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  #1  
Old 05-03-2022, 05:55 PM
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1983 Mercedes 300D running very hot after cooling system service

Hello! I've owned my car for about a little over a year now, and it has always run at a pretty steady 80-85C while driving, but now I am running into a strange issue with my car. I got my timing chain and tensioner replaced at the shop about 4 weeks ago. They drained my old coolant out (which I am unsure of the type) and put green stuff in, and I noticed my car was running slightly hotter than before, particularly while idle it would now quickly creep up to about 90-95C until I started driving again. I decided to flush the cooling system and put a new Mahle thermostat and gasket in, and put Zerex G-05 in. After doing that, my car now runs at about 100C and goes up to 110C under stress (long uphills), and I cant for the life of me figure out why.

What I've Noted:
- old thermostat was practically orange
- upper radiator hose is now extremely hot to the touch, I cant hold it for more than half a second
- lower radiator hose is warm, but I can keep my hand on it without burning myself
- 99% sure there are no air bubbles in the system, Ive been driving with the new coolant and heater on for about a week now
- Heater is cold in the morning, but hot in the afternoon

I really hope to get this cooling issue fixed, and hope I can get some useful info here. If any other questions I could answer to help diagnose can help, please ask! Thank you for reading

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  #2  
Old 05-03-2022, 07:00 PM
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Put the old thermostat back in or replace it with another new one.

Use an infrared thermometer to check for cold spots, cold spots indicate a restricted passage which doesn't have either any hot coolant or less flowing through.
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  #3  
Old 05-03-2022, 11:01 PM
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I have recently been through the same issue.....

The original Mercedes thermostats came with an arrow etched onto the thermostat to point the direction "up". The thermostat was supposed to mounted with the arrow at the topmost position in the thermostat housing. Inside the thermostat there was a "ding" on the mating surface of the inner thermostat seal. This "ding" was supposed to allow air and water through the thermostat even when it was closed. This allows the thermostat housing to bleed air through it before the thermostat opens up. With no air bleed in the thermostat spring/element facing the block gets trapped in an air pocket and the air does not provide enough heat capacity to open the thermostat. So you get a hot running engine. The original Mercedes thermostats no longer have the "ding" to provide the required air bleed. Your Mahle thermostat also does not have this "ding".

The solution to your problem is to drill a 1/8 inch hole in the thermostat at the top, but not obscured by the rubber gasket. I would also drill another 1/8 hole at the bottom of the thermostat. Make sure the holes are not covered by the rubber gasket once installed in the thermostat housing.Drilling a single hole at the top of thermostat provides a solution that serves the same function as the original Mercedes thermostat "ding". The current Mahle and Mercedes thermostats are identical except that the Mercedes thermostat still has the arrow to mark the position of the "ding" which they no longer have. So if you drill your Mahle thermostat you are just as well off as if you drilled your Mercedes thermostat. Drilling two holes in your thermostat, one at the top, and one at the bottom, allows the air bubble to dissipate faster. The lack of a "ding" in the thermostat is a recent Mercedes/Mahle quality control problem.

I will have more to say about this later as I am still conducting experiments to sort out exactly how many holes you need to drill and what size. However, if you drill the two 1/8 inch holes as I described, you will be out of the woods.

As another note, the man who shall remain unnamed in Bellingham Washington bleeds his cars by loosening the coolant hose going between the auxiliary pump and the block (just above the alternator). This was a significant benefit to getting the system bled. I bleed my cooling system with a vacuum bleeder, which also works well once you have the thermostat drilled.

Last edited by carock; 05-03-2022 at 11:11 PM.
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  #4  
Old 05-04-2022, 12:18 AM
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Quote:
Originally Posted by Sugar Bear View Post
Put the old thermostat back in or replace it with another new one.

Use an infrared thermometer to check for cold spots, cold spots indicate a restricted passage which doesn't have either any hot coolant or less flowing through.
Followed your advice and it worked like a charm. My old thermostat had no issues beforehand, I wish i never replaced it. I guess the new one I put in was just a little menace. Thanks for the help! Car is running much cooler now
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  #5  
Old 05-04-2022, 02:52 AM
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Thanks for the info provided! When I do eventually need a new thermostat I will be sure to do some drilling into the new one.
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  #6  
Old 05-04-2022, 08:04 AM
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Originally Posted by Roamer24 View Post
When I do eventually need a new thermostat I will be sure to do some drilling into the new one.
Don't even think about drilling holes in your bypass thermostat. The W123 engine features a built-in bleed passage in the thermostat housing.
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Old 05-04-2022, 09:00 AM
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X2 bad bad to add holes.
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  #8  
Old 05-04-2022, 09:53 AM
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Have you checked whether the newer thermostat was a different temperature than the old one?
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  #9  
Old 05-05-2022, 01:50 AM
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If you check the bleed passage in the thermostat housing, you will see that it only bleeds air from the FRONT of the thermostat, to the cylinder head exit. This does nothing to get rid of. a bubble covering the BACK of the thermostat element facing the block. That is why Mercedes put the "ding" in the OEM thermostat. When that ding is not there, you need to drill a hole of suffer the consequences. If Mercedes and Mahle were offering the correct thermostat you would not need a hole, but they are not doing that now.
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  #10  
Old 05-05-2022, 10:11 AM
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Quote:
Originally Posted by carock View Post
If you check the bleed passage in the thermostat housing, you will see that it only bleeds air from the FRONT of the thermostat, to the cylinder head exit. This does nothing to get rid of. a bubble covering the BACK of the thermostat element facing the block. That is why Mercedes put the "ding" in the OEM thermostat. When that ding is not there, you need to drill a hole of suffer the consequences. If Mercedes and Mahle were offering the correct thermostat you would not need a hole, but they are not doing that now.
Pictures of the difference?
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Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (113k)
1991 350SD (206k)
1991 300D (228k)
1993 300SD (291k)
1993 300D 2.5T (338k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (265k)

Past Diesels:
1983 300D (228K)
1985 300D (233K)
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  #11  
Old 05-05-2022, 12:12 PM
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It is really hard to get a photo of the "ding"!
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Old 05-06-2022, 08:29 AM
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Quote:
Originally Posted by carock View Post
If you check the bleed passage in the thermostat housing, you will see that it only bleeds air from the FRONT of the thermostat, to the cylinder head exit. This does nothing to get rid of a bubble covering the BACK of the thermostat element facing the block.
If that theory is correct, it would require that any air trapped in the engine escape through the (drilled) hole in the thermostat and then travel DOWN the lower radiator hose to the bottom of the radiator. How, exactly, does that work?
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Old 05-07-2022, 12:32 PM
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Coolant exits the cylinder head, travels down through the radiator and travels up into the lower radiator hose where it gets trapped by a closed thermostat. The bleed hole built into the thermostat housing will route air and a small amount of water to the cylinder head exit.

Coolant flows through the thermostat and into the water pump.

If the thermostat is closed, the back of the thermostat can sit in an air or steam bubble and it may not open.

The Mercedes thermostat is supposed to come with a "ding" pressed into the moving part of the thermostat seal. This allows a small amount of coolant to bleed through even when the thermostat is closed. This coolant will relieve any air pocket on the back side of the thermostat.

Some of the latest Mercedes and Mahle thermostats are not being shipped with the "ding". The Mercedes units have an arrow etched into the thermostat that points "up". This arrow is supposed to locate the ding on the top of the thermostat seal when in use. The latest Mercedes thermostats I have received have the arrow but not the ding. These faulty thermostats make bleeding the thermostat housing extremely difficult and result in symptoms the OP described. The solution is to drill at least one small hole in the thermostat housing.
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  #14  
Old 05-07-2022, 07:18 PM
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Originally Posted by carock View Post
The bleed hole built into the thermostat housing will route air and a small amount of water to the cylinder head exit.
When adding coolant to a cold engine, is there anything to prevent coolant from filling the engine via the built-in bleed passage? Is there anything that obstructs displaced air from being bled to the radiator and then the reservoir?

If the built-in bleed passage doesn't work, what is its purpose?

If holes in thermostats are so essential, how has my 617 engine been cooling normally for the past 40 years with unmolested thermostats?
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  #15  
Old 05-08-2022, 05:19 PM
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Quote:
Originally Posted by tangofox007 View Post
When adding coolant to a cold engine, is there anything to prevent coolant from filling the engine via the built-in bleed passage? Is there anything that obstructs displaced air from being bled to the radiator and then the reservoir?

When filling the engine with coolant the water level gradually rises from the bottom. If you look just below the thermostat there is a short coolant hose. Coolant rising in this area will trap air on the back side of the thermostat.

If the built-in bleed passage doesn't work, what is its purpose?

The bleed passage built into the thermostat housing only bleeds air from the front of the thermostat. This is great, but the rear of the thermostat contains the wax element that expands and opens the thermostat.

If holes in thermostats are so essential, how has my 617 engine been cooling normally for the past 40 years with unmolested thermostats?
The correctly built Mercedes thermostats come with a "ding" built into the thermostat seal that acts just like a hole drilled in your thermostat. There is an arrow etched into the thermostat to show you where that "ding" is and lets you locate it at the top. Since the Mercedes thermostats being shipped today no longer have the "ding" you will need to drill a small hole to serve the same purpose. This is a quality control problem with Mercedes and Mahle thermostats being shipped today.

If you replace your thermostat and find that you have an overheating problem where it appears that your new thermostat is not opening then it is likely that there is air trapped on the back side of the thermostat. Drilling a hole in the thermostat will solve the problem.

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