I'm not happy with coolant temps on my new to me 300D, and while things are greatly improved after replacing the rad and leaky expansion tank, it's still creeping to 100c mid day in traffic. I feel it can do better.

So I pull the thermostat to check. This is not how I remember a thermostat looks like.





I've read about the holey thermostat body mod, But I dont remember the OM603 'stat having a tail like that. Or any thermostat I've seen having a tail like that.

Anyone know why it appears to not fully block the bypass hole when it opens?

Will be replacing it with the BMW M50 71c 'stat, hope it improves things.

The drilled holes indicate that some idiot tried to fix the cooling problem by drilling out the flange. This is never a good idea. The bypass valve is normally open like this, only the center part blocks the passage. The outer ring is cone shaped, and the openings allow some of the flow to be redirected to the back of the valve. The idea is to balance dynamic flow force so that the wax motor has less work to do. Here's a photo of an un-munged Behr.

A 71C thermostat won't fix your problem. If the thermostat is opening (which I doubt), then the car is running hot for some other reason. Your best bet would be to install a new 80C thermostat.

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The thermostat is designed to allow the engine temperature to rise at a controlled rate. When the thermostat is closed the coolant flows through the water pump by pass hose. If the thermostat does not open that will cause the engine to overheat. Test your new thermostat by placing it in hot water with a cooking thermometer to ensure that the thermostat opens at the proper temperature. Other reasons for over heating include a plugged radiator, faulty water pump, blown head gasket, non operational fan, etc.

Is the thermostat brass? Curious as to how it took on that copper color? Was the trans cooler leaking into the old radiator? Head gasket leak? Probably worth doing a good cleaning flush of the cooling system and then monitoring to see if you have pressure or additional discoloration, leading to other contributors to your cooling Temps

__________________
Stable Mates:
1987 300TD 310K mi (Hans)
2008 Jeep Grand Cherokee OM642 165k mi (Benzrokee)

As already stated, a cooler thermostat isn't going to fix the problem. If the temperature is only creeping up in traffic or slow movement, the clutch for the fan is dead. With a good clutch and a functional cooling system, the coolant temp rarely ever moves even on the hottest days with the A/C running.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Check what fluid is actually in there, if its just water it could cause rust from block but also not be as efficient in cooling - 50/50 mix of spec coolant will drop temp further.

Thermostat when closed dead heads the flow (pump is not really a pump but impeller, similar to central heating system) to get engine up to temp asap.

Check also for air trapped perhaps when doing a flush.

__________________
David


1996 Mercedes S124 E300TD - 129k - rolling restoration project -

1998 Mercedes W210 300TD - 118k (assimilated into above vehicle)

tru dis

As stated if only creeping up in traffic then look at fan clutch.

__________________
Jim

YEAH BABY!!! i love it.

Yeah lol, someone has been ditzing around in there for sure. Thanks for the explanation of the design and the pic! With that tail design, the necessary implication is that when the wax motor pushes the tail to block the bypass, it isnt really blocked completely? Not like with a thermostat that has a standard flat disc on the tail?

I agree a 71c thermostat wont fix any problems, more of it needs a new un-futzed thermostat.


Thanks, that was my thinking as well. Good idea to check before install. Car is not overheating but I feel the system is marginal at high load at high ambient temps so investigating all components are in good working condition

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There was evidence of a neglected system, probably because of that. I dont think it ever had head problems or a cooler/rad leak though. I've driven this car extensively and no abnormal pressurization and coolant discoloration so far. Will be watching out for that though.

This is my thinking as well, so I checked the fan for that, shut off the engine with the IP stop lever and the fan stopped spinning as soon as the engine stopped. Spun it by hand and it was firm. Electric fan goes low and high speed as well. Which is why I started with the rad and leaky expansion tank. That was good for a 10c reduction across the range immediately.

I agree, a low opening thermostat wont solve anything a properly working thermostat would.


It was running on green coolant when I got it. System was clean but there were traces of rust from a previously neglected system.

I replaced the rad and expansion tank and filled it with MB blue coolant 50/50. All that cut 10 deg across the board right off.

I inspected the drained 2 day old coolant as I plan to reuse it after the 'stat change. It looks to be clean. Ran a finger in the stat housing and no slime/rust, but it was noticeably stained from a previously neglected system.

No air in the system during testing, will watch out for that after the new stat.

The fan clutch can still be "firm" but slipping enough to have crap airflow. My SDL was that way, passed the "shredded newspaper" test with flying colors, but the clutch was still crap. Would easily go over 100C in traffic. New clutch/fan from a 606 turbo and temp never moves.

The electric fan should never operate at high speed if the cooling system is working properly. High speed is pretty well reserved for overheating conditions. Low speed is for the A/C. Engine cooling should be taken care of by the clutched fan.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

agreed. it may make it all right. With a compromised stat like that a benz will run hot when the outside temp is warm and cold when it is cool.

The person who drilled the holes was trying to solve a hot run problem probably but now you have changed the rad so it is likely to be fixed with a new 80 degree stat. The engine should run at about 82 unless loaded by climbing, running fast or towing a trailer.

__________________
[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

Interesting experience, thanks for sharing it.

That is next on the list if the 'stat doesnt do enough.

I am wondering what the intent was for the 606 11 blade fan. Was it to move more air or make less noise? I seem to remember reading less blades usually move more air?

In any case, good to know it worked for you.


I sure hope so!

We'll see, putting the new thermostat in a bit.

Incorrect conclusion. The center of the bypass valve still closes the passage fully. The "wings" and the outer ring are just there to direct flow in order to build a little pressure behind the valve. It's a weird design, but it seems to work. I really think that thermostat is toast, and if you replace it your system will perform better. If not, then the problem is elsewhere.

Some of the impellers were made from plastic which can break, also have a look for belt tensioner spring in case weak causing slippage.

__________________
David


1996 Mercedes S124 E300TD - 129k - rolling restoration project -

1998 Mercedes W210 300TD - 118k (assimilated into above vehicle)

The 606 fan has slightly deeper pitch to the blades than the metal 603 fan, the clutch has a slightly different engagement curve, and being plastic weighs a bit less so less stress on the bearings and clutch internals. Otherwise, the primary improvement is stronger airflow at idle and low RPM. The difference with the A/C system is noticeable. The other issue is cost. A quality 603 clutch was well north of $140 when I last shopped. A quality 606 clutch was $90 with a fan blade adding another $20. $30 doesn't seem like a lot of money (and it isn't), but it was enough to push me towards the slight upgrade that the 606 clutch/blade offers.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)