hey guys im now completly at a dead end. My 1978 300D non turbo is overheating. I have replaced the t-stat, water pump, brand new Nissens radiator. I have my climate control bypassed and just did a oil change with new oil pan gasket I know this doesnt relate to the cooling system but im throwing all my cards on the table.

At idle or while driving the temp guage just keeps climbing. The top radiator hose is hot, the lower hose is warm, the top of the t-stat housing is hot.


Any suggestions would really help me guys, like I said im stuck.

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1978 300D non turbo

Sounds like it's time to verify the gauge/sender accuracy.

How high does it climb?

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as the car warms up it slowly climbs all the way to the top.

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1978 300D non turbo

I would get one of those infrared temperature gauges and verify the car's gauge. The gauge in both of my cars read incorrectly.

where is the best area to point the infared temp reader?

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1978 300D non turbo

#1.Thermostat housing.
#2. Cylinder block between the injectors.
#3. The temperature sensor (driver side cylinder head)..



.

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1985 300SD
1983 300D
1984 190D
2003 Volvo V70
2002 Honda Civic

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Another thought is to check that the thermostat is installed correctly and opening as it should. They can be installed backwards.

It has gotten to the point I always boil test my new thermostats.Thats with a thermometer in the water before installing them. I have the time to do so.

I just want to make sure they are opening somewhere near where they should before installing them. I started this when the boxes said made in China. In my case and non mercedes related I hope these chinese thermostats will have simular total life cycles compared to what the originals had before failing.

My logic is that I do not want to continue to attempt to find the cause of the overheating by going onward if it was just a new thermostat that was bad as well. For what it is worth I have never gotten a bad new chinese thermostat yet. On the otherhand I am not installing them on a frequent basis.

These cars are very old so it is no shame to check the accuracy of the temperature reading system as mentioned.

I understand wanting him to check his tstat, but if the tstat was the problem for the OP that would mean the first tstat would have to be bad or installed improperly along with the second one. I think the first thing he needs to do is verify the gauge is accurate. If the gauge checks out then delve in tstat and proper flow.

dirkfunk, you say you bypassed the climate control. Care to explain this more or show us pictures? Did the bypassing of the climate control happen to coincide with the overheating? I am assuming you mean bypassing the electrical control of the climate control and not the actual coolant flow?

How long has the overheating problem occurred? How long before you changed the water pump, radiator and tstat? I assume there was no change before and after?

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2009 ML320 Bluetec
1985 300CD
1981 300TD

Past Mercedes
1979 300TD
1982 300TD
2000 E320 4Matic Wagon
1998 E430
1984 300SD
1980 300SD

Does the temp go back down if you kick the heater on full blast? These engines don't generate tons of heat, so that can be enough to cool them off noticeably.

As for t-stat failing, all it would take is to get a bad new t-stat, which has been known to happen.

Other than that, all I got are general cooling system things:

• How about the oil temperature? These engines pump so much oil around that they're partly oil cooled. The temp gauge doesn't measure oil temp, and if the oil thermostat isn't opening, then the cooling system is having to dissipate all that extra heat.
• Is the system holding pressure? If not, it could be locally overheating and raising the temp of the block near the sensor. When my water pump went, I discovered my system leaked pressure all over the place. I fixed the pressure leaks and the water pump and did the flush all at the same time, so I can't tell how much of a difference the pressure leaks were making.
• What water/antifreeze mix are you running? Mine was too high on the antifreeze and it didn't cool right. Worked much better once I got it down to a 50% mix. If you have the green coolant, you might also try some cooling system "snake oil" that worked for me -- NAPA Cool. I only ran it for a while until I did the citric flush, but it seemed to actually help.
• Have you tried doing a citric acid flush? Maybe the interior of the passages are all scaled over and not transferring heat well?
• I know you replaced the water pump, but with what? When mine went, I did some digging and found the price difference was in the fins -- the OEMs had spiral fins, but there were some with just straight partial fins. Fin shape makes a pumping efficiency difference, especially at idle.

If it isn't false indication as others have said, my guess is that it might be a failed oil thermostat, since it climbs both stopped and while moving.

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'83 300DTurbo http://badges.fuelly.com/images/smallsig-us/318559.png

Broadband: more lies faster.

How long have you had the car?
If the old T-stat was installed upside down and you replaced the same way...

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On some nights I still believe that a car with the fuel gauge on empty can run about fifty more miles if you have the right music very loud on the radio. - HST

1983 300SD - 305000
1984 Toyota Landcruiser - 190000
1994 GMC Jimmy - 203000

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hi, i'm not claiming to be an expert by far, however i had similar unhappy hot car issues when i moved from NY to California (suprisingly before then i never had problems in 100 degree NY weather) i think high heat + humidity is certainly better than dry heat.
anyway - i went to walmart and found a "special" race car coolant that claims to lower your running temp 20 degrees. i bought it and put it in my radiator.
it keeps my engine at a steady 80. again, not claiming to be an expert, just throwing it out there. good luck!

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1985 300D - "Desi"
1981 300D - "Pepi"
1984 240D - "Sticky"
1985 300TD - "Flitstone"
1984 300TD - "Frenchie"
1987 300TD - "4481"

Is your fan clutch working correctly?

Did you check the inside of the water pump housing when you changed the water pump?
The "Running surface" inside is sometimes damaged by corrosion and/or cavitation.
This can diminish the water pump output quite a bid.

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76 240 D. Bought in 1998 for $25.
85 300D. Got it for free with a bad engine. ( Sold )
60 Unimog 404. What was left of it, was given to me. Now powerd by 617A.
88 560 SEL. Bought without engine and trans. Now powerd by 617A.
67 250 SE. Cuope. For resto or sale.
64 220SE. For resto.

when i bypassed the climate control servo i disconnected the electric wires left the vacuum lines plugged in and bypassed the coolant flow.

im in the process of rechecking the t-stat and the guage using a non contact temp reader.

this all started when my water pump was leaking out of the weep hole. ever since then it has been overheating. The servo has been bypassed long before this issue.

thanks for all the replys I will post back with further news.

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1978 300D non turbo

I see you are in Carlsbad. Nice part of the world, wish I lived there

See this tread about the fan clutch:
Fan clutch confusion .

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I choose to be happy, even when the Mercedes is being awkward, lol