On my 1980 300sd at 45 mph the temp gauge maintains 93 to 95 degrees. As you speed up to a steady 55 mph the temp gauge goes to 100 degrees. It maintains the temp as you drive. If you drop down to 45 the temp drops.
The engine has a recored radiator. New water pump, new auxiliary water pump, new 80 degree thermostat and new hoses. The engine was flushed and new antifreeze has been installed. All of this within the last 200 miles. The auxillary water pump has ceased to function within the last 2 days along with the a/c which has also been repaired with in the same time period.
I am stymied as to what is happening. On my 300 wago if you got into traffic the temp would go up as the speed decreased. When the pseed increased the temp would drop. This is opposite to what we are experiencing with the SD. The fan belts were also replaced. Any suggestions

Fan clutch?

A bad fan clutch will usually cause low speed overheating and no problems at highway speed. I had a high speed overheating problem before and it turned out to be inadequate flow/heat transfer in the radiator, corrected by installing a new one. Maybe the recoring didn't go as well as it should have? The coolant mix is 50/50, right?

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Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 400,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 22,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.

something is seriously wrong there. I would replace the thermostat, that is easy and there have been cases where a new thermostat turned out bad.
Then look into the radiator, etc.

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!

wait a minute...does it immediately go up and down with speed (or RPM)? that could be purely electrical.

assuming its not electrical, what is the ambient outside temperature when this is happening? this is similar to the behavior I experience on my 300D with my radiator blocked after about 60 degrees F ambient temps (I block the radiator in the winter to keep coolant hot). if that is the case, your thermostat isn't opening fully, and as others have suggested it needs replacing.

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'85 300D, 'Lance',250k, ... winter beater (100k on franken-Frybrid 3 Valve Kit)
'82 300D, 'Tex', 228k body / 170k engine ... summer car
'83 300TD Cali Wagon 210k, wife's car

Still some air trapped in the cooling system?

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1981 240D 143k 4 spd manual -SOLD

2004 VW Jetta TDI 5 speed 300k -still driven daily

This sounds like 'radiator insufficiency' which could mean a partially-plugged radiator. I know the radiator is 'new', but can you feel across it to verify that it is flowing through all of its passages? Let's be sure that the 'recore' isn't a dud...

I have the same problem, but more severe. Not to hijack the tread, but I would like to see if anyone else has the same problem, or hopefully fixed it. This is an 85 300TD.

My temp will be at 110-115* cruising at 75. Around town 85-100*, usually closer to 100*. Some times the needle will jump lower or higher by 10-15*. Air in the system? I have replaced the t-stat, fan clutch, water pump (original was good), cap, and reservoir tank. I also will get no heat idling, to lukewarm heat at speed, or when I rev the engine. I am stumped on this one. I don't think its a head gasket as it holds pressure overnight, there is no oil in coolant-coolant in oil, no steam or drips out the tail pipe, and coolant level stays the same.

The heat will go from cold at idle to lukewarm at speed. If you slow down or are at idle it could be cold, or hot, or lukewarm, depends on how it feels at the moment. If it is hot, it will be hot for no more than 30sec, then go to lukewarm or cold.

mine did the same thing and the fan clutch solved all the overheating problems. hot when moving...hot under boost..hot going up hill..ect

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1993 e300
1995 e320
1994 e320
2006 s500 4matic
2004 Jeep wj overland
2001 Ducati 748
2004 Honda shadow aero

you likely have multiple problems.

check you auxiliary coolant pump. it may be seized blocking the flow through the heater core. you can check it by either testing the terminals with an ohm meter (infinite resistance bad, .7ohms bad, around 10 ohms is good) or just put the ACC on defrost and put you hand on it with the key in the run position. you should hear and feel it running. that's probably the heat in the cabin issue.

second, as i advised the OP, get an IR temp gun an make sure these are really the temps you are seeing on the head. that really helps narrow down. you'd hate to do any more (like replace the radiator) if all it is a faulty sensor. or, you'd make a different move if you knew those temps were 20C less.

go from there and let us know...

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'85 300D, 'Lance',250k, ... winter beater (100k on franken-Frybrid 3 Valve Kit)
'82 300D, 'Tex', 228k body / 170k engine ... summer car
'83 300TD Cali Wagon 210k, wife's car

My cars are really old . When I experienced hot under load (speed or uphill)
but dropping to great temps in idle or cruising I immediately assumed bad flow
of coolant and needing to clear internal blockage.

In my case it has always been blocked airflow through my old radiators.

Turns out I never needed a radiator replaced and just need to clean the
fins and dirt, debris, and hair like material clogging up the radiators.

Hair never biodegrades, so this has it's own problems.

There is a radiator cleaning product, but I have experimented with hair clog melting drano/drain cleaner. The stuff really disssolves hair.

Lots of shop vac in blow and suck mode, compressed air - I eventually got
the fins really clean. And the radiator cools as it should.

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80 300D 340K Owned 30 yrs
83 300SD 440K Owned 9 yrs - Daily Driver 150mi/day
02 Z71 Suburban 117,000
15 Toyota Prius 2600 miles
00 Harley Sportster 24k
09 Yamaha R6
03 Ninja 250

x2. A bad t-stat will not allow such vast temp fluctuations just based upon vehicle speed.

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Thanks,
Mark in NC

"Spark plugs?...We don't need no stinking spark plugs!"
1985 300SD "Der Silberne Schlitten" 420,000 mi


Wish these were diesel:
2003 Ford Club Wagon 130,000 mi

Warmed up the car and used a IR gun across the fins to see if there was any blockage. The radiator is good with the same temp or close in all areas. Pulled cable off the temp sensor behind the IP and the boss who is behind the wheel noticed an immediate drop of temp. Like 15 degrees. Ran for awhile with it reconnected and noticed flucuation still. Ordered new temp sensor today. Lets see what happens now.

When you pull the connector off the temp sensor, the gauge should have dropped to zero, not just 15 deg. Proboably not the sensor but rather an electrical problem in that circuit, most likely a ground problem per your 1st post..

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked