99 E300D is overheating. It went to the red line so I turned the climate control to hi heat and the temp dropped to 95C. Shut heat off and the temp climbs after a few minutes. I am able to control the temp by using the climate control on hi heat.

The aux fan is working, the fluid level is good, and I can see the shaft for the water pump spinning.

So I changed the thermostat. Still no good. Same exact problem.

What else is in the system? Bad radiator?

Thanks,

Mike

Does it do it while cruising at interstate speeds or at stop lights?

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Jim

It happened on the highway. But I did not try to duplicate at stop lights.

The belt seems fine. It increases speed when accelerating the engine.

The viscous fan does not increase speed as much when accelerating the engine. Can barely tell if more air is flowing. I'm holding my hand near the air flow. Is there a way to test this fan?

If it is running at elevated temps at highway speeds then that is not the fan clutch. Is the coolant level where it is supposed to be? Does the temp drop at idle? If it does then the fan clutch is probably ok. Is the a/c running during this time? If yes and the outside air is over 95dF is the electric cooling fan in front of the condenser running?

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Jim

Not sure where level sould be.

It was originally just above the mark looking straight down the filler of the reservoir.

1.5 gallons drained out during thermostat replacement. I added a total of 1.25 gallons to the reservoir and the top hose of the radiator.

It was 85 deg outside. Set A/C to 65, drove 5 miles, around 3 or 4 stop lights, speed limit is 40 on this road. Engine temp is stable at 90-95.

Get on highway. 5 miles later at 65 mph the temp begins to climb.

Pull over, the electric fan is running on high, the engine temp is at 120.

Turn climate control on HI. Temp comes down quickly. Stabilizes at 90-95. Drove back home with climate control on HI.

I had to add an additional pint of antifreeze after the test drive.

That is because the air is working it's way out of the system. You have to get all the air out of the system for it to cool properly.

What was the temp doing prior to thermostat replacement? Why did you replace it?

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Jim

Verify coolant temp. Sounds like a blockage.

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Your information that the heater can extract enough heat to return the system to a safe temperature is critical. It says that the radiator is not doing its job, either due to an air bubble that has not yet worked its way out of the system or a clog somewhere in the radiator or the lines that feed it, or a failure elsewhere in the system.

You could try draining the system, disconnecting the top and bottom radiator hoses, and try running garden hose water through the radiator to check for an obstruction. If the radiator seems OK, look for blockages in a hose, thermostat mis-installed, water pump failing, etc.

Finally, you have not shared with us information on what the car was doing earlier, that is, did this problem come on gradually over the last several days/weeks/months, or did it start suddenly one day having been fine the previous day?

Jeremy

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

Jeremy5848 - the overheat problem came on suddenly. However the engine normally runs at 80C. This summer it has been running at 90-95C.

engatwork - I replaced the thermostat because the coolant level seemed ok, the electric fan works and the belt seems ok

I drove the car today. 75 deg outside, set a/c to 65, drove 8 miles through town, stop and go, engine temp held at 95C

Drove on highway for 5 miles, temp held at 95C.

Is it possible that the thermostat fixed the problem, after I added the additional coolant (1 pint) after test drive? Is the coolant at the proper level? It is filled just above the mark when looking straight down into the filler hole of the reservoir.

I have a feeling if it were 85 deg outside, the car will overheat. I'm gonna take the car for a longer highway drive today.

Just took the car for another drive. The car wants to overheat on the highway only. Around town it is fine.

Can control the temp with climate control on HI.

Did you look at the coolant again?
Odd that the W210 is overhesting as its "overcooled", massive radiator, 2 electric fan and the beld fan...
Is the pump workin OK?

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E300TD year 2000. RUSTY SOLD
cost a fortune to maintain on the road
but run well on WVO
Second Merc died due to corrosion ( NOT rust) How can mercedes get away with that for so long?
Third lasted a month then went away...
Fourth now... Corroded too...

It is either an air flow or coolant flow issue. I have not read in enough detail the history why the thermostat was replaced. I'd test the thermostat and and insure all the air is out of the system. Was it cooling ok prior to the thermostat getting changed?

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Jim

Have you used any "Bar's Leak" recently? I took the top tank off a radiator I replaced and found it completely blocked with the stuff. I hook up a garden hose to the bottom outlet and water just seeped out the top. I used a cut down hacksaw blade to clean out each tube. I re-installed the top tank and the radiator is back in service in another car.

If your heater core can remove enough heat to bring your system back to normal temp, then your radiator is not doing it's job. At freeway speeds you have plenty of air flow without any fan assistance. Your radiator is blocked, either externally or internally.

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Greg
2012 S350 BlueTEC 4Matic
2007 ML 320 CDI
2007 Leisure Travel Serenity
2006 Sprinter 432k
2005 E320 CDI
1998 SLK230 (teal)
1998 SLK230 (silver)
1996 E300D 99k, 30k on WVO
Previous:
1983 240D, on WVO
1982 300D, on WVO
1983 300CD, on WVO
1986 300SDL 237k, 25k on WVO (Deerslayer)
1991 350SDL 249k, 56k on WVO - Retired to a car spa in Phoenix
1983 380 SEC w/603 diesel, 8k on WVO
1996 E300D 351k, 177k on WVO

GregMN - Have you used any "Bar's Leak" recently? No
Your radiator is blocked, either externally or internally. I agree but have not inspected for external blockage in detail.

engatwork - It is either an air flow or coolant flow issue. Airflow seems to be ok. The viscous fan moves freely when cold, but stiff when hot (that is too funny!) The elec fan works fine. Sometimes the temp indication jumps a bit. Usually it is stable 95C. Maybe the radiator has some internal blockage and the flow is being disrupted?
I have not read in enough detail the history why the thermostat was replaced. I'm an idiot!
Was it cooling ok prior to the thermostat getting changed? YES and NO - YES - the high temp came on suddendly. I was lucky to notice.
NO - the engine was running cooler before the summer. No doubt. Around 80C, now at 95C.

Olivier - Did you look at the coolant again? Three times since driving about 20 miles. I think it is ok. I dont know where it should be. It is filled just above the mark when looking straight down into the filler hole of the reservoir. Is that ok?