How is an oil leak from IMS area an indication of failure?

I've seen several threads where it is claimed that an oil leak from the IMS area is indicative of an IMS bearing failure.

Not even taking in to account the possibility of an RMS issue, surely an actual leak from the IMS bearing cover has more to do with it's O ring?

In addition to this it is advised to run a replacement bearing without it's own seals so again this suggests that the cover should be oil tight?

I think if oil is getting to the IMSB cover seal then the seals on the IMSB have worn out as the oil must have come through the bearing.

This may mean the bearing itself is worn - any play or movement in the bearing would cause increased wear to the bearing seal leading to allowing it to pass oil.

But yes if oil is getting past the cover seal that would also need replacing.

If oil is leaking from your IMSB cover seal then at least you know your IMSB has some lubrication!!!

Theres many trains of thought regarding seals or no seals on replacement bearings, or just leave one seal off.

It's not necessarily - the seal failing can literally be just that, nothing more.

Re: How is an oil leak from IMS area an indication of failur

Giff said:

I've seen several threads where it is claimed that an oil leak from the IMS area is indicative of an IMS bearing failure.

Not even taking in to account the possibility of an RMS issue, surely an actual leak from the IMS bearing cover has more to do with it's O ring?

In addition to this it is advised to run a replacement bearing without it's own seals so again this suggests that the cover should be oil tight?

Click to expand...

Leaking IMS cover is not a sign of failed bearing. It is a sign of failed cover gasket. Look up on the design of the IMS.

I guess what I'd don't understand is if the engine design allows oil to both sides of the bearing.

If oil only flows to the engine side then as wasz said leaving that seal off would only oil the bearing.

I have noticed the LN cover have an improved seal design and runs an open bearing that may suggest that a damaged std bearing with broken seals could provide too much oil for the std cover to handle.

If this was the case then an oil leak could indeed be indicative of a failing bearing.

Personally I would avoid the LN bearing option like the plague. Ceramic bearings carry their own problems and they only suggest that they last 40k.

If the oil is coming out, then the oil is getting in.

For an IMS that is good news.

Probably your RMS weeping.

The RMS is fitted to the engine crankcase but the crankshaft is fitted into t separate carrier which is then bolted into the crankcases and as a result there is often some miss-alignment. This allows the seal edge to sit in a different lateral position around the crankshaft journal and effectively encourages oil to slip under the seal edge as it rotates and escape. Some are in line while other can be up to 4 thou or 0.1mm out of line - so this can be a cause of oil leaks as can a worn seal.

Later seals have a self aligning outer skin design which helps the seal face settle in the right position - but then they removed the spring from the new seal - so they often eventually leak as well if the crankshaft happens not to be perfectly in line.

The oil leaking from the IMS support can come through the shaft, the "O" ring or even the three bolts holding the housing to the crankcase as they are tapped through into the sump area lower than the oil level and are often more lose than the sealant they supply on them when they are new and more than that can make the gap up. They often need extra sealant to prevent leaks.

The seal on the bearing is more of a dust shield and has no direct influence on leaking as there is oil all around the housing being splashed by the chain running at up to 40 mph in an oil bath. The seal is not directly next to the crankcase anyway. The seal on the Ims bearing has nothing to do with external oil leaks.

The most common cause of a leak around the housing is an "O" ring (or flat section seal) shrinking and hardening around the outside of the spindle housing.

Overall it is not the best design in lots of ways and if in doubt and you have access to it - best check it out and however you refit it use plenty of sealant!

Baz

it's just the case when the oil contains some cuttings. Then the bearing might be suffering.

I was under the impression that both the single row and dual row IMS bearings of later cars were sealed units. The bearings are not designed to be lubricated by oil from the crankcase, so oil running through them is not a good thing surely?!?!

From what I've read this seems to be the problem!

Going back to the cover's oil seal, I received "101 Projects for your Porsche 911" today and it describes the later double type oil seal.

Is this a worthwhile update to the earlier O ring type cover? Can the later cover be retrofitted?

Sadly there doesn't seem to be any info on what type of cover you have based on year / engine number.

just had my seals done this week. Oil leaking from the bell housing - picked up at MOT. The car was due a Major service so I waved goodbye to it for a couple of days and got some other bits and pieces done at the same time (corroded exhaust bolts replaced etc..). It's the first time out of OPC and the bill will be circa £1200, which I think is good value for all that work.
It still pi$$es me off though that such a 'high status' manufacturer can make a product which such poor designs - and uses exhaust bolts that cannot live up to European weather - ffs £20 more for proper bolts isn't going to kill their profit margins.

The IMS bearing sits away from the crankcase wall so has little or nothing to do with external leaks.

The grease filled bearings are a weird technical decision that I don't agree with.

Grease is a good lubricant if the temperature is not too hot and there is no oil nearby – if it is contained and the bearing is not overloaded. Grease filled bearings need more space to carry grease and wear particles..

The small early bearings have a very thin outside track and it can easily lead to a tight fit if the tolerances and interference fitting squeeze it too tight.

While it specifies OK for all calculated normal loads – a mistake younger designers often make it relying to much on spec books and not enough on practical experience and the shock loads due to the chain flex and snatch are much higher and near the limit to being too high for the bearing size.

As a result some engines and bearings grind away small pieces of metal during running in and this mixes with the grease to form a type of grinding paste that then wears away the balls and track.

How they are driven, the tolerances, how they were fitted, how often the oil is changed etc etc all contribute to the failure rate – still low at probably around 10% over several years but too expensive to fix to ignore.

The 'seal" is actually only a type of plastic dust shield so as the bearing wears and the 'swarf" runs out it wears the shield until oil can also drip in. If that happens to flush out the grease and swarf before failure the oil then lubricates the bearing and extends its life – if not yo have premature IMS bearing failure.

All this occurs inside the crankcases and not at the external face.

To prevent oil escaping from the bearing cap spider support there is an 'O" ring on the spindle and an external 'O" ring or plastic ring. They shrink with age and harden allowing engine oil to seep out.

The threads for the three fixing bolts pass right into the sump below the oil level and oil also seeps past the threads over time. New bolts come with a sealant on the threads but usually not enough to seal the thread so additional sealant is advised.

The Porsche spindle on the early small bearing was also compromised for strength by having a groove cut in it to accommodate an 'O" ring seal (very poor pure engineering practice).

Because of the shock loading even ceramic bearings can fail.

Basically any small bearing is marginal for designed loads (if you know what they are).

The larger later bearing is a better spec but still has the grease and shields fitted. Bearings with shields and running in oil are better because the oil flushes out wear particles to be collected by the filter and cools the bearing.

The chains nearby run up to 40mph in an oil bath so throw oil in all directions at the bearing area to provide excellent lubrication.

Oil seeping from the spider housing will only coincidentally be there as the engine ages and not be a sign of a problem with the bearing itself.

Later IMS with larger bearings only run with later crankshafts due to a change in the chain design from roller to hivo.

Failure rates like this are too high to ignore unless you have financial reserves to pay for the engine repair. Various ways are available to over the potential expense of a rebuild however there are some other weak spots with similar failure rates – so an engine failure may still occur from say a cracked liner, scored bore, and of lesser frequency a cracked head or chain failure – sometimes a crank bearing failure.

As you cannot predict which (if any) failure you may experience - it could therefore be a better bet to cover against any kind of failure and get the other weak spots dealt with during the rebuild.

Porsche warranties (limited in age etc) may cover the whole engine (and therefore it will last for a lot of ,miles again). Some others schemes like ours cover all weak spots during a rebuild for whatever reason (for a very small extra charge) and may be worth considering as an alternative especially as there is no age, mileage or cost limit applied.

The greatest loads to the IMS seem to occur at low speeds and therefore it is not always careful drivers sympathetic with their engines – that avoid failures.

Baz

bazhart said:

The IMS bearing sits away from the crankcase wall so has little or nothing to do with external leaks.


Baz

Click to expand...

The culprit for my leak was indeed the O ring

As it's a little confusing with 3 (or 4 seals) in that location and even though Baz correctly explains the layout, it's always good to see a picture.

The oil seal for the IMSB cover, red seal.




A IMS bearing with both seals removed.



And seals fitted.