Hello!
Last week, I drove my '83 300D up to my mom's house for Thanksgiving...and while I
was up there, I developed a coolant leak. Before driving the 300 or so miles home, I
went to look at what had started leaking. After some digging around under the hood, I
found that it was the coolant bypass hose.
This is a short but wide hose, located behind and under the thermostat housing, behind
the neck to the lower radiator hose. The above picture was taken from the right side of
the engine, after I had removed the air cleaner assembly. It's VERY difficult to see with
the air cleaner assembly in place. I took this picture after loosening the hose clamps.
This hose connects the thermostat housing to the water pump housing, and of course is
a dealer/online only part. So, I jury-rigged a hose clamp and carefully drove the car
home.
After getting home, I ordered the replacement hose. Since removing and replacing this
hose requires you to remove the thermostat housing, I also got a replacement gasket
that goes between the thermostat housing and the cylinder head.
The hose is Mercedes part number 900271 042012, and the gasket is M-B p/n 616 203
02 80. As you can see in the photo, the hose I got was 12 inches long; considering how
short the hose needs to be, I cut a piece off....and I estimate that I now have enough
hose to do this job 5 or 6 times if needed. Good news is, these were all the parts I
needed. This would have been a perfect time to replace the thermostat; however, I had
replaced it within the last month so I figured I didn't need to pull it out again. Good news
is, it's possible to remove the housing without unbolting the cover that holds the
thermostat in place, so I didn't need to disturb that seal
Here's what the thermostat housing looks like from the front, with both radiator hoses
removed. The bolts to remove the housing from the cylinder head are not visible, but are
on the upper left, behind the bracket that holds the a/c line.
I have a ratchet on one of the two (13 mm head) bolts holding the thermostat housing to
the cylinder head. The other one is visible just in front of the EGR pipe. The front one's
a bit difficult to get to with the EGR pipe in place, but it is doable. You can unbolt the
a/c line from the bracket if you want, although I didn't see a need to.
The thermostat housing is removed! You can see, from both sides, what you're working
with here.
Here's what the hose looked like when I pulled it out. Surprisingly, it came out without
any trouble (or any effort, for that matter). Even more surprisingly, this means that this
is what the hose looked like before I pulled it out, including the chewed up section. How
in Hades did I get 300 miles out of that hose?!?
This is what things look like with the thermostat housing mounted and the hose NOT
installed. As you can see, there's really very little space in between the two fittings. If I
had known this in advance, I might have tried to cut some of the hose and try running it
that way. Sorry it came out so blurry; I didn't realize it had come out that bad until just
now.
Putting everything back together is really the reverse of disassembly. Make sure you
clean up the gasket mating surfaces before re-installing. I also put a small bead of
Permatex on each side of the gasket before installing, just for peace of mind. There's
no given torque specs for the bolts holding the t'stat housing to the head, but keep in
mind that the t'stat housing is aluminum, so you don't want to tighten it up TOO tightly.
I borrowed liberally from this article, but I thought that people might appreciate
a write-up that included pictures
On that note, sorry for keeping the pictures so large.
I thought it would be good to keep them as large as possible so that people could see
detail. However, I know it makes this thread a bit large to open for someone with a
slower connection, and also made me format the text in a weird way so that people
wouldn't have to scroll from side to side to read everything. I figured the trade-off was
worth it.
Hope this can help someone in the future
Last week, I drove my '83 300D up to my mom's house for Thanksgiving...and while I
was up there, I developed a coolant leak. Before driving the 300 or so miles home, I
went to look at what had started leaking. After some digging around under the hood, I
found that it was the coolant bypass hose.
You must be registered for see images attach
This is a short but wide hose, located behind and under the thermostat housing, behind
the neck to the lower radiator hose. The above picture was taken from the right side of
the engine, after I had removed the air cleaner assembly. It's VERY difficult to see with
the air cleaner assembly in place. I took this picture after loosening the hose clamps.
This hose connects the thermostat housing to the water pump housing, and of course is
a dealer/online only part. So, I jury-rigged a hose clamp and carefully drove the car
home.
After getting home, I ordered the replacement hose. Since removing and replacing this
hose requires you to remove the thermostat housing, I also got a replacement gasket
that goes between the thermostat housing and the cylinder head.
You must be registered for see images attach
The hose is Mercedes part number 900271 042012, and the gasket is M-B p/n 616 203
02 80. As you can see in the photo, the hose I got was 12 inches long; considering how
short the hose needs to be, I cut a piece off....and I estimate that I now have enough
hose to do this job 5 or 6 times if needed. Good news is, these were all the parts I
needed. This would have been a perfect time to replace the thermostat; however, I had
replaced it within the last month so I figured I didn't need to pull it out again. Good news
is, it's possible to remove the housing without unbolting the cover that holds the
thermostat in place, so I didn't need to disturb that seal
You must be registered for see images attach
Here's what the thermostat housing looks like from the front, with both radiator hoses
removed. The bolts to remove the housing from the cylinder head are not visible, but are
on the upper left, behind the bracket that holds the a/c line.
You must be registered for see images attach
I have a ratchet on one of the two (13 mm head) bolts holding the thermostat housing to
the cylinder head. The other one is visible just in front of the EGR pipe. The front one's
a bit difficult to get to with the EGR pipe in place, but it is doable. You can unbolt the
a/c line from the bracket if you want, although I didn't see a need to.
You must be registered for see images attach
You must be registered for see images attach
The thermostat housing is removed! You can see, from both sides, what you're working
with here.
You must be registered for see images attach
Here's what the hose looked like when I pulled it out. Surprisingly, it came out without
any trouble (or any effort, for that matter). Even more surprisingly, this means that this
is what the hose looked like before I pulled it out, including the chewed up section. How
in Hades did I get 300 miles out of that hose?!?
You must be registered for see images attach
This is what things look like with the thermostat housing mounted and the hose NOT
installed. As you can see, there's really very little space in between the two fittings. If I
had known this in advance, I might have tried to cut some of the hose and try running it
that way. Sorry it came out so blurry; I didn't realize it had come out that bad until just
now.
Putting everything back together is really the reverse of disassembly. Make sure you
clean up the gasket mating surfaces before re-installing. I also put a small bead of
Permatex on each side of the gasket before installing, just for peace of mind. There's
no given torque specs for the bolts holding the t'stat housing to the head, but keep in
mind that the t'stat housing is aluminum, so you don't want to tighten it up TOO tightly.
I borrowed liberally from this article, but I thought that people might appreciate
a write-up that included pictures
On that note, sorry for keeping the pictures so large.I thought it would be good to keep them as large as possible so that people could see
detail. However, I know it makes this thread a bit large to open for someone with a
slower connection, and also made me format the text in a weird way so that people
wouldn't have to scroll from side to side to read everything. I figured the trade-off was
worth it.
Hope this can help someone in the future
