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Old 05-02-2002, 12:43 AM
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Question 82 300D cooling problems

Hi everyone. I really need some help here on my 300D. My problem is that my car has been running what I feel is to hot lately. It is running between 95DC and 105DC under normal driving. I started up a very steep incline the other day and it went on past 105c, so I promptly turned around and proceeded back down the hill! The car has around 200,000 on it and runs great except for this problem. The clutch fan seems to be working well, I installed a new MB stat today and flushed the radiator (with water, as not to damage it with anything to strong). None of this did anygood. As a matter of fact just to check, I filled the system with 100% water to see if this cooled the car down.... it did not. My electric fan is not functioning but I think the car should run cooler than this on the highway. By the way, I've narrowed the electric fan problem down to the coolant fan switch, but I can't seem to locate it.....duuuuuh! I anyone has any idea where I might find this creature I would love to know this as well. I also installed the MB stat before I was informed that the arrow needed to be pointed up, could this be the problem? Anyway what I would like to know is.....where do these cars run normally temp wise, at what temp does the electric fan kick on at, and does anyone know why my car is running this warm? I live in Waynesville, NC so the temp this time of year is usually between 70-80F.

Thanks in advance,

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Old 05-02-2002, 09:27 AM
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Waynesville is a nice area. I have vacation place near Brevard so am aware of the good weather you have up there.

Your cooling issue can be complicated, unfortunately. I read many posts from people who have done about everything possible and still can't get the temp down to where they want it ( most seem to like the 85 c area). But...many times I have seen very experienced people say that 100 c is ok. More than that on regular basis is another story. My car runs between 90 and 100 but never over 100.

Some ideas that you might pursue:

- take the radiator off and clean it. in between the rad and the a/c cooler trash can and does collect. getting the rad off is pretty easy but you do have to disconnec the trans cooler lines. if you do this, replace them with new ones that have wire on the outside.

- buy another thermostat and install it. they can vary somewhat.

- replace waterpump. not too difficult or expensive and a good preventive maintenance thing anyway if yours is old.

- make sure valves are adjusted correctly.

I wish you luck. Search under various related topics and you will find all kinds of info. Apparently, at the worst end, something can be wrong inside the engine that just prevents cool operation.

Columbia, SC
'85 300CD @ 150k miles
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Old 05-02-2002, 09:35 AM
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Join Date: May 2001
Location: Manhattan; Shelter Island
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What's good to use as a coolant flush? Prestone makes something called Super Flush which you pour into your cooling system and drive around with for several hours. I was going to use this stuff when I drain and fill my cooling system soon, but will refrain if you guys tell me it's junk, or if there's something better out there. Thanks.

1983 300D (parked for four years)
2012 VW Sportwagen TDI Manual
2001 Miata SE
1962 Chevrolet Corvair Rampside
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Old 05-02-2002, 01:49 PM
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Join Date: Jul 2001
Location: San Antone
Posts: 408

A bad radiator cap can also cause problems.

The thermostat needs to be removed and checked because they are sometimes bad, even new ones. The stat begins to open at 80-90 C/175-193 F and is fully open at 90-94 C/193-200 F. Check the stat by first attaching a string or small piece of wire to the stat that does not interfere with the valve, immerse the stat into water at 200-205 F (stir the water to ensure even temp and the stat does not contact the side of the pot), the stat should open fully in about 1 minute or so (also, if you still have the old stat, then check it and compare to the new one). If the stat does not open fully, then youv'e probably found the problem.

A test for your radiator/engine/cooling system. With the stat out, close up the cooling system, fill with water to the correct level, leave the radiator cap off, run the engine, see how long it takes to heat up at all (I just flushed my colling system and it took about 1/2 hour at 1,500 rpm to reach 130-140 F). This test will indicate if the radiator is not flowing as it should indicating possible blocked passages and/or blockage in the engine or another part of the cooling system.

Since you're fooling with the radiator and if you are so inclined, pull the radiator out before the above test and reverse flush it with water from your hose at the highest flow rate with the garden hose inserted into the lowest connection on the radiator (lay the radiator flat with the bottom/lowest hose connection facing up, insert the hose there to force water through the cooling passages, and the water/crud will flow out the filler neck and top hose connection). Reverse flushing will hopefully force any loose crud out the top and through the fill neck/top hose connection (won't work for encrusted crud). You reverse flush because the normal flow is from the top down and reversing the direction of flow should remove loose crud, plus you can see how much water flows out - at the highest pressure/flow on my hose almost no water flowed out the bottom hose connection where the garden hose was inserted.

The flushing and running test w/o the stat is not the same as what a radiator shop will do, but it will give you an idea how things are working to narrow the problem.

With the stat out, the radiator flushed, and you've checked for flow by running the engine - you can flush the entire cooling system using citric acid from M-B. Citric acid will loosen encrusted crud and really clean your entire cooling system. Before the citric acid flush you have to de-oil the cooling system. M-B sells a detergent or you can use dishwashing detergent (like Palmolive, but not a lot). With the stat out and the cooling system together, completely drain the cooling system using the radiator drain cock and the engine block drain plug located on the right side of the engine - viewed from the driver's seat - behind the exhaust/intake manifold, refill with a mixture of detergent and water till the system is full (about 2 1/2 to 3 gallons is the cooling system capacity). Run the engine to 80 C/175 F at a fast idle (about 1500 rpm) and hold the temp for 5 min. or so with the heater control set to high temp. and fan on high speed (it took me an hour to reach 175 F). Drain all the detergent/water out using the drain cock on the radiator and on the engine block. Flush the engine two times with fresh water by filling the cooling system with water, run at fast idle till warm (about 130-140 F), let cool to below 125 F, and drain the entire system. Repeat with fresh water. I also took the heater hose connected to the back of the head off while draining the cooling system and flushed using the garden hose for both de-oiling and citric acid procedures - I flushed both through the engine and through the hose for the rest of the cooling system. After the second fresh water flush is done, make a batch of citric acid and water mixture to re-fill the cooling system with. Run at high idle (about 1500 rpm) to 80 C/175 F and hold for 15 min. with the heater on high temp and fan at high speed. Let cool to 125 F and drain the entire system. Flush the cooling system three times with fresh water like for the de-oiling procedure - I used distilled water for the last two flushes. Then re-install the stat with the arrow pointing the right way (this can make a difference) and re-fill the cooling system with a mixture of 55%antifreeze/45% distilled water.

Do not overfill the cooling system. Too much coolant will prevent good flow through the radiator and also cause the engine to run hot.

When was the last time you changed both fuel filters? Partially clogged fuel filters can cause less fuel to reach the injection pump and then less fuel to the injection nozzles causing a lean fuel mixture which can also cause the engine to run hot.

Also, fuel contaminated with algae will clog the fuel filters causing a lean mixture and a hot running engine. Algae can clog the filters to the point where no fuel reaches the injection pump and the engine will not run or stop running if you are driving. When you remove the inline fuel filter look for black/grey colored bits of crud in the filter screen (if you have the opaque plastic inline fuel filter, then break it apart to inspect it, completely surround the inline filter with a shop rag and hit it with a hammer to break it open). Algae contamination is more common than you might think and can be picked-up from a contaminated hose at your gas station. Diesel Doctor by BioBor is the cure if you have algae in the fuel (look up an old post I replied to for a lot more info about algae and how to combat it). Algae contamination must be treated ASAP, my algae infestation killed my engine because of the lean mixture causing me to have to rebuild it.

The aux. cooling fan's fuse should be checked. Take the fuse out and inspect it because the fuse can look good in place, but when pulled out the metal portion falls apart. If the fuse is OK, then disconnect the two wires located on the AC drier. Connect the wires together and the aux. fan should run. If not check the aux fan relay - called the ice cube relay. My relay on the '77 is located on the left side wheel well inside of a small plastic box - your relay may be located on the firewall, driver's side.

Good Luck!
America: Land of the Free!

1977 300D: 300,000+ miles

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Last edited by tcane; 05-02-2002 at 06:18 PM.
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Old 05-02-2002, 03:58 PM
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Join Date: Aug 2001
Location: USA
Posts: 8,102
Note: On the W123 (your car) the auxiliary electric fan will ONLY run when the A/C is on, and refrigerant pressure is high enough to trigger the switch on the receiver/dryer. There is no connection to the cooling system at all. Mercedes designed the main fan (and clutch) to be able to cool the car without the aux fan. The aux fan is there mostly for low-speed A/C performance, when there may not be enough air flowing over the condenser.

In later cars (W124, W126) the aux fans did have a coolant temp switch, on my car it turns the aux fan on at 105C.

Oh yeah, Tom's info was good, check all that stuff too. And don't forget to clean the fins on the radiator AND condenser with compressed air or water spray, removing the radiator if necessary. An amazing amount of bugs & gunk gets in there!

Boise, ID

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Old 05-02-2002, 06:35 PM
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Thumbs up Thanks for the info!!!!!!

Guys, I really do appreciatre the info, I'll definitely dig a little deeper into the problem and thanks again for you time and knowledge.

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Old 05-03-2002, 12:47 AM
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Join Date: Feb 2002
Location: SF Bay Area
Posts: 131
I had a similar overheat problem about a year ago. Changed the water pump and overheat problem went away.


1985 300D Turbo

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