W123 thermostat replacement - how much coolant?
 

Good evening all;

Need to replace the thermostat in my '85 300TD (operating temp never gets above 60 degrees).

How much coolant do I need to have on hand to replace what is lost when I pull the hoses / thermostat housing? (Need to lay in supplies, because driving to get it after the fact will not be an option)

Thanks in advance,
Dave

if you do it with the car cold, remove the cap first then re-tighten it completely. This will create some vacuum in the system when the fluid tries to drain out.

one gallon of coolant should be enough

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Draining the coolant into a clean container worked well for me.

If you drain the radiator into a clean container, you can reuse the coolant. No significant "loss" should occur.

Hi
I just did this job on my 1985td - I bought 2 gallons of 50/50 and actually used 1 & 1/2 gallons.

It is nice to see the temp up to 80 - 85C where it belongs.

Joseph

Hi, everyone!

My 1981 300D seemed to never get warmer than 60 or so, so I figured I needed a new thermostat. Ordered one, had it installed, and now, nothing seems different -- it still barely exceeds 60 degrees. I wonder what the problem is...Any ideas? Could it be the gauge? Thanks a lot!

Have you verified the new thermostat was installed in the correct direction? It could be the gauge, how doe the heat feel? Do you have access to a IR temperature "gun" to directly measure the temperature at the thermostat housing?

i have not verified the direction. i didn't put it in...can i check it without draining the coolant? the heat works well and is as hot as any i've felt...it's not so cold here in az but it can make the car very hot. i don't have access to a gun.

The gauges do get out of calibration. I would think it is rare for VDO's, but still it happens. I have one the shows too hot, most of the time (90-100).

After thorough IR measuring, it became clear that the engine was running perfectly cool (79-81) were the coolant exits the engine.

It caused me some headache assuming the gauge would be the last thing to be inacurate. Just saying. :rolleyes:

Yes! They do go out of adjustment.

I replaced my three gauge cluster this past summer due to a leaky oil gauge and the temp read hotter. My operating temperatures we always 80-83 now it is a consistent 90. IR gun and swapping in the old cluster confirmed that the new (well older gauge) from the 82 I pulled it from is less accurate.

Try and find a friend with an IR temp gun. That can confirm if your engine is truly cold.

I bought an IR thermometer for $20 at Harbor Freight. Well worth to have in your tool box.