Thermostat replacement

[Sven]

All,

Is there a trick to removing all 4 of the thermostat housing bolts (2 of the bolts are in too close proximity to the injection fuel lines and can't get a socket or good wrench hold) on a 280sl?  Is there an option other than disconnecting all of the injection lines to get solid access to the bolts?  The previous owner changed the thermostat seasonally, but it just doesn't seem that simple to a novice like me.

The car stays runs at a temperature of 180-200 in prolonged idle and stop and go traffic, but aims for the red zone once on the open road.  I figure I should start basic with replacing the thermostat.  Also, could an air bubble in the coolant system display these symptons.

Thanks in advance.
Sven

[Ben]

I was able to release the clamp that holds the lines together and get a slim allen key in there !

However the bolts can be very tight and one of mine rounded out. I soaked them all with PlusGas and then removed the coolant hose from the top, and also the bottom of the 'stat housing and opened the two 13mm bolts ( I think !) and took the housing off completely. I was then able to open the top part easily !

If I was doing it again this is how I would do it from the start. You will need a new housing to block gasket a s well as the "O" ring for the 'stat top cover !

Regards,
Ben in Ireland.
'64 230SL 4sp.

[hands_aus]

When I replaced the thermostat in my 250sl I broke off one of the bolts and I ended up having to remove the housing and take it to an engineering workshop with all the correct tools to remove the rest of the bolts.
This was a pain but it was the best solution given the location of the injection fuel lines.
I replaced the original steel bolts with stainless steel ones but I was advised that normal high tensile steel bolts are better, so I replaced the SS bolts.
If you remove the water pipe (connects to the housing via a banjo bolt) from the thermostat housing to the inlet manifold you will probably need new washers there too.

Bob Smith (Brisbane,Australia)
RHD,1967 early 250 SL, auto

[bpossel]

Hi Sven,

If the PO changed the thermo each season, then you should be lucky and the bolts won't be frozen.  Remove the injection lines clamp(s).  This provides a little more wiggle room.  I use a few different tools to remove my thermo bolts: small socket; small ratchet wrench; and even a screw driver.  One of my bolts has a slot for a screw driver.  Not sure why?

When you install the new thermostat, make sure that the air hole on the thermostat is pointing forward (needs to be sitting at the highest point to release the air).

Take your time and it should work out.  Good Luck!
Bob




bpossel  (Memphis, TN.)
'71 280SL  /  '97 E320

[Sven]

Followup.  I had a litte bit of time to mess around with the thermostat housing over the weekend.  3 out of the 4 bolts were easily turned (couldn't get good access to fourth).  As recommended, I proceeded to remove the thermostat housing and quickly realized I had to remove the alternator to get access to the housing bolts/nuts.  I ran out of time (2 kids under 5 yo) before I could manage to remove the alternator.  I only was able to see two bolts securing the thermostat housing.  Is that correct?

[hands_aus]

yes

Bob Smith (Brisbane,Australia)
RHD,1967 early 250 SL, auto

[bayleif]

I did this just a month ago. Wish I had read this discussion first! I ended up removing or loosening all of the injector line clamps. I then disconnected the lines closest to the thermostat housing, I think they went to cylinders 5 & 6 and the line that goes to the manifold cold start valve. That gave me enough room to get a large screw driver into the slotted bolt on the thermostat. I assume it was slotted because I'm not the first one to do this on this car. I used an open end wrench on the square shaft of the screw driver while bearing all of my weight down on the screw. Fortunately, she broke free (instead of just breaking).
Now I didn't know about the little hole in the thermostat, so if it's facing front it's only by accident. I did have a duce of a time bleeding the air bubbles which I do by loosening the banjo fitting on the front of the thermostat housing. I always know when I have a problem because the idle does not change when the engine heats up.
The new thermostat, now that I got it working, solved my problem. It wasn't the worst thing that I have ever done but I'm not too excited about the prospect of doing it all again. How important, now that I got it working do you think it is to get the small hole facing toward the front?

Chuck Bartlett
1969 Signal Red 4 Speed