole they fitted into has often changed shape and often has "pick up metal" (compaction) on the surface so removal and refitting can result in an even tighter and more out of round fit for the replacement bearing than the original and if it is done while the engine is still together it is difficult to feel if the fit was right on assembly.

Two weaknesses therefore were the fit of the bearing and the resulting metal wear and grease and how quickly one ran out and the oil managed to get in. A further problem is making things worse by trying to fit a replacement in situ.


If the IMS bearing has survived a long time and high mileages the original fit will have been OK and the seal will by now allow more oil in - so they can then last a very long time.


Replacing the bearing in situ can strain the chain, become too tight and even damage the chain runners - so although you get a new bearing - it might not make things better (and bearings are made with different qualities and clearances) but because of the different fits in the housing a very high quality tight bearing may be worse than a lower quality looser one (since the greater clearance of the loser one can accommodate out of round housings better).


However the small single or double row bearings are verging on being marginally too small for the loads applied and the larger replacement bearing takes more loads comfortably and has a thicker outside diameter (so doesn't distort as much on assembly) and seems much longer lasting despite still having a seal and being grease filled from new.


Removing the seal allows more fresh cool oil in and more debris out - so seems to improve things.


We see relatively few IMS bearing failures so hence if it has already lasted high mileage it might be best to leave it unless the engine is stripped when the fit of a replacement bearing can be more easily established or it can be replaced with an IMS with the bigger bearing (and no seal).


Numerous replacements available have better or worse outcomes. Rollers take more radial loads but less axial ones, ceramics can fail because they are too brittle if there are a lot of vibration loads present, a plain bearing is probably best of the same size replacements but requires an oil feed and is therefore more expensive.


The larger ball bearing without a seal - so far seems the best all round compromise to solve a difficult issue technically and in practice at a modest cost (if the engine is stripped) and a plain oil fed bearing (if not).


Baz

Great insight and explanation Baz. Thanks for posting. Seems like it is a luck of the draw, though thankfully relatively rare! Seems there is no right or wrong answer to avoid occurrence, so literally 'fingers crossed and hope for the best' you are not the x% failure affected.

Seems the only 'no brainers' is regards the seal / or oil feed system to maximise oil flow and change the IMSB when being overhauled.

I've got to agree with Griftter though. The 'market perception' is that the 996 has significant weakness in the engine. I would say 50% of people I talk to at car shows and meets - not normally, but including Porsche specific, ask me if I have changed my IMSB and why am I waiting. Many say they wouldn't buy a 996 unless it had been changed.

Perhaps it is the financial consequences of failure that grabs the headlines rather than frequency that fuels the perception...

...unless you have a spare £10k plus sat doing nothing then it remains my view, despite whatever you take from this data, that anyone would be mad to contemplate buying a 996/7 with the M96 engine without engine work....

I am inclined towards a view that buying a leggy nice condition model which has had some money spent on it in the usual places and then getting the engine rebuild represents an excellent proposition however...

Same goes for every single car on the road. Oil pump failed on Les' Boxster the other month and destroyed the engine. Are Boxster owners mad too now? I once owned a Cavalier and the oil pump failed and it spat a con rod right through the block and radiator. All engines have the chance of failing and when they do they cost a 4 figure number to rectify.

Baz's post is interesting because it's the first time I've read an authoritative view that manufacturing tolerances might at least in part explain why some engines fail prematurely. Thanks Baz.

I bet if we had enough data we could identify roughly which 'batch" is more at risk. Anecdotally it's always seemed to me that 97-99 engines are most at risk from IMS (those fabled iron-coated piston engines...did the tooling for the bottom end change at the same time?...) but of course it may be that many more were produced in that period hence more observed 'failures". For sure, as Baz explains, the dual row bearing seems no better, anecdotally. I'm equally confident that most that would fail prematurely will have done so by now. But mud sticks!

Anyway we'll never know. You do your research, pay your money and take the risk. Or not.

Look on page 1. Most the failures reported here were registered within a month or so of each other.

Griffter said:

Baz's post is interesting because it's the first time I've read an authoritative view that manufacturing tolerances might at least in part explain why some engines fail prematurely. Thanks Baz.

I bet if we had enough data we could identify roughly which 'batch" is more at risk. Anecdotally it's always seemed to me that 97-99 engines are most at risk from IMS (those fabled iron-coated piston engines...did the tooling for the bottom end change at the same time?...) but of course it may be that many more were produced in that period hence more observed 'failures". For sure, as Baz explains, the dual row bearing seems no better, anecdotally. I'm equally confident that most that would fail prematurely will have done so by now. But mud sticks!

Anyway we'll never know. You do your research, pay your money and take the risk. Or not.

Click to expand...

Maybe I've picked you up wrong or looked at the graph wrong. Is it not the later 3.6 cars that have recorded more failures?

3.6 is higher failure rate. But don't forget, 3.6 was more torpue & bhp so higher load on the bearing.....and C4S was by far heaviest model so even more load.

Got ya now Alex 👍

Not saying that's why more 3.6s failed but it may have been a contributing factor.

NedHan79 said:

Maybe I've picked you up wrong or looked at the graph wrong. Is it not the later 3.6 cars that have recorded more failures?

Click to expand...

Sorry, yes you're quite right. I had only the 3.4 litre engines in mind but I didn't actually say it! 3.4 litre engines have had IMS failure, but none are recorded here, in fact I don't remember reading about a current (as opposed to historic) 3.4 litre IMS failure for years. I was just speculating as to why some went pop and others didn't, and why failures seem rare now - despite the hype and fear!

I think Baz summed that up perfectly well. When I inspected mine around 95k the seal had some thick tar like oil behind it but the bearing was like new with no play. I pinged the seal off so fresh oil now lubricates it and nearly 40k miles later still running well.

Given european sales of @

31k between 1997 and 1999 and;
85k between 2000 and 2005

and considering subsequent survival rates coupled with owners proud enough to be part of the 911UK community I'd say it's reasonable to see more 3.6 failure reports of 3.4 based on numbers alone - there are simply more 3.6's in existence to go wrong!

Not the only probability factor agreed, but a major one to take into account nonetheless.

It's a numbers game.

Unless you've owned your 996 from new you don't truly know how that car has been treated by previous owners. The odds are, better than most marques - something to take comfort from.

The 996 is a victim of the internet forum generation. Don't believe the hype, eyes wide open and enjoy them for the performance bargains it's their destiny to remain. We, the owners, have the advantage here!