I wanted to document an issue that is becoming more and more common on the M112 & M113 engines now that the years are starting to pile up.
Over the past few years... I have heard of many individuals dive into a head gasket job because of "oil in coolant" or "coolant in the oil". Many perform the entire head gasket job only to see the issue remains.
I have noticed this issue as well on 5 different engines now. 3 of my own personal vehicles and 2 from friends/family.
The reality is: the head gasket itself rarely fails. Most usually, the cause of this issue is one (or both) of the 2 main rubber o-rings that seat between the timing case and engine block.
There were reports years back (when I had my repair shop) of this "rare issue". But I would like to reassure everyone here: this is no longer a rare occurrence. Due to the nature of what happens, this issue will become ever-more common as we move forward. Here is why: the material used for those o-rings actually swells over the years. On one side, that o-ring is exposed to coolant. On the other, it is exposed to oil (even though the factory applies a very thin coat of silicone, that silicone breaks down and eventually gets onto the o-ring).
Once this o-ring swells enough, it literally swells INTO the passage-way where the coolant flows and eventually... gets pushed out of it's spot by the coolant-flow and washed out-of-place and ends up somewhere in the engine.
It usually comes to rest in the cylinder head.
Even a couple engines that did NOT have this issue - but were tore down for other reasons: those o-rings were not far from having failed the same way.
Here are some pictures...
Just removed this timing cover from my 1998 ML320:
![Image]()
Notice this passage is missing the o-ring?
![Image]()
This is where I found the o-ring... this seems to be the most common place it comes to rest: the cylinder head.
![Image]()
A comparison of the original o-ring next to a new (MB OEM!) one.. Notice how severe the swelling is? The o-ring literally swells up INTO the pathway of coolant until the coolant washed is out of it's position. Luckily, they apply a tiny amount of silicone from the factory on the oil side, so when this happens - the leak/contamination is often more subtle and not a catastrophic event. Nonetheless, it WILL begin to leak oil into the coolant or coolant into the oil.
![Image]()
Again, these particular photos are from my 1998 ML320. HOWEVER - I have seen this failure (or about-to-fail) on just about every 112/113 engine I have opened up over the past 3 years (and I have opened several). So this issue affects any chassis with the M112/M113 engines.
Keep in mind also: these o-rings are essentially the same material that your oil level sensor o-ring is made of. If you have ever had to change the oil-level-sensor o-ring, you can bet these timing cover o-rings are not in much better shape! And consider this: after 150-200k miles.. virtually every M112 or M113 engine has had to have the oil level sensor o-ring replaced by now.
I believe over the next few years, this issue will crop up more and more and these o-rings continue to swell and get washed out-of-place. As this happens, many will be led to believe that the head gasket has failed when it has NOT. Unfortunately, if you happen to pull the timing cover and find one of the o-rings missing, the only safe thing to do is pull the heads to ensure you do not leave this floating around in the block as it could cause serious issues eventually if left where-it-is.
Anyway.. I wanted to get this out there so others know: if you pull the heads.. PULL THE TIMING COVER AS WELL!!! Chances are almost 100% that - given the age of these motors now - those o-rings are either near-failure or have already failed.
Hope this helps others.
Maybe make this a sticky somewhere?
Tags: M112, M113, Cylinder Head Gasket, water coolant in oil, oil in water coolant, still leaking, timing cover, leaking
Over the past few years... I have heard of many individuals dive into a head gasket job because of "oil in coolant" or "coolant in the oil". Many perform the entire head gasket job only to see the issue remains.
I have noticed this issue as well on 5 different engines now. 3 of my own personal vehicles and 2 from friends/family.
The reality is: the head gasket itself rarely fails. Most usually, the cause of this issue is one (or both) of the 2 main rubber o-rings that seat between the timing case and engine block.
There were reports years back (when I had my repair shop) of this "rare issue". But I would like to reassure everyone here: this is no longer a rare occurrence. Due to the nature of what happens, this issue will become ever-more common as we move forward. Here is why: the material used for those o-rings actually swells over the years. On one side, that o-ring is exposed to coolant. On the other, it is exposed to oil (even though the factory applies a very thin coat of silicone, that silicone breaks down and eventually gets onto the o-ring).
Once this o-ring swells enough, it literally swells INTO the passage-way where the coolant flows and eventually... gets pushed out of it's spot by the coolant-flow and washed out-of-place and ends up somewhere in the engine.
It usually comes to rest in the cylinder head.
Even a couple engines that did NOT have this issue - but were tore down for other reasons: those o-rings were not far from having failed the same way.
Here are some pictures...
Just removed this timing cover from my 1998 ML320:
Notice this passage is missing the o-ring?
This is where I found the o-ring... this seems to be the most common place it comes to rest: the cylinder head.
A comparison of the original o-ring next to a new (MB OEM!) one.. Notice how severe the swelling is? The o-ring literally swells up INTO the pathway of coolant until the coolant washed is out of it's position. Luckily, they apply a tiny amount of silicone from the factory on the oil side, so when this happens - the leak/contamination is often more subtle and not a catastrophic event. Nonetheless, it WILL begin to leak oil into the coolant or coolant into the oil.
Again, these particular photos are from my 1998 ML320. HOWEVER - I have seen this failure (or about-to-fail) on just about every 112/113 engine I have opened up over the past 3 years (and I have opened several). So this issue affects any chassis with the M112/M113 engines.
Keep in mind also: these o-rings are essentially the same material that your oil level sensor o-ring is made of. If you have ever had to change the oil-level-sensor o-ring, you can bet these timing cover o-rings are not in much better shape! And consider this: after 150-200k miles.. virtually every M112 or M113 engine has had to have the oil level sensor o-ring replaced by now.
I believe over the next few years, this issue will crop up more and more and these o-rings continue to swell and get washed out-of-place. As this happens, many will be led to believe that the head gasket has failed when it has NOT. Unfortunately, if you happen to pull the timing cover and find one of the o-rings missing, the only safe thing to do is pull the heads to ensure you do not leave this floating around in the block as it could cause serious issues eventually if left where-it-is.
Anyway.. I wanted to get this out there so others know: if you pull the heads.. PULL THE TIMING COVER AS WELL!!! Chances are almost 100% that - given the age of these motors now - those o-rings are either near-failure or have already failed.
Hope this helps others.
Maybe make this a sticky somewhere?
Tags: M112, M113, Cylinder Head Gasket, water coolant in oil, oil in water coolant, still leaking, timing cover, leaking
