Hi All, thanks so much for all the responses over the past week! You all have given me a path forward with diagnoses. Please see my specific responses below!
Quote:
Originally Posted by Sugar Bear
Welcome, An 85 300TD, great car. Mine was 904 blue with palomino, it was nice but the NE tin worm reclaimed her @ 307k.
Couple of things...
1 I wouldn't be comfortable with what the dash gauge shows.
Put black electrical tape on whatever you aim the IR thermometer at to get more accurate readings.
Treat it with CRC ThermoCure, it works wonders.
Don't exceed 50% coolant.
RedLine WaterWetter emulsifier helps.
On a hot engine just after shutdown does the fan clutch offer resistance or freewheel? It should freewheel cold and build resistance hot.
If you have a used thermostat, gut it and retest. I used only Behr or Wahler stats.
Remove the gauge sensor, clean both ends, tighten the contacts and retest.
The temp should be a degree or two above the thermostat rated temp. You should be able to see it open by watching the gauge for a hiccup near the rated temp.
Air pockets are a possibility, fill the system with the nose raised, slowly and with the heater on full temp.
Again, CRC ThermoCure has worked very well for me and actually saved the original block in a valuable collectible car.
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Thermocure seems very good, and I've seen this recommended across a few different threads! Perhaps I'm uninformed, but wouldn't I be able to tell whether the block needs flushed by the coolant being dirty? Mine came out super clear. Also, your suggestion about the fan clutch is a good one, thank you. I will see how long it takes to spin down after the engine is powered off hot. Here's a pic of it. I noticed some VERY thick sticky buildup on the front face of it. Is this an indication of the fluid leaking out?
Quote:
Originally Posted by Frank Reiner
Shern is spot on about the temp differential across the radiator; the water flow rate is so low that a great deal of heat is being exchanged. More than is intended.
Why? Again, as Shern noted, the thermostat may be at fault. The thermostat is the flow control valve for water flow.
Two other possibilities: 1) An unintended restriction of flow in the system (the thermostat is an intended restriction), such as pieces of an old gasket, or a rag. 2) As noted above (post #5) the pump function may be insufficient.
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I went ahead and ordered an official, fancy-shmancy Mercedes thermostat and will install it ASAP. Mostly just so others can't say I didn't try it
. If that doesnt take care of it, I suppose its time to find the blockage. I totally agree with you and Shern, there is a low coolant flow situation. Thank you!
Quote:
Originally Posted by vox_incognita
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Thank you for that chart! I did measure the sender and the resistance values roughly matched across the IR thermometer measurement and dash gauge reading. So safe to say, the dash gauge is accurate to about +/-2*C.
Quote:
Originally Posted by Shern
Unless you're experiencing sub-arctic conditions in Maryland, 75C to 37C is a massive delta. You've got a flow restriction. What brand is your thermostat? I bet it's aftermarket.
That's the first place I'd look.
ETA: checked my notes. Typical rad delta should be 10-15C. You're hovering around 40C. Re: aftermarket thermostats, a pot of boiling water is a dramatically different environment to a thermostat housing in a running vehicle. The reasons why are pretty intuitive. Spring force is just one of several key variables. Get a Behr or Wahler and good luck.
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Yeaaaahhh you guessed it, the P.O. replaced the thermostat with an aftermarket one. Replacing with a MB official one ASAP! Good catch on that temperature delta.
Quote:
Originally Posted by Frank Reiner
Inasmuch as the gauge indication and the IR indication (at cylinder head) are in rather close correspondence it seems fair to say that the gauge is OK.
So what's going on? The water pump on the subject engine is of relatively small diameter, and has a conical impeller. The cone of the impeller is matched by a conical surface in the pump housing. If 1) the impeller is too far from the housing surface,[due to mis-assembly during build/rebuild], or 2) the conical housing surface has become corroded/eroded, the flow capacity of the pump/housing package will be significantly reduced. A low rate of water flow will result in higher temperatures in the cylinder head.
How good is that MB pump? At a pump speed of 4500 RPM it will pump 80 gallons/minute. [That flow rate was observed during a dyno run when that pump was installed on a non-MB engine.]
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Thank for the info on the flow from the water pump! I happened to take a pic of the inside of the water pump housing. Could you please take a look and let me know if the conical surface in this housing looks totally trashed?
Also, here's a pic of the water pump installed. Anything out of the ordinary with my install? I did notice there is no banjo bypass tube on the water pump housing. Maybe this is an '85 thing? I've seen a few photos of cars missing this. The plugs in the holes for the banjo bolts look factory...
06-08-2025, 12:57 AM
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