Have you ever had More wrenching fun with your diesel Mercedes than you originally intended to? Yeah, we all have. My fun today consisted of having a few spare hours of time to wrench around, and deciding to go poking into my warped bypass hose situation.

I went under the hood, removed my air cleaner hoses to gain some greater access, and found the hard plastic one to be cracked. The fun begins. Then, come to find out, one of the air cleaner mounts is broken, which explains the rattling sound.

After wiping off oodles of sludge and grease from the thermostat housing after removing it, I had about half an hour of fun using an individual razor blade to remove the old thermostat housing gasket to arrive at bare metal for the new gasket. There must be a better way to remove a gasket that doesn't involve a razor blade, whether it's mounted in a handle or not. The bypass hose was very warped, and I'm surprised I didn't have a major water leak which resulted in an overheated engine or a blown engine to be honest.

Then, I randomly decided to remove the U-shaped tube with the two banjo bolts (the one located above the fan), to see if I needed to clean it out and clean the area inside the threaded hole. I don't know off the top of my head what this tube does, but I decided to let the tube with the bolts, as well as the entire thermostat housing assembly and bracket which holds the AC hose, soak in a large vat of Evapo-Rust that I have had going.

I actually have three large containers of this stuff, each with a few gallons in it, with different levels of grease in them. I put them in level one (greasiest level) for a day, then rinse off the grease. Then level two and one, and finally I take the final product, which is relatively grease free by this point, and put them in my large Ultrasonic parts cleaner which has clock cleaning fluid in there to get the micro grease off (only to later be reapplied by my leaking engine). Sometimes I use the Evapo Rust in the Ultrasonic, especially with aluminum parts. But for brass or steel the clock fluid works best.

So I plugged up my cylinder head hole with a rag as well as my two large coolant hoses and decided to call it a day after a couple of hours of fun. Who knew that the hardest part of this job would be cleaning off greasy parts and scraping off an old gasket. It does a number on your knees.

I've got a 4-ply silicon based hose of the correct diameter and length which I purchased off fleabay to replace the stock hose. They market them for turbocharger intercoolers, but the quality and feel/flexibility of it was surprisingly good (not to mention 4-plies of stainless mesh) and I know I'll never have to replace it again for another 15 years. The new thermostat (Meyle of course) is in stock ready to go in.

Now I am wondering whether I should replace the coolant temperature sensor, and the rabbit-ears sensor which are attached to the thermostat housing. Would anyone recommend this? As opposed to, say, using a bench grinder with a wire wheel attachment to 'buff' out the brass of the sensors and have a fresh mating surface? I'm thinking if it aint broke don't fix it, just make it better, because these sensors I believe go for about $40 each.

One of the two ears on the rabbit ears sensor was capped off by me, I believe it was because I deleted my EGR valve. I have no idea what the other ear is for.

So that's what my fun with w123 consisted of today. I always seem to bite off more than I can chew, or at least when I go to fix one thing, it can never be just that one thing that gets fixed, but also other unrelated things next to it that I didn't intend to open my wallet for.

Money spent:
-$6 housing gasket
-$7 4-ply bypass hose
-$18 thermostat
-$5 air cleaner mount
-?? used U-shaped air cleaner hose off fleabay
-?? two sensors (rabbit ears and water temp)
-three to four hours of time

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The silicone bypass hose should survive fine. What size did you buy and did it fit tight? I used 5/8" silicone "heater hose" in place of the electric "aux pump" and is still primo after ~3 years.

You can find info here. As I recall, the electrical temp sensor disables the cabin fan until the coolant is warm, so as not to blow cold air on rich people. Unnecessary to most of us bottom-feeders with these cars today. I don't know if anyone has bypassed it (perhaps ground the sensor wire?). The vacuum sensor is for EGR, so you could just plug the hole since you removed other EGR stuff. I would certainly clean off the sensors and re-use since they wouldn't be hard to replace later.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans

Someone replied! Nice!

The size I bought was 42mm by about 4 inches long. I had to cut it to the length of the original (plus about a centimeter so I could measure once and cut twice).

Yeah, so I bypassed the EGR. However the rabbit ears sensor has two tubes sticking out of it. One of them was plugged at the time I bypassed it. What about the other one? Plug that one as well? There is a vacuum tube leading...somewhere, that emanates from it.

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the "rabbit ear" sensor and replaced it with a brass pipe plug.

Rocky, may I ask why? At some point a few years ago when I deleted the EGR, I blocked off one of the two tubes after following the EGR delete instructions. But the instructions didn't state to block off the other tube

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to simply clean up the engine compartment and improve access.

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Sev, I'll contribute to your thread.

I just replaced the bypass hose on my 1984 300D. It wasn't leaking, but I removed the T-stat housing to ease getting at the timing chain tensioner. The old rubber hose was swollen a bit, so probably near its life. I used 40 mm ID (~1.5") silicone hose. I bought a 3'L piece on ebay for $18 w/ free shipping, so set for life. Most U.S. cars use 1.5" ID hose for the upper radiator, and you often need to trim it, so that could also be a source, but I wanted something that would last a long time in that location.

Note I previously used 5/8" silicone hose when removing the evil aux water pump. It is more flexible so was able to fit the different diameter tubes on each end. The turbo inlet (off in photo) is also blue silicone hose (to a 1985 CA air cleaner), so that whole side is a purty blue.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans