Coolant leak at coolant temp sensor... again.

Howey said:
Sorry i fail to see any reason as to why you are an "Idiot"?
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I’m an idiot because it was a HORRIBLE job last time... and I didn’t manage to fix the problem. Only an idiot would voluntarily have another go!

I’ll pick a lovely day, get a beer and maybe remove the undertray so when I drop my 10mm socket and 2 of the bolts I’m not fishing for 90 mins with an extensible magnet like last time.

Hopefully the job will be easier on attempt 2...


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On the bright side, my replacement expansion vessel DID alert me that the coolant level was low today!

Although it only came up for a minute or so and when I pulled over the vessel was pretty much empty. (I had checked it before I set off but I assume it emptied once I ran the engine a bit - silly me).

It seems a bit temperamental when it will alert me to a low level. Even when I unplug the sensor the car doesn’t immediately alert.

Does it normally only check it infrequently or will it only alarm after a delay or should it alert immediately the level falls below the line and stay alerting until fixed?


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Did you have any luck with sorting this? Now the weather is warmer and the kids have gone back to school, I’ve got some motivation to change my thermostat (again!) to see if I can improve the warming up time on my TDi. I’m going for a Gates type (TH11287G1) as they seem to be recommended, which one have you gone for?
 
Well hopefully that’s the cause of this identified and sorted... fingers crossed.
 

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A bit of advice to anyone who is having an issue of leaking from this coolant flange area - and it isn’t fixed by replacing the coolant sensor and seal.

The coolant flange is very cheap to buy, particularly compared to a new gasket alone - and comes with a gasket. About £8.

You need a deep 10mm socket to remove a nut from a threaded rod. A normal socket won’t do, an open socket might. I got one about 50mm long and that did the trick. Access is horrific otherwise.

A mirror on a stick is really helpful to visualise the area. It’s not an easy place to get your head into.

Remove as much as possible from above it to improve access.


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Glad it is finally OK. Mentioned back in post #2 about carefully checking the housing. A seal is always in a groove, so the missing part of the housing required the housing ( coolant flange ) replacing. If you have had a tandem pump leak diesel I would automatically replace this housing.
 
audifan said:
Glad it is finally OK. Mentioned back in post #2 about carefully checking the housing. A seal is always in a groove, so the missing part of the housing required the housing ( coolant flange ) replacing. If you have had a tandem pump leak diesel I would automatically replace this housing.
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Let’s not jinx it! It looks good... so far.

The housing looked okay the first time but when I pulled it off again today and paid particular attention to the facing that section of edge came away with only a little pressure - it was cracked all along a few mm down.

The replacement seal itself cost more than the whole housing. Just change all of it!


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Just done a 300mile trip over the weekend. Rough, hilly terrain and about 11hours of driving... and the coolant is still perfectly on the line.

Well done @audifan. Next time anything even looks like being a ‘seal leak’ I’m going to check the cost for the whole housing first.


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And just so everyone knows. This part is common across ALL the diesel engine codes ANY, AMF, BHC and ATL.

Audi part number 038 121 132 C
 
Ouch!

Mine was £7 from AUTODOC and I thought it was cheap but that’s INSANE!!!

I did see a video where someone had milled a solid aluminium replacement of the same/similar part for a TT because this flange is apparently a common failure on higher mileage Audis.

Now that I’ve done it twice (and have a mirror on a stick, an extra deep 10mm socket and some hose clamp pliers) it’s actually not that bad a job. Also having cleaned the area thoroughly makes things a lot easier too.


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What makes it worse is that if even a small amount of diesel leaks down from the tandem or a fuel pipe and sits on top of the coolant flange it will get at the rubber seal. The seal is rated for water and not diesel and starts to swell slightly. This in turn exerts pressure on the thin walls of the seal channel groove. Very small cracks start that grow and split the base of the channel and now you have the start of a coolant leak. This may take months or years but the result is the same - coolant loss. As for a fix to the problem, PERHAPS a different material seal that is both diesel and water resistant, a metal gasket around the edge of the rubber one, an all metal flange, a spray shield over the flange to divert any diesel leak away from the flange, or just treat the part as a consumable item and plan to replace it if replacing the temp sensor and or the thermostat. Or when you do your coolant change ( you do change your coolant don't you? ) replace it then. Plastic it not the best material, especially the plastic Audi use for long term durability against heat and cooling cycles, then add any extra issues like fuel contamination or impact damage then the components can fail unexpectedly. There are a few plastic coupling pipes on the FSI that are currently seeing this very issue. Once we are aware it is a problem then we can try preventative maintenance while we can obtain replacements, but as the parts get scarce then an alternative must be found.......
 
Pinkythelabrat said:
Just done a 300mile trip over the weekend. Rough, hilly terrain and about 11hours of driving... and the coolant is still perfectly on the line.

Well done @audifan. Next time anything even looks like being a ‘seal leak’ I’m going to check the cost for the whole housing first.


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Hi Pinky, are there any tips you can give me on pulling the sensor out, when replacing the coolant flange? Did you use a small screwdriver or anything to unclip it? I’m changing my tandem pump so access is a lot easier for me, but no idea how to pull the sensor out the back. Did you disconnect the battery to do it?
 
Remove the securing clip and pull the sensor out. It is only held in by a thick O ring. Make sure you know the orientation of the socket so the wiring fits back on correctly.
 
froggy said:
Hi Pinky, are there any tips you can give me on pulling the sensor out, when replacing the coolant flange? Did you use a small screwdriver or anything to unclip it? I’m changing my tandem pump so access is a lot easier for me, but no idea how to pull the sensor out the back. Did you disconnect the battery to do it?
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If you are replacing the coolant flange then no need to remove the sensor in the car, just do it on the bench. Even better if one is already fitted to the new flange.
 
Catnip64 said:
If you are replacing the coolant flange then no need to remove the sensor in the car, just do it on the bench. Even better if one is already fitted to the new flange.
Click to expand...

I'm taking a break from the driveway at the moment, and having had a flashback to reading something along the lines of my current issue I've searched for "coolant sensor o-ring+a2oc.net", and it brought me straight here.

Have got the temp coolant sensor housing off in an attempt to try to get the project car usable for the return trip Scotland to Plymouth. There's been a very gentle dribble of coolant that dries onto the gearbox since before Christmas that I've been unable to trace as it is never wet - I suspect it only comes out when fully up to temperature.

As noted above, getting to the two hex bolts on the threaded rods is a bit of a pain, and there always seems to be a brake pipe from the ABS unit or a/c pipe in the way of the handle of the locking pliers to get the spring clips holding the pipes on.

The housing has come off to reveal that the o-ring that mates onto the block surface is completely flat / compressed. There is a very small mark of dried coolant directly underneath the mark on the block where the o-ring was located, which is higher up than any other marks I've seen, so I'm hoping that it is this seal that has been allowing coolant to blow past.

As noted by Graham above (@audifan ), one of the earlier jobs I had to do on this car was replace a leaking diesel pump, so I am hoping that it was the effect of this that has slowly caused the seal to fail. A couple of the rubber pipes were replaced at the time owing to diesel contamination and going soft.

The L-shaped metal coolant pipe below this also looks a bit crumbly (and I have a brand new replacement for this), but I am going to try reassembling once I've cleaned up the block surface and hope that this has solved the primary problem for now.

Ironically, I bought the new housing when the deal price posted was posted above just in case, so it has been waiting until now to be fitted!

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It was all going so well.

Got the housing (with sensor installed) back onto the block and tightened down, then replaced lower and rear coolant pipes. The rear was a real struggle to shimmy into place as it was being fouled by the smaller coolant pipe that runs from the block behind the sensor housing, underneath the main pipe to/from the cabin heater, then upwards to the top of the expansion vessel.

I realised that loosening this off may allow the rear pipe to sit down properly, so I undid the top end of the expansion return pipe and then bent this downwards. Sure enough the rear pipe thenfreed up to sit home. At this point there was a loud bubbling noise as the return pipe drew in air, and a large amount of coolant poured out of somewhere at the back of the housing. I couldn't tell though if it was coming from the engine end of the return pipe, or the housing to rear pipe interface, but it kept going for a few seconds as the expansion tank emptied. Worth noting at this point that I hadn't yet refitted any of the spring pipe clips.

I've undone the rear pipe from the sensor housing again, but I cannot see if there is a crack in the pipe, or if the liquid was coming from higher up - but I'm now flummoxed. More irritatingly, I have a box of spare coolant pipes for a Webasto AMF somewhere - but I couldn't find it when I started the job and I'm wondering if it is in my other A2 that is at the garage awaiting tracking... aaaaaa!!!

You'd think with all the frustrations that being a home-mechanic involves that someone would consider setting up an establishment staffed by professionals who do this stuff for a living.
 
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