'79 300TD / W123 Running Temperature

As it's been getting colder, I'm noticing that my engine isn't warming up like it used to, and I wonder if there might be something else going on.
The temp gauge used to sit just above 175 when I got my car a couple of
months ago, maybe 180 or 185. It has been getting colder, but not that much colder, and now unless I'm driving on the highway, it tops out at maybe 160.
A possibly-relevant modification I've made is to bypass the ACC servo 'cause it was leaking, but I can't imagine that cabin heat itself would be enough to keep the engine that much cooler.
Could this mean that I need a new thermostat, or is there something else I should look for? I remember reading that the engine should run at least 175 degrees in normal operation, for proper combustion, so I'd really like to take care of this.
thanks, -tom!
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That is sign of bad thermostat. Get a new one.online or at the dealer.
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Tiger wrote:

thx.
-tom!
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The car I got last week ran too cold. My local Napa store had one in stock for about $25, got it out of the box, made in China!. Carquest, Kragen, Autozone, all Chinese. I got a German made Behr online with the seal for under $12 and free shipping. Changed it after work today, both brass "arms" had cracked so it was wide open.
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Stupendous Man wrote:

I finally got around to swapping it out. It appears fairly certain that whoever did the job last time considered that what they were doing was a "hack job." :) The seal was ripped and there was all this red sealer stuff in there all over everything. RTV or something? Anyway, scraped all that stuff out and there was a milky-white, but solid something in there, in the vent hole, and just sort of "around." To guess- it looked like solid Crisco or maybe even from the veggie oil that I've been running, but there was no other signs of contamination; the coolant came out clean. Anyway, it's all cleaned up now and here's fingers crossed that the Behr seal that came with the 'stat is sufficient to prevent leaks.
I couldn't figure out how to drain the coolant properly. I assumed there'd be a valve on the bottom of the radiator somewhere, but nothing was obvious. The top of the radiator says "Behr," and it matches the wife's (she's got an '85 D, mine's a '79 TD), but nothing popped out at me. Beyond the main "cold water" line off the bottom of the radiator, it appears that there are two more much smaller lines going to the transmission, so I figure the transmission cooler is in there somewhere (maybe the little radiator in front of the "main" radiator?) How do I drain this thing, anyway? ...or do I just have to pull the hose (as I did, but on the thermostat side so I didn't have to drain the whole thing), or is there a valve there somewhere that I'm too blind to see?
thanks, -tom!
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Tiger wrote:

AutoHaus says there was a change in the thermostat "for models with engine serial number through 12-067785." Where is this serial number?
In letters about an inch tall on the driver's side of the block, there is the following marking, cast into the block: "R 617 011 09 01", but that doesn't seem to jive with the '12' followed by six digits format on the AutoHaus website. Below that are the digits "04E 8". There are also numbers on the front of the block which appear to be 617 016, and some other numbers which I can't make out (but there are only four of them). Maybe this is the part that says 617 912, though, and the angle and grime make it look different.
So- is my engine serial number 110901? That would seem to match up with info in the shop manual, which says that the first version was through Feb 1978; my car was built in September of that year, so if this is the case, then the second version of the thermostat is definitely what I need. I'm just extra confused because of the grouping of the numbers and the fact that the engine is apparently a 617.912, but the '912' part doesn't appear to be showing in the apparent serial number.
thanks, -tom!
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That number on Autohausaz refers to the last 6 digits of your VIN number.
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Tiger wrote:

Huh, they seem to go out of their way to say "engine serial number," whereas other things on their site specifically say, "VIN".
Are those numbers that I saw on the side the engine serial number? They seem to be cast in, rather than punched like most serial numbers. Not really sure where to find the engine numbers.
thx, -tom!
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