Oil Cooler Thermostat Replacement
 

Since I installed a set of engine instrumentation sensors on my car (Engine Instrumentation Project) I noticed that I rarely get above 150 degrees oil temperature, and never above 180. Since the oil cooler thermostat in the oil filter housing is set to open at around 200 degrees it would appear that my thermostat is stuck open. Corroborating this observation is the fact that the temperature rises gradually and not all at once as would be expected when the thermostat opens.

So anticipating the need to replace the thermostat I tore apart my original filter housing (I had to take off the OM617 filter from the 123 chassis and replace it with one from a 126 chassis to get steering gear clearance which is why I had a spare one floating around)

Other than taking the filter housing off the engine, it's a pretty simple and straightforward task.

The first step is to take off the valve seat ring. To do so I just used a stout needle nosed pliers and a crescent wrench.

http://www.peachparts.com/shopforum/...1&d=1437608398

With the ring removed the spring retainer, spring, return flow locking valve and its spring come right out.

http://www.peachparts.com/shopforum/...1&d=1437608992

Then the thermostat and its spring seat slide right out.

http://www.peachparts.com/shopforum/...1&d=1437608992

Here's the parts, laid out in order, for the thermostat assembly.

http://www.peachparts.com/shopforum/...1&d=1437608992

So sometime soon, I'll take the car off the road for a series of maintenance projects and pull the oil filter housing with the bad thermostat and swap in the thermostat from my "spare" housing.

I didn't see a whole lot of documentation on R&R of the thermostat, so hopefully this will be helpful for those contemplating this procedure in the future.

Nice, thanks! I've never heard of an oil t-stat failing, very glad it failed open and not closed.

Aren't you going to test the T stat for opening temp before install? I would. Easy to do with a PID controller, SSR and a heating element.

Here is a note in my FSM for the 617.95 engine under section 18-125.

Remove functioning thermostat only at temperatures
below 60c, since otherwise the thrust pin will
be pushed out.

Never pull thrust pin out of a wax thermostat.
since otherwise proper function cannot be guaranteed.

Charlie

sent from my pos computer

It was 28C here yesterday, so I think I'm safe....however, I think your point is that to try to test it, I would need to get it to 95C would push out the thrust pin making the thermostat worthless.

I assume what the FSM was stating, is to change it when the engine is cold.
Testing it would destroy or damage it.

There is info on testing it in the engine.

section 18-120

To obtain a favorable operating temperature of engine
oil at a faster rate, a thermostat with an opening
temperature of 110c (formerly 95c) part no.
117-180-00-75 has been installed in oil filter since
December 1979

checking

1. Exchange oil dipstick for flexible heat sensor
part# 116-589-27-00

2. Run engine at increased speed and watch
Telethermometer.

3. At an oil temperature of 110 - + 4c or 95 - + 4c
depending on thermostat installed, a distinctive
increase of oil temperature on oil cooler should be
noticeable (manually).


under installation

6. Tighten valve seat ring by means of
socket 35 - 45 NM.

7. Peen valve seat ring in both recesses on oil filter
housing by means of a cross chisel.



I took one apart a few years ago, and ground down a Socket to have the 2 pins
so I could remove the Seat Ring.

The other deep recess is the by Pass valve.
I used a socket that is used (I believe) to adjust the tie rod ends on some older equipment.
It sort of looks like a large slot screw driver.


Thank You for posting the pictures, I haven`t seen this posted before.

Charlie

sent from my pos computer

Thanks for the write up and pics. I suspect the thermostat in my 78 300sd has failed shut, as the oil cooler never even gets warm to the touch, but the oil temp rises in any thing other than mild weather.

I finally got a spare filter housing to pull the thermostat out of, as apparently new thermostats are NLA. Now just need some spare time to make the swap.

W.

More
 

I pulled the oil thermostat apart again on the spare oil canister to investigate further.

First I decided to do a quick test to see if the plunger in the thermostat module would move with a change in temperature - it didn't.

At 80 degrees F it measured 41.91mm
At 0 degrees F it measured 41.87mm

It's not intuitively obvious (at least to me) how the thermostat works. I studied it for awhile and determined that the thermostat sits in a piston that moves up and down in the bore. At a particular point the plunger moves outward which in turn uncovers a port in the bore (think 2 stroke exhaust port as a model) which allows the oil to go to the cooler. This appears to work on a linear expansion model where the port being uncovered starts the flow, unlike a water thermostat which opens as the spring pressure is overcome by metal expansion to open a valve.

So next I wanted to see what the difference was with the piston with the thermostat in vs out. I carefully measured the piston depth without the thermostat module (bottom of the bore, totally closed) and then again with the thermostat in. Here are the results

Without module - 21.55mm
With module - 14.98mm

I then noticed that there appeared to be some wear on the plunger indicating a range of movement based on varying temperatures. I guess I would expect that this would be at the more closed position (hiding the wear of the plunger) at ambient and 0 degrees F. But since it's ostensibly "stuck" in an out position, I'm now wondering if the spare module is bad too. After all if it's stuck in the outward position that would tend to imply that the valve is stuck open.

http://www.peachparts.com/shopforum/...1&d=1437680991

Now since the difference between the two positions with and without module is 6.57mm and the wear mark is 4.59mm that puts the bottomed out module at 2mm differential which might be a reasonable neutral point, just don't know.

And just for the record, it's indeed the later version with the 110C opening temperature.

http://www.peachparts.com/shopforum/...1&d=1437681038

I guess the next step is to pull the filter canister off the formerly spare engine (now just bad engine) and check that one out for comparison.

Aha
 

Well, the problem is solved. The thermostat on my spare engine shows what is the proper, fully collapsed position for the plunger at ambient temps. This is the 95 degree module however.


http://www.peachparts.com/shopforum/...1&d=1437683392

And for the inquiring minds, here is the bore of the thermostat. Note how the bore runs deep for the 160 degrees or so to the left (between the three left-most "tabs". The right-most 200 degrees or so (except for the "tab" that keeps the piston centered is cut away deep). There is about 1/4 inch or so at the bottom that the piston rides in for a full 360 degrees - the closed position. Once the plunger begins to extend, the piston rises to uncover a path for the oil to flow through the cooler.

http://www.peachparts.com/shopforum/...1&d=1437683596

This makes 2/3 of my engines with a stuck-open thermostat. Without my engine instrumentation sensors, I would have never known. I'm either extremely unlucky or these fail with greater frequency than anyone realized. Fortunately, I drive a lot and for large distances and this profile has likely limited the negative effect of condensation building up in the oil to the detriment of the engine. I'm sure that even though the engine sump oil is only at 150F normally, as it transits the valve train, bearings, turbo etc. it sees 100C and higher, driving off condensation. Nevertheless, I'll change this out at first opportunity.

I guess the conclusion is - check your oil cooler thermostat for proper operation.

If I didn't have my oil temp sensor, I'd be inclined to get something like this to put down the dipstick tube to verify things are working properly. Dirt cheap, but it only needs to work once -

http://www.peachparts.com/shopforum/...1&d=1437685006

It looks like part# 117-10-00-75 has been replaced by
Part# 117-180-01-75.

http://www.peachparts.com/shopforum/2704828-post41.html

Here is the parts diagram and part# with prices from that dealer in Atlanta.

Mercedes OIL FILTER


Here is the wholesale price from the dealer in Monterey, Ca.

http://www.partswebsite.com/benzpartswholesale/oemparts/mercedes-benz-137/1171800175.html

Charlie


sent from my pos computer

Interesting that part is still available - the stuff I was reading indicated it was NLA

Thanks for the heads up.

Thank you Charlie!

W.

Mach4, your posts go back and forth between deg F and deg C. My point: did your test in post #8 really measure from 0 deg F to 80 deg F? That doesn't seem like much of a test to me, but perhaps you were using a "less than set-point" temperature to avoid damaging the t-stat?

Sorry about the C-F dichotomy...I have my Gauges reading in Degrees F but MB references everything in C.

Yes, ambient was 80F and the freezer was 0F. The manual talks about not removing the thermostat module above 60C (there we go again) so I was just testing to see if I could get any discernible movement in the plunger across a 80 degree temperature delta.

Thanks for clarifying, love the thread! I'm going to have to test both my cars for this issue.