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Old 09-17-2003, 04:52 PM
mike690003's Avatar
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Join Date: Apr 2003
Location: Miami,FL
Posts: 807
300E running hot...HELP!!!

Hello Board,

My 300E has been running high temps(110 C at idle with A/C on) for a while. I thought my coolant needed to be flushed. Well I flushed the coolant, added a radiator flush additive(Prestone Heavy Duty) and despite all that my car still runs hot. My aux fan kicks in at 100C, but the temp still hits 110 C, but it does not hit the 120 C mark(thank goodness!!) I am afraid that the constant pressure and heat (110C) will hurt my head gasket. I have done this so far:

New Thermostat 1 week old(87 deg)
6 month old aux fan
2 year old radiator
50/50 mixture of coolant and water

still no improvement

Is the position of the little hole in thermostat critical for proper operation??

Should I have my system professionally flushed??

1987 mercedes 300E
1995 e320 conversion(hated the 300e grill)
HID/Xenon (D2S)
Keyless Entry
Monochromatic Paint (Custom Blue)
Smoked Tails
Flat Badged (front)
Debadged (rear)
custom "carbon fiber" console
18 inch HP EVO rims
Sold! Now I drive a Monte Carlo SS

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Old 09-17-2003, 05:12 PM
yal's Avatar
yal yal is offline
Join Date: Oct 1999
Location: New York, Long Island
Posts: 2,704
The little hole in the thermostat should be on the highest point when the thermostat is seated.
You should also take of the radiator cap and run the car with the heat on high so that you can purge the system of air.
You might also want to change the radiator cap if its the original.
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Old 09-17-2003, 07:00 PM
engatwork's Avatar
Join Date: May 2000
Location: Soperton, Ga. USA
Posts: 11,234
Does it do it idling around or out on the interstate at speed?
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Old 09-17-2003, 07:39 PM
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Location: Monrovia, CA
Posts: 490
A llittle more information might help. What's the ambient temp when you're seeing 110 with the A/C on? Have you had any work done on the A/C lately, i.e. recharge, adding oil, converting to R134, etc.? What kind of driving are you doing - open-road cruising or stop and go city or freeway traffic? Does the fan eventually bring it down to under 100 and shut off?

When idling, try putting the trans in neutral. This removes some of the load from the engine. You could also try putting it in neutral and raising the idle speed (via the gas pedal, of course) to about 800-1000 rpm. This will get a little more air moving past the radiator without adding significantly to the engine load. I've used this on my '86 300E to bring the temp down noticeably on really hot (105-110 deg f.) days here in So. Calif.

I've also been running Evans NPG+ instead of regular coolant for the last 30K miles. It's a nonaqueous propylene glycol coolant that has a boiling point of 375 deg. F and runs at 0 psi system pressure. Check our their website at:

BTW, don't get the NPG and NPG+ confused. NPG+ is a direct replacement for the coolant in your car. NPG is an earlier product designed primarily for racing engines. It's much thicker and requires different radiator, pipes, hoses and coolant pump.

Depending on where you are and what kind of winter temps you experience, you might want to try a more water-diluted mixture of antifreeze and water if you don't need the freeze protection. The more water in the mix, the better the heat transfer and hence the better the cooling effect will be. I think you can go as low as 70/30 and still have adequate corrosion protection. Check the label on the coolant container. That's another good thing about the NPG+; no water, so no corrosion.

There's also Redline Oil's Water Wetter, which reduces surface tension dramatically to bring more of the water in contact with the internal metal surfaces to improve heat transfer. It can be used with either straight water (usually required for racing purposes) or with a water/antifreeze mix. For me, it was a toss-up between Water Wetter and NPG+ for a while. I checked out every post I could find on various discussion boards and researched them both on the web. I think either one is a good way to go, but I liked NPG+ better for a number of reasons.

Here's a link to Redline website:

All in all, though, 110 deg.C isn't going to hurt anything in your engine. That's why the red zone starts as high as it does.

This is from a previous post by tkamiya:

T-stat will start to open at 85 to 89C and fully open at 102C
Fanclutch will cut in at 96 to 104C AT THE CLUTCH
Electric fan will come at full speed at 107C and off at 100C
A/C will cycle at 116 to 118C and fully shut-off at 119-121C
Pressure cap will open at 130C

Hope this helps.


Disclaimer: I have no connection with anyone who makes or sells any of the products I discuss in my posts. These are just my personal opinions. Some are based on my personal experience and some are not.
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Old 09-17-2003, 11:23 PM
drbrandini's Avatar
Join Date: Sep 2001
Location: Wrightsville Beach NC
Posts: 1,512
New rad cap, also its not totally unheard of to get a falty thermostat.
Good luck

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Old 09-17-2003, 11:39 PM
mike690003's Avatar
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Join Date: Apr 2003
Location: Miami,FL
Posts: 807
Thanks a lot guys!

Well I think that I probably have air trapped in my system, I will try to get the air out, and report back with the results.

When I refilled the cooling system, it took only 1 and 3/4 gallon of coolant/water to fill it up. I think the cooling system in a 300E holds 9.6 liters(more than 2 gallons).
1987 mercedes 300E
1995 e320 conversion(hated the 300e grill)
HID/Xenon (D2S)
Keyless Entry
Monochromatic Paint (Custom Blue)
Smoked Tails
Flat Badged (front)
Debadged (rear)
custom "carbon fiber" console
18 inch HP EVO rims
Sold! Now I drive a Monte Carlo SS

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Old 09-18-2003, 12:56 AM
Mike Murrell's Avatar
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Join Date: Feb 2000
Posts: 2,141
I tried to locate the original comment from M.B.DOC(old post), but was unable to do so.

If I remember correctly, some older 103 motors have a tendency to have trapped air which can cause overheating.

Elevate the front of the vehicle, remove cooling system pressure cap(radiator cap), start engine and let it warm up awhile. Watch the overflow tanking for bubbling/gurgitation.

For a diyer, a good set up ramps might help with the elevation of the front of the vehicle.
Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC

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Old 09-18-2003, 01:29 AM
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Location: SF Bay Area
Posts: 18,745
The M103 has a couple of bleed ports where the valve cover meets the head near the dipstick handle. A 14mm sockets gets them out. I think you only have to remove one since they lead to the same passage. Keep track of the copper crush washers.

Given the reverse p-trap configuration of the lower hose (in a W126 anyway), I pump the highest point of the lower hose repeatedly while filling the cooling system.

Check your reservoir cap as has been suggested. If it sticks to the neck when you remove it, something's not right.

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