Hey guys,

I am really, really angry. Really, really, really angry.

I payed $5500 for a supposedly "perfect" 300D. With 100,000 original miles.

Well, so far I have spent $500 repairing oil leaks, and now, $2200 chasing a slight over heating problem.

Anyway, the car is running at 92-97C constantly, in light driving in cool weather. It almost hit 100C when climbing a long hill. It doesn't ever run hotter than that.

List of recent fixes:

-Replaced thermostat three times
-Replaced temp sensor and wiring
-New BEHR radiator
-Replaced headgasket (old headgasket had coolant leaks, and leak between two cylinders)
-Water pump is in excellent condition
-Water pump housing is not pitted, in excellent condition
-New fan switch (turns on at 100C)
-Has good compression and passes coolant pressure test (when cold at least)

My mechanic thinks that the head or block has a pinhole or crack.

I don't know if I am willing to spend any more money on this problem right now, so I am looking for low cost solutions. How about a fan switch that comes on at 80C? How about a "fix leak" product? How about running without a thermostat?

Cassidy

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Cassidy
1982 MB 300DT - Running Great @ 104K!

1972 MB 220D - RIP @ ~200K (Dropped prechamber)
1992 MB 190E 2.6 - SOLD @ 145K

I was chasing a similar "too hot" problem on my 300D and did much of the same tests and replacements as you. Nothing changed. My solution? Buy a 300SDL and give the 300D to my son. Now it does not bother me as much.

Seriously, I think wilth all you have done, as long as there are no other symptons and the car does not actually overheat, the problem could be with the calibration of the guage, or it just runs hotter than some. There has been a lot of discussion about this over the last few years.

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Alan Hamm
'87 300SDL 277K Miles
'89 560 SL 68K Miles

100C? What's the problem? As long as it never looses coolant and doesn't peg the needle in the red, I wouldn't lose any more sleep. You'll achieve better economy with this engine running at 90C (193°F) than at 80C (176°F) and, it's better for the engine to run a little hotter too.

The gauges in these cars are notorious for being poorly calibrated. SWMBOs 300D and my 300TD both have new, identical 90C thermostats (tested to open at 90C in a pot of water on the stove), freshly flushed cooling systems and all new hoses yet her car always indicates 100C and mine always indicates 80-90C.

If you're really fretting, I'd pay another $99 and buy a non-contact thermometer. Then, go run the snot out of that car, stop in a safe place and take the temperature of the top radiator hose or at the head where the temp sensor screws in. That will be the REAL temp.

Raytek non-contact thermometer: http://www.contractorstools.com/raytek_mt2.html

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Never a dull moment at Berry Hill Farm.

"How about running without a thermostat?"
That is not what you want to do. Specifically nixed by the FSM... I have posted pictures of the thermostat configuration in the past.....search will bring it up....
I think you need to consider pulling the core plugs ( freeze plugs) and block heater plug...and the drain plug.... and using a 90 degree head on a high pressure sprayer to knock loose and evacuate any rust or grime which has accumulated in the water cavities of the block. You will probably need to make and use a shop vac to help evacuate the stuff... the drain hole is too small....and would restrict the stuff so it would precipatate out before draining with the water through that small hole.
In the old days when an engine was rebuilt it was standard procedure to take the block to a vat and get it boiled out... then start up putting it back together.... I think too many times partial rebuilds are done to our engines due to lots of factors...and then at some point the accumulation of gunk in the water passages makes them ultra sensitive to extra heat load.
The Factory Shop manual specifies boiling out the block in a rebuild.... but many people don't read the FSM....

Is this car losing any coolant? Does it ever 'boil over'?

If it isn't loosing coolant and consistently runs at 100C with no other problems, you should get some or all of your $2200 returned because that mechanic is either ripping you off or he's a moron.

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Never a dull moment at Berry Hill Farm.

Cazzzidy,

Follow the above advice, these temp reading systems (sender, wiring and gauge AS A SYSTEM) are notorious for being not accurate. The only way to get an accurate reading is to use a laser digital thermometer. Any good mechanic should have one.

R Leo gave you the correct instructions about where to read the temps.

There was a post about 1-1/2 years ago about the resistance of the sending unit vs. temp. It doesn't take much of a temp change to modify the resistance.

After speaking with my buddy (who works on satellites out here in SO CA) about resistance changes of wire when the temperture changes, I'm suprised our gauge systems are even close.

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84 300DT Puke Yellow. Totalled after 438,000
84 300DT Orient Red. 169,000 (actual mileage may vary)
2002 Explorer EB (wife's)

I too chased a running hot problem for months after many differant repairs i just gave up. Last week i decided to change all the lights in the dash. After reinstalling the cluster presto temperture gauge now reads normal. It must just have been a bad connection at the gauge.

i have one w123 that runs 90+ and one with 450k that runs at
80. if your system really is overheating then your temp will inch
up, not stay consistant. so it seems to me.

the solution to overheating problems isnt a mechanic but a
radiator shop.

don

One other thing to consider is an air
bubble in your system somewhere.

Easy fix with the right tools.
Have a mechanic pull a vacuum on your
system and this should get rid of the
air.

Louis.

BTW: My 84 TDT runs about 95-97 at normal
60MPH but doesn't climb.

Have you checked the clutch on the engine-driven fan?

Double ditto on R Leo.

Don't sweat it. Drive it.

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1979 300D 220 K miles
1995 C280 109 K miles
1992 Cadillac Eldorado Touring Coupe 57K miles SOLD
********************
1979 240D 140Kmiles (bought for parents) *SOLD.
SAN FRANCISCO/(*San Diego)
1989 300SE 148 K miles *SOLD

I would have to agree with everyone else, just don't worry about it and drive it. My car runs at about 90*C to 100*C, but before my waterpump went out it ran at 80*C-90*C, I was worried about it for a while, but just drive it the way it does. Even on the hottest day the car doesn't go over 100*C and the aux fan never comes on

Are you absolutely sure that there isn't too much air in the system(An air pocket) ?

When I refill the 300D cooling system I take the upper radiator hose loose at the radiator and finish filling that way then put it back on fast.

First time I ever refilled a 300D I did it through the tank cap only and it ran too hot. Let it cool and filled through the hose...problem gone.

Your car running at 100 C doesn't seem too bad, but if it goes above that ...

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Diesel-guy

I did a good Sam stop tonight for a 240D on I75 south.
The man was in a panic, thought it was over heating.
I pulled my non-contact thermometer out of the trunk and shot the head, block and upper radiator hose.
Temperature reading was 89C with A/C running.
I looked at his gauge which read 120C, no wonder he panicked.
I showed him how to use a non-contact thermometer and where to buy one.
He pulled $140.00 from his wallet and bought mine on the spot.
I will buy another this weekend.
This is a tool I recommend to many owners, gas and diesel.
Check this thread also http://www.peachparts.com/shopforum/showthread.php?threadid=97165

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ASE Master Mechanic
asemastermechanic@juno.com

Prototype R&D/testing:
Thermal & Aerodynamic System Engineering (TASE) Senior vehicle instrumentation technician.
Noise Vibration and Harshness (NVH).
Dynamometer.
Heat exchanger durability.
HV-A/C Climate Control.
Vehicle build.
Fleet Durability
Technical Quality Auditor.
Automotive Technical Writer

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

https://www.boldegoist.com/

as already mentioned, confirm the actual temp at the engine coolant outlet.
you do not have ac on I assume either...

gauge error is where I would look next...

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79 240D my current toy
42 years a Diesel addict

240D sold
250SE sold
220D sold
280C sold
280S sold
300D (2) sold
300CD sold
300DT sold
300SD sold
380SL sold