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  #1  
Old 02-04-2002, 10:00 AM
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overheating

Sorry to bring up a topic that has been discussed in several threads, but I still have some questions about normal coolant temps. vs. overheating.

My 85 300D has a brand new radiator, new thermostat, new coolant temp sensor, and climate control works great. Cooling system has always been flushed every two years. Before I put in the new thermostat, coolant temps on the highway were less than 80C (too low). Since I put in the new thermostat, here is what happens:

At outside temp. of 32F, at normal highway speeds, ACC off, coolant temp will be 95-100 deg. C, with some spikes above 100. As soon as I put on the heater (on max.) coolant temps drop to 85 - 90 deg. C. When coolant temps are in 100C range, the top of the radiator feels very hot, but the bottom of radiator and return hose feel cold. Either the radiator is doing its job, or there is lack of circulation. Belts are tight, water pump is good.

I put in a second new thermostat, thinking that I may have a bad one, but conditions are identical. So now I have an extra new one.

I would expect that with cold outside temps, coolant temp would be at steady 85C. However, according to Mercedes, thermostat is fully open at 95C

It seems odd that circulating coolant through the heater core would have such a dramatic effect on coolant temp. while the radiator low end is cold. I would guess that with a properly functioning cooling system, the effect of the small heater core would be noticable, but minor.

According to the Mercedes manual, temps in the 95 - 100 range are normal, and the manual says that under heavy load and hot weather, temps may go above 100. But I feel realy shaky about these temps.

Some previous comments in other threads seem to implicate a bad fan clutch. However, with my car, I cant get the radiator hot enough directly in front of the fan clutch to cause it to engage. Maybe this will only happen during hot weather.

The conditions I describe may be absolutely normal (I suspect this is the case, and commented on previous threads as such). The fact that the radiator is cold during cold weather likely means that I have enough "reserve cooling capacity" to keep coolant at appropriate temps during hot summer ambient temps.

I guess my specific questions is this:

When you folks had put in a new thermostat, what would your temperature guage indicate at normal highway speeds?

Thanks,

Mark

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  #2  
Old 02-04-2002, 10:59 AM
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Mark, have you owned this car since it was new ? Have you ever taken the freeze plugs out and looked or semipressure sprayed into the water cavity in the block to see if there is a build up rust/grime ? Greg
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  #3  
Old 02-04-2002, 02:52 PM
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85 deg C +-1deg always. Make certain that your cooling system is completely filled with water, there are no internal air pockets and that the mixture is 50/50. Also, did you flush before the change? the block too? Is your water pump aftermarket?

ATLD
Adam
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Old 02-04-2002, 03:32 PM
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I have experienced the exact same issue as you. My temperature did the same as you explained, I replaced the thermostat 3 times, and the same result as you explained. I am concerned considering what RunningTooHot has found. Do you have any light thudding, nailing, starting issues, or poor fuel milage? I also checked the temperature right in front of the thermostat and it read 179 degrees F. This tells me that the cooling system is working properly. Next question, is the sensor wrong?? Do I have a temperature issue in the area where the sensor is??

DieselNation
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Old 02-04-2002, 05:35 PM
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I've had the same problem, but it's fixed

Overheating problem solved
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  #6  
Old 02-04-2002, 07:18 PM
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Normal temps

My car (83 300d) always had a temp right on 80c except in cold weather. If the outside temps were 40 or less the highest the temp would get to would be about 60c. I changed the thermostat with one from this site, German made but no star, and now my temp is right at 90c no matter what. Highway, in town, air on or off, 60-95 mph etc. It just never goes above 90c. Heater works much better when it's needed. (about 5 days a year ). I think the old thermostat which had a star on it when removed was the one that came with the car. After 230k + miles it was time to replace.

Jim

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67 200d sold
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Old 02-04-2002, 07:51 PM
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Come spring I recommend y'all go flush out the system really well and fill it with a 50/50 mix of MB Antifreeze and water.
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  #8  
Old 02-05-2002, 09:32 AM
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thermostat

Thanks....

I dont think the cooling system needs flushing....I do the citric acid routine every two years, and I rarely see any crud come out....inside of system is clean, brand new straw-colored MB coolant is clean. Radiator is new.

This morning was very cold out...15 deg. F. Yet when I turn off climate control, and run car on highway, coolant temp. is 95 - 100.
The only thing that seems to make sense is a bad thermostat (two bad ones actually). Based on CaptKirks previous thread, I picked up an AutoZone Whaler this morning and will install tonight....Ill keep you posted.

Thanks
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Old 02-05-2002, 11:46 AM
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I think if you talk to an MB mechanic who knows his stuff, or read the service manuals closely, it may be that the "rated" temp of the t-stat is when it *starts* opening, not when it's *fully* open. Therefore the OEM t/s may be working correctly and the aftermarkets are not. I'd stick with the OEM stuff myself, but ordered online to avoid the stealer's ripoff prices (often 10%+ over MBUSA list price!)

I'll have to look this up in the service manual tonight, but I'm pretty sure it's correct...


HTH,
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Old 02-05-2002, 01:35 PM
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you are probably correct

I've read all the MB manuals, and understand that what I am experiencing may be normal, but it is not clear from the manuals. However, it doesnt seem right that when outdoor temps are in 15 Deg. F range, coolant temp with heater off is dead on 100deg. C (212 F). 85 to 90 C seem more approprate ?????
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Old 02-05-2002, 01:52 PM
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Yes, 85-90C is what I'd expect too. Here's a silly question, but which temp sensor did you change? The one for the dash temp gauge is right near the glow plugs, not on top of the motor by the thermostat.

Any chance the fan clutch is failing in a way that it doesn't spin properly when not "engaged", meaning it's just barely freewheeling when cold? Also, when the engine is stone cold, is there high pressure in the cooling system (10-20psi)?
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Old 02-05-2002, 02:35 PM
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Heres what i did

I have actually replaced both temp. sensors on the intake manifold side...one was temp. sensor for idle control, the other (near first glow plug) was for coolant temp guage in instrument cluster.

The fan clutch is original and may actually be functioning improperly. However, I dont have a problem with overheating at idle. Hot coolant temps occur when at full highway speed....during recent cold weather here (15 deg F), I dont need the fan.

Ill check on pressure in cooling system.
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Old 02-05-2002, 02:49 PM
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One last thing. Is your radiator clean? As in bug-free? Try hosing it out with a stream of water, and/or compressed air. And definitely check out that fan clutch...
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  #14  
Old 02-05-2002, 03:15 PM
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Radiator is brand new

I put in a new aftermarket (Nissen) from fastlane a couple months ago (not because of overheating, but because i found slightest trace of trans. fluid in the coolant....replaced radiator, and replaced trans fluid)

I do need to check out fan clutch. Question: what is the primary function of the fan/fan clutch setup...is it to ensure adequate air movement at idle speeds and around town type of driving...e.g. at low engine rpms?? Or does it also have an important function at higher speeds, or does is "drop out" at higher speeds since air flow is probably more than enough due to vehicle movement. I read the discussion in the MB shop manual, but it wasnt clear to me.
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  #15  
Old 02-05-2002, 04:34 PM
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Quote:
Originally posted by gsxr
I think if you talk to an MB mechanic who knows his stuff, or read the service manuals closely, it may be that the "rated" temp of the t-stat is when it *starts* opening, not when it's *fully* open. Therefore the OEM t/s may be working correctly and the aftermarkets are not. I'd stick with the OEM stuff myself, but ordered online to avoid the stealer's ripoff prices (often 10%+ over MBUSA list price!)

I'll have to look this up in the service manual tonight, but I'm pretty sure it's correct...


HTH,

Nope: 80C is the OPERATING TEMP of the Thermostat, that is the temperature at which the thermostat is fully open. The gauge should read a few degrees over 80 when everything is working properly. Since there could be a few degree variance in the thermostat (not a very exact science) and it needs to be a degree or 2 over 80 to stay fully open.

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