M129 Heat Exchanger

My 250SE Cabriolet had a feature I was not aware of.    Underneath the injection pump is an oil/coolant heat exchanger.   This was also an option on late 230SLs.    Instead of a separate cooler that the M130 engined cars have, the heat exchanger helps warm up the oil when the engine is cool and cool it down when the engine is hot.

There are lines from the oil filter housing that go to the heat exchanger, as well as coolant lines that come from the side of the block.    An example is pictured below.  The picture is not my heat exchanger.  It is a second hand unit available on eBay.   From all I read on the Pagoda forums, the M129 Heat Exchanger is quite effective.  The 250SL runs quite cool with it.   The heat exchanger is now NLA, but the part number is 000 188 44 01.

M129 Heat Exchanger

I would have been blissfully unaware of my heat exchanger except for a growing oil leak.     The oil leak was coming from the braided flexible oil hose that runs from the far side of the M129 heat exchanger to the oil filter housing.

When my mechanic investigated further, he also discovered that the coolant outlet on the engine block was corroded.

I started looking up what parts I needed to buy to fix the oil leak and make sure the heat exchanger was working properly.    On my car, the longer hose had a short metal section, the braided flexible hose and then another short metal section.    But in the EPC this was one hard line.   There was no mention of any other system.

M129 Heat Exchanger

I started doing some further research and I found a post on the W113 forum. It described how in the paper parts manual, there is an earlier setup which had been superseded. The older setup was never brought forward into the EPC.

In that section you had the two hard lines (127 180 00 27 & 127 180 02 27) plus the braided line (198 180 01 82).   This was replaced by 127 180 03 27 which was the single metal hose.  308 in the diagram above.  In case you’re wondering 127 180 01 27 is the shorter hard line for the other outlet of the M129 Heat Exchanger (302).   This never changed.

While I had the backup of taking the existing braided hose to a hose shop to have the ends re-used for a new hose, I first checked to see what parts were still available.   Since the braided house was first used on the 300SL, it is still available.   The surrounding hard lines are not, but mine were perfectly fine.    Additionally, the replacement part 127 180 03 27 is also still available.   Neither were particularly expensive.  Both are probably cheaper than having a hose company re-use the ends of my existing braided hose on a new one.

I decided to go with the 300SL part.  I had a few reasons for this.  Firstly, there is very little space around the M129 Heat Exchanger.  Getting a flexible hose in and connected with the engine in situ was probably going to be far easier.  This was going to save me labour time.   Secondly, the parts cost was less than half.  Thirdly, I figured the flexible hose had a bit of give in it for vibrations and parts not being perfectly aligned after 60 years.    And finally, I liked the idea of having 300SL parts on my car.

M129 heat exchanger

While the car was at the workshop, I had new rear shocks fitted and the power steering pump adjusted so it wasn’t so noisy.   My oil leak is gone. I’m also sure the heat exchanger working properly will help the car cool on very hot days.    I also learned a lot about a feature of my car I never knew it had.

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