W123 Running Hot (90Cish) ... Flush or Drain/Refill

[catalba]

Recently I've noticed my 1985 Mercedes 300DT (300,000 miles, 2nd owner) is running a little hot. Of course, it's hot in Atlanta in August.

Today I ran a little test ... Ten miles in the heat of the day WITHOUT A/C on. It averaged around 90C, with small spikes going up near 100 when stopped at intersections.


Did the same test this evening; same route, same distance -- but the temps were much cooler in the evening (around 80 ambient temp). I got the same results: The car averaged around 90C with small spikes when stopped at intersections. Throwing on the defroster does drop it down to about 85C.


Here's why it matters ... in two month's time, I'll be driving this ol girl 2400 miles from Georgia to the West Coast.

I've purchased a new thermostat, new radiator plug and a new cap for the radiator expansion tank.

My question is this ... is it too risky to run a proper flush with a long trip looming? Would it be better to simply drain and replace the coolant (which I believe is green) when I replace the thermostat?

(In the three years' I've owned the car, I have not flushed the radiator and cannot attest to the last time it was done. The radiator in the car does not appear to be original, meaning 30 years old.)


Any and all advice greatly appreciated.





... And before you think driving an '85 W123 across country is foolish ... my wife will be driving her ML behind me. Now THAT is risk.

[leathermang]

Have you checked your viscous fan performance ?

[catalba]

Originally Posted by leathermang

Have you checked your viscous fan performance ?

I have not. Are you talking about the fan clutch? How does one check it?

Thanks for your reply.

[leathermang]

yes.. the fan clutch.... I am not sure how to check it properly since I do not have one on my 240... lots of good info in archives....or wikipedia it....

So you are worried that a ' proper' flush.... I take it you mean citric acid.... might eat through something ?

[catalba]

Originally Posted by leathermang

yes.. the fan clutch.... I am not sure how to check it properly since I do not have one on my 240... lots of good info in archives....or wikipedia it....

So you are worried that a ' proper' flush.... I take it you mean citric acid.... might eat through something ?

Hmm ... The visco fan is always on and increases speed with acceleration. It's never not running when the engine is running.

[leathermang]

Checking for proper working is not that simple.....
it should be overriding at speed where the incoming air is going faster than it would be pulling air in.... and then engage well at idle...where you are having your heating spikes...

edit.. if you were to replace it.... the FSM says when stored it is supposed to be in its working position... good luck confirming the parts house knew and followed that one....

[catalba]

Originally Posted by leathermang

Checking for proper working is not that simple.....
it should be overriding at speed where the incoming air is going faster than it would be pulling air in.... and then engage well at idle...where you are having your heating spikes...

edit.. if you were to replace it.... the FSM says when stored it is supposed to be in its working position... good luck confirming the parts house knew and followed that one....

Do you think changing out the thermostat and replacing the coolant is a good idea? And yeah, I'm worried about a proper citrus flush before a long drive.

[leathermang]

Yes I do. and do it sooner than later so you have time to drive it and find if all is ok before taking off into the hinterland...

NOT changing the coolant with some of these cars can also eat through things like the heater core...

[catalba]

Originally Posted by leathermang

Yes I do. and do it sooner than later so you have time to drive it and find if all is ok before taking off into the hinterland...

NOT changing the coolant with some of these cars can also eat through things like the heater core...

Thanks. I'm going to try to drain the radiator and block, gently run some water through, swap out the thermostat and expansion tank cap with new, refill with the correct coolant mix.

Just fearful a big blast of a flush might dislodge things and lead to failure on the open road.


Thanks for your reply.

[Rogviler]

Don't overlook the simple things, like if there's a bunch of stuff stuck between the radiator and condenser (or in the fins of either). When mine was overheating it turned out to be dead leaves filling about half of that cavity.

-Rog

[uberwasser]

The factory service manual describes how to test the viscous fan clutch:

Start the engine when cool. Run it at 4000-4500/rpm while in park or neutral. Best to do this by pulling on the throttle linkage by hand, under the hood, so you can hear and feel what the fan is doing. When the coolant reaches about 90-95*C the fan should engage. If it's working correctly, you can't miss it. It will be both a audible change (i.e. it gets loud!) and you can feel the air flow change.

With that in mind, I'm not sure your situation is that concerning. Your vehicle's thermostat hasn't fully closed off bypass mode until nearly 95*C anyway. So it's not without reason for your engine to reach that. Generally, anywhere in the range of 85-100*C is considered normal operating temp. When filled with a proper mix of antifreeze (to raise boiling point) it is also just fine, per the FSM, for these cars to reach 125*C during heavy/hard driving. So you're still far from any danger zone.

In the end, you may as well replace the thermostat (test it in a pot of hot water first, to avoid installing a thermostat that is bad out of the box) and change the coolant (fill with Zerex Z05). It will help ease your mind as you head out on your trip.

Check my signature for the link to guides on iFixit that I've written to see how to do both the coolant change and thermostat replacement, if it's your first go around.

[jbach36]

You better watch Kent Bergsma's video on Youtube about w123 radiator problems with the neck of them that goes into the hose failing. That could seriously ruin your day.