signs of failed thermostat
 

I am helping a friend with her '74 450SL. One of her problems is that the engine is running real hot. I just drained the radiator and the block, as her old coolant was disgusting looking and the coolant to water ratio was off. So I refilled the system, ensuring a 50/50 mix.
Anyways, here is my question. I added all that she would take and let the car idle a good long while. The temp gauge gradually crept up to well above 210 Degs. The upper radiator hose got very hot, but the lower hose was cold. I mean, not even luke warm. This was even after an indicated 200+ degs. The engine seems to have a reverse flow cooling system. The lower radiator hose goes to/comes from the thermostat.

I plan on taking the T-stat out and do the boiling water test. In the mean time, what are everyone's thoughts on this. From what I read in the service manual, the t-stat should be opening at 160 degs. 160 degs, I certainly would have felt when I grabbed the lower rad hose.

I drove the car last night, on the highway and the eng. temp stayed in the 200-215 range. Though it was at night and much cooler. My friend said that on some of these 98 deg days we have had, the temp got up to just below the red line. My thought is that the t-stat is partially closed.

Any thoughts and tips would be greatly appreciated.

chances are that if the coolant looked so bad that its way past the time to change the thermostat. If you are gonna go through the trouble of pulling the thermostat out you should just replace it.

I agree, but I want this to be the fix. The trouble now is draining all this new coolant and some how putting it back into the car once I have replaced the thermostat, if in fact its bad. As I mentioned I plan on placing the t-stat is a pot of water and see at what temp, if any, it opens. I have heard of this test, but have never done it. How much should a good t-stat open?

Since I have never dealt with a reverse flow radiator before, should I have been able to feel a fair amount of heat going through the hose that connects directly to the t-stat.

In a traditional t-stat set up, once one felt the upper radiator hose get hot, that meant the t-stat opened. This as I said is a different animal.

you willlose less than a pint changing thermostat

I think you are missing the obvious. If that is the original radiator then it is 38 years old! You said the coolant was disgusting. What do you think the inside of the radiator looks like? At least I would take it to a radiator shop that has a heat gun and check the radiator from top to bottom.
Or bite the bullet, spend $300 on a new radiator and thermostat and be done with it! :D
Anziani
'93 320CE 205K
'95 E420 125K

I actually have a video borescope, so I will look inside the radiator via the upper and lower hose connections.
While I have the t-stat out, I will try and see how the water pump looks.
As for the price of the radiator.....try $829 and that is from our host. This radiator is all metal and is huge.

Has anyone had any luck with those cooling system cleaners?

Thanks for the tips/ideas on what to look at.

Ok, got the T-stat off. First, the radiator inside looks really good. No crud build up or calcium/mineral deposits. This radiator at somepoint in it's lif has been seen by a radiator shop. The outfits sticker is on the rad.

The t-stat is nice and clean and so is the area behind in the wp housing. Right away though, I noticed that not only was the deg mark there (75c) but a date 03 95. This t-stat is a wheee bit old.

Did the hot water test. Per the FSM, the t-stat is supposed to open at 75C (167 f). At about 170 degs, the t-stat opened a smidge. Nothing that I would consider enough to let a significant amount of water through, about an 1/8 of an inch. Anyways as the temp went up, it opened more. It didn't fully open until about 205 degs. I also notice a small amount of rust or what looked like rust on the center pin of the t-stat, as it started to open.

Per the FSM, at "operating temp", the t-stat should be partially open and fully open at 203 degs. At this point, all coolant should be flowing through the upper hose, down through the rad and up the lower hose, through the tstat on its way to the wp.

Thoughts. Mine right now is to replace the t-stat based on the evidence of rust on the center pin and that the t-stat is 17 years old.

Just form the record, I contacted radiators.com and got prices in the $300 range. They offered OEM and after market models.
Anziani

If you still have the old metal radiator, I would consider it a "heritage" item worthy of repair or at least a clean out and flow test, before considering a replacement. The old-school way was to "recore" it, which returns it to new condition and sure there are still ways of getting this done.

Be sure the thermostat is fully closing too, as a thermostat that never fully closes can be as bad as one that never fully opens. If you are seeing score marks on that center post it is a hint that it might be jamming in certain positions as well.