Oil Cooler Thermostat Replacement - PeachParts Mercedes-Benz Forum

mach4 · #1 07-22-2015, 07:50 PM

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.

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

Then the thermostat and its spring seat slide right out.

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

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.

Maxbumpo · #2 07-22-2015, 08:57 PM

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

funola · #3 07-22-2015, 10:27 PM

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.

charmalu · #4 07-23-2015, 01:37 AM

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

mach4 · #5 07-23-2015, 09:46 AM

Quote:

Originally Posted by charmalu

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.

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.

charmalu · #6 07-23-2015, 11:37 AM

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

funola · #7 08-04-2015, 04:38 PM

Quote:

Originally Posted by charmalu

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

Hey Charlie, I pulled the pin out and your pm is full!

Walter K · #8 07-24-2015, 02:01 AM

Thank you Charlie!

W.

Maxbumpo · #9 07-24-2015, 10:22 AM

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?

mach4 · #10 07-24-2015, 10:34 AM

Quote:

Originally Posted by Maxbumpo

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.

Maxbumpo · #11 07-24-2015, 10:54 AM

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

mach4 · #12 07-24-2015, 12:12 PM

I always learn so much in doing these troubleshooting and problem solving processes. My experiment in putting the thermostat module in the freezer and comparing that to the plunger length at ambient was based on a completely erroneous understanding of the mechanics of the design. I was assuming a linear expansion process as might be used in a thermal expansion bimetal thermostat or a gas bellows thermostat, but alas this uses a phase-change process. Wax that is formulated to melt at a particular temperature is placed in a contained space with a plunger. When the wax melts it expands greatly forcing the plunger out and when it solidifies it contracts withdrawing the plunger. No wonder I got no movement in my quick test.

Here's a drawing of the essential design.

Of course that explains why the manual states not to disassemble the thermostat in the oil filter housing until it's cooled to 60C.

OM617YOTA · #13 07-24-2015, 03:18 PM

Awesome writeup, thanks Mach4.

I would think testing for a failed oil thermostat could be as simple as touching the oil cooler. If it starts heating gradually, immediately after your engine is started, vs. staying cool for some time and then going up to temp all at once, you likely have a failed thermostat. An infrared non contact thermometer would probably work well enough to diagnose a problem as well.

barry12345 · #14 07-26-2015, 07:44 PM

Who checks their oil temperature? Besides the risk of condensate build up I suspect the effective viscosity of the multi grade oils may remain too low.

It apparently requires oil temperatures in specific ranges to give the higher viscosity ratings claimed on the can. I have often wondered why too many of these engines seem to fail at lower than expected miles.

Because of their odometers I kind off attributed failures to odometer turn backs or periods where the odometers where left inoperatable for long periods of time.

Now I wonder with the large sump capacity on these cars and the generally higher operational temperatures of more modern engines. If we may not be getting in general the higher claimed viscosities of the modern oil?

If so it may be important to have that oil thermostat functioning properly. There have been failures on these engines that I suspected may have been long term low oil viscosity issues.

It would be interesting to get some temperature data from the cars oil coolers as well with a reader. With proper viscosity of the multigrades being reached the oil drag factor may drag down fuel milage a little though as well. One last thought was if the car is used for basically only short trips what viscosity is occurring with the oil when it is colder. For example a 10w 30 may not see a higher than 20 weight viscosity when the oil is cooler.

Now on the other hand since there is so much obvious oil drag on these engines when cold. Perhaps viscosity starts high and drops to no lower than say 30 at the specified temperatures. Based on hot oil always seems thinner than cold.

These engines at least I always thought where designed for a constant 40 weight single grade oil for average spring summer and fall use. Some examples I suspect At least judging by their general appearance have probably reached high total milage as well. Maybe 700k but of course there is no way to really verify if it was done on the original non rebuilt engine or not.

funola · #15 07-30-2015, 10:52 AM

mach4, have you done anymore work on this and what's your plan going forward?

I am looking into the oil temps of my 85. I made a type K thermocouple dipstick probe, drove 45 min on highway, pulled over and took a measurement. It was 181F at the top of the oil in the sump, as I pushed the probe deeper, temp slowly decreased till it touched the oil pan, where it dropped to 167F. I did this multiple times so there is no mistake about it. I know you said you are convinced your oil temp measurement is actual but my measurements indicate otherwise. If I had a sensor at the drain plug where you have yours, I'd bet it will be lower than 167F since it is cooler there due to the greater mass of the drain plug compared to the pan, dissipating more heat to ambient.

I have a complete spare oil filter housing that I plan on doing some testing on. I do not want to disturb the T stat (before doing the test) and will be testing it in the housing. In figuring how to test it, I poured water and filled the cavity where the T stat is and it did not leak out, indicating the T stat is closed. The plan is to fill the cavity with fresh motor oil, heat the oil up with a PID controlled 200W heater element and observe the temp at which the oil starts to leak past the T stat, which would be the T stat opening temp. What do you think?