My 1982 300SD engine takes very very long time to start getting just warm.
City traffic or highway will not make difference to a engine and coolant system to heat. I tried to put a cover over the radiator but it make no difference.
After driving almost 10 miles, the thermometer will not be above 50C.
Now that the winter is around it get even worse not having hot coolant to heat the air in the cabin.
On the other hand my 1984 300SD is just good on that. This 1984 driving just 5 miles local road it reach 80C very easy and I have a good heat in the cabin.
Can or not be the engine thermostat? Any other possible issue?
Thanks

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Nyck - TX - USA

T-stat highly likely. Stuck open will keep the engine from heating up fully.

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'85 300TD "Puff The Magic Wagon" - Rolling Resto
'19 Mazda CX-9 Signature - Wife's sled
'21 Morgan 3-Wheeler P101 Edition
'95 E300d - SOLD
'84 300TD "Brown Betty" - Miss this one
'81 240D "China Baby" - Farm grocery getter

But you also need to check the temp via separate method.

Get a laser temp reader and check multiple spots on the radiator, t-stat cover, metal coolant lines at the firewall, etc.

You may have a bad temp sensor that just isn't telling you the truth.

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'85 300TD "Puff The Magic Wagon" - Rolling Resto
'19 Mazda CX-9 Signature - Wife's sled
'21 Morgan 3-Wheeler P101 Edition
'95 E300d - SOLD
'84 300TD "Brown Betty" - Miss this one
'81 240D "China Baby" - Farm grocery getter

It is a good idea but I did test most of the coolant lines and all ares where coolant travels to. It is really no getting hot.
I left my car running in idle for more than 1.5 hour and engine and radiator were barely worm.

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Nyck - TX - USA

Idle will take a LONG time to warm up a diesel... but you most likely have no thermostat action... either the stat is stuck open, or it's not there...

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1987 300TD
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Change your thermostat. They aren't expensive.

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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.

You should be able to (VERY CAREFULLY!) touch, feel, or put your hand near different parts on your motor and tell that the cooler engine isn't getting anywhere near the temp of the warmer engine, without complicated equipment.

I'll add a +1 for thermostat. Good thing it failed open instead of closed! Moving parts are designed to work together at certain tolerances, which are only achieved at operating temperature. Extended run at lower temperatures isn't good for them.

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617 swapped Toyota Pickup, 22-24 MPG, 50k miles on swap