Water Leak Advice
 

Hi All,

Just after some advise about a water leak I discovered after work this evening. There seems to a small damp patch under the plastic pipe between the thermostat and the engine inlet/outlet ......whichever it is (on the left side of the V on a KV6 engine). But when you look under the engine, you can clearly see a drip....drip....drip coming off the oil sump.

I don't think I'm loosing much from that plastic pipe, but could it be coming from somewhere else?

Thanks in advance

Replace all the plastic components while you are in there, itll be one of them that has fractured.

Hi Simon,

What you have is a common problem with the KV6. What has happened is that one or more of the 'O' rings on the plastic coolant parts has flattened and coolant is escaping into the 'V'. This gives the impression that the plastic parts have split but they actually have not.

You'll need just four new 'O' rings (CDU3858) and, after reassembly, ensure that the strange plastic clips on the straight pipe are pushed fully outwards to make close contact with the water pump and thermostat housings. Crucially this simple operation should prevent a recurrence and the drips on the sump will magically stop! :D

Simon

Do not refit the old plastic pipes or stat, they will have suffered some form of degradation.

Without wanting to dig up all the history of the degradation suffered by the coolant issues, can you post pictures please of the pipes when you have removed them.

I'm with Trikey - even if it's only the 'o' rings I'd still replace the plastic pipes whist I was in there, it's a false economy not to. :}

If you can afford it, I would replace them with stainless steel ones, too. :D

Simon (Redmaestro),

Just to give you a bit of history on this problem. This was the original belief when coolant leaks were found below the thermostat housing. We all know that plastic takes a very, very long time to degrade, certainly in excess of the life of a 75/ZT! This led me to search for the truth. I found that the plastic pipes and thermostat housing do not degrade and I have proved this by refitting mine more than once. I asked on the forum for evidence of this alleged degradation or splitting of the thermostat housing joint as some claim. Only one person came forward who pointed to the sort of manufacturing marks which you see on every plastic component if you look carefully.

You can renew the plastic parts if you like, but you are wasting your time and money. Four ‘O’ rings and setting the straight pipe clips properly is all that is needed to rectify this problem.

Simon

These are bold statements indeed Simon, especially when you consider that Andy is a specialist in thermoset polymers.

So let's get back to your theory, in terms of O ring failure, what you refer to as "flattening", can be more accurately described as "compression set".

This is caused normally by excessive temperature development, causing the O ring to harden and lose its elastic properties, or volume swell of the O ring due to incompatibility with of the elastomer material used in the O ring material and the fluid it is sealing.

So if we assume the mechanical size and specification of the designed use of CDU 3858 is correct, the failure type that you describe due to "movement" must be caused by breakout friction, where the two parts sealed by the O ring are subjected to movement and causes abrasion or tearing.........no that doesn't sound right you talk about "flattening" occurring and not abrasion.

Now because the cooling system pressure is in operation well below 100 psi, and considering the cap is designed to vent at around 21 psi, it can safely be classified as low pressure, and because of that the system pressure alone is not great enough to mechanically activate the seal, thus the O ring depends solely on the resilience of the elastomer to both the temperature it is being subjected too, and it's chemical resistance to the fluids it is in contact with to retain its original compressive force.

So which is it going to be then.

A) Incorrect specification of dimension for CDU3858 by MGR.

B) Thermal cycling properties incompatible with shore hardness of elastomer used in CDU3858

C) The resilience of the elastomer used in CDU3858, being incompatible with the fluid it is placed in contact with, i.e. OAT coolant.

You may like to note that none of the above reasons for failure include mechanical movement of the two parts being sealed by the O ring, as this simply does not fit the model of failure you describe ;)

Brian :D

Brian,

A) I have considered this. CDU3858 is 32.5 mm ID x 3 mm cross section. The next available larger size off the shelf was 32.92 ID x 3.53. The protrusion was too great for the clearance into the port, the ‘O’ ring bowed out and the force required to fit it resulted in the housing fracturing. It was possible to fit 33.0 ID x 3 but this also leaked after a period in service. This suggests that the size of CDU3858 is appropriate, but see also the first paragraph of B).

B) There are many KV6 engines which have not suffered this leakage; only a small proportion have. It therefore follows that thermal cycling in the normal operational range of the engine cannot be responsible.

As far as abnormally high temperatures are concerned (115 degrees C and above), my engine has never experienced this yet I have had recurring leaks. The leaks therefore cannot be attributed to abnormally high temperatures.

C) I know of four KV6s which have used OAT at 50% concentration all their lives and have never suffered leakage. Additionally, I have fitted EPDM ‘O’ rings (a different material to that used for CDU3858) and the leak still occurred. It therefore follows that there cannot be an incompatibility between OAT and the original ‘O’ ring material.

So it is clear from this evidence that the cause is something other than your A), B) and C).

It is easy for anyone to sit at their computer and theorise but if it was as simple as that, there would be no need for experimentation or trials. We all know that even the finest scientific brains in the country test their theories and are often proved wrong. Brian, you have just missed an open day at the National Physical Laboratory. The next is in two years’ time. I recommend that you pay them a visit when you’ll be able to have fascinating discussions with the academic staff on a wide variety of technical subjects.

Simon

Apologies for the bad photo of my laptop screen. For some reason I can’t screenshot the details.

Simon, hopefully this will convince you that oat does degrade polyamide plastics (the nylon parts as used in the cooling system)

And please don’t think that just because the test temperature is 138 deg c, that this is the reason for the reduction in strength of the plastic components, I have witnessed the degradation in the classroom at greatly reduced temperatures (Admittedly over a greater length of time)


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