this literally has started happening overnight.
when the car is sitting at idle, the gauge climbs up to within a millimeter or two of the 120 mark. if i have the car in park and rev the engine, the needle drops down to above 80 within seconds.
driving around town at 25mph+ and the needle stays at normal too.
what the heck?

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1981 NA 300D 310k miles

fan clutch?

i've read that this could be a ground issue.

any coolant leaks? coolant level is ok?

Only 1/4 Serious about possible diagnosis.

BUT,Think about it...
The only difference is the "Need" for higher engine [and thence W.P. Pulley]
R.P.M. s

I forget does the W123 have an Heat Activated,fluid filled,Viscous Fan Clutch?

If so,the increase in R.P.M.s would also increase the Fan's
(And mayhaps it's "Pull" of the incoming Air).Only if the Clutch is locked.

BUT,If your cooling system "Needs" the extra R.P.M.s @ Idle I'd suspect
you also have Liquid Flow Problems...

How many Gazillion Dead Insects are trapped in front of/between the condenser/radiator interface ?

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'84 300SD sold
124.128

First thing, you should do....Is pull over, let the engine cool and check the coolant!

Has any maintenance ever been done to the coolant system?

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Belt slipping or broken

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Graham
85 300D,72 350SL, 98 E320, Outback 2.5

coolant level is OK, no repairs have been done to the system in almost two years since i replaced the radiator. the belt was replaced with an alternator replacement a few months ago. theres no weeping or wetness at the water pump. i've read about the fan clutch thing now. is there any way to test this? there seems to be conflicting information. i have a 6 blade fan.

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1981 NA 300D 310k miles

When the engine is cold the fan should turn with modest effort and not have any wobbliness. It should not spin freely.

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

thanks. i'll have to check tomorrow morning when it's cold, but from what i remember the last time i tried something like that it spun pretty freely

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1981 NA 300D 310k miles

Carrot test, is one that always works.....Put a carrot in the fan, when the car is running....Carrot stops the fan, bad clutch......Though, after all these years....Its always wise to just replace it, as preventive maintenance...

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Take a jumper cable from the negative post on the battery to the engine block... Then start the car and let it idle... See if it fixes the rise in temp.

The ground strap should be tight on the transmission bellhousing, but if it's loose, the starter draw causes resistance to form and the temp climbs electrically...

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

What? There is no starter draw once the car is running.

This is a problem with flow...either air or water. Short the aux fan switch while the car is idling. If the temperature comes down, it's air. Otherwise, it's water.

If it's air, one possibility is the fan clutch, but this seems unlikely since it cools off as higher RPM's. Check for pluggged radiator fins, and if applicable, pull the a/c condenser forward a bit and see if dirt is trapped between it and the radiator. Also make sure your idle speed is in spec, if the car idles low, you will have no air flow.

Water could mean the thermostat is stuck partially closed. It could be a problem with the water pump...a slipping belt or a chewed up impeller. Or it could mean the radiator is partially blocked.

I mean, that the draw from when the car is started heats up the connection creating resistance between the strap and the loose bolt...

trust me... it's very common. simple to eliminate the possibility... just clamp on a ground cable. if the problem stops, it's the strap. if it does nothing investigate the coolant flow issue...

if it's happening right after starting the car, there is NO WAY this car generates enough heat from startup to idle to raise the temp that quick... but a lack of ground will certainly do it.

__________________
John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

if this problem is after driving a while, and spontaneously the car begins to overheat at idle, I agree, it could be water pump, or belt issue... a stuck thermostat would also do it, and revving the motor pushes fluid through the heater system to cool the motor.

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

And we're all sure that this is not just an electrical problem with the gauge? At idle the alternator turns very slowly too, so any grounding problems would be more exaggerated than when it's revved up and can provide more power. We know bad grounding causes a high temp reading...

I have a super high reading on my temp gauge when I turn off my car warm. Let's say the temp is at 90c on the gauge. If I turn my car off, and turn the ignition back on a few seconds later, my temp gauge is at 105. I start the car, the temp gauge will be at 110, and the moment I rev it up it'll drop down within seconds to 90c again.

My example is not nearly as extreme as the OP's, but since it all sounds similar it might just be a gauge that's off.

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"The MB W123 is so bulletproof, you can drive them forever. Which is a good thing as it takes that long to get anywhere."
Betsie: 1984 W123 300D (hobby, 280k miles)
Myrla: 2001 Mazda Protege 2.0 ES 5spd (daily driver, 130k miles)
The Turd: 2007 Toyota Camry (wife's car, 118k miles)

I intermittently have the same issue.

if it is a ground issue. what is the fix? wiring under the hood, or behind the cluster?

easy fix? or new harness?

thanks