Hi All!

I have recently purchased a 1985 300TD (222K mi.) that seems to have the common semi-kinda-sorta-overheat problem. The engine temperature generally settles around 95C-100C per the dash gauge. I blasted the parts cannon since I wasn't sure of some of the cooling system history, and the gauge reading didn't budge an inch. I've reviewed a ton of threads regarding this issue, but I'm looking for a sanity check before I continue troubleshooting...

Do you folks think this temp gauge reading is accurate vs what I'm measuring with my IR thermometer? See below for photos of the measurements.

Here are the parts I've replaced thus far:

• Radiator (Nissen)
• Water Pump (Kent Special)
• Thermostat (P.O. replaced <500mi ago)
• Sender (P.O. replaced <500mi ago)
• Pressure Cap
• Coolant (Green stuff replaced with Zerex G05)

Some other details:

• The cooling system holds pressure @15psi for a few minutes, @8psi over an hour, and @5psi indefinitely. No fluid leaks as far as I can tell.
• At 95C-100C the cooling system pressure feels normal. Firm-but-squeezable top hose which returns to soft after engine cools.
• P.O. did a partial flush-out of green coolant and replaced it with G05. I then drained that concoction (it was clean and clear) and added approx. 2.5gal of G05 premix during my parts replacement and futzing.
• Filled coolant tank till radiator overflowed and top hose till overflowed w/ car front-end up on stands. Did NOT see a 'burp' while running/filling. No coolant loss observed while I've owned the car.
• 80C Thermostat was boil tested and opened fully between 80C and 90C. Opened similarly to a second thermostat I had on hand)
• The car runs great and shows no other symptoms.

Here are some temperature measurements after bringing the engine to temp with a 15 minute romp around town:
Gauge Reading:

Temp sender / block area: 92.4C

Cylinder Head Top Surface: 96.7C

Thermostat Housing: 90C

Radiator Inlet: 75.2C

Radiator Outlet: 37.0C

Oil Cooler Inlet: 75.8C

Oil Pan: 68.6C

That is exactly how my 1981 300SD runs all the time. And I am the original owner.

Looks normal to me.

Welcome to the forum. That would bother me as well.

It looks like you are carefully looking at the right things, and have checked them. I don’t know much about how to use the IR sensor readings, whether your readings are normal or not. The readings don’t appear to show overheating, but I’m not certain of that. I don’t have any more to add.

__________________
Marshall Welch
Seattle, WA
1982 300D-T

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.

__________________
"Rudeness is a weak man's imitation of strength" - Eric Hoffer

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.]

Again, looks normal to me.

Can we get some more input from folks driving an SD these days? Does this not look normal to anyone else?

I appreciate the perspective! There are so many threads on this topic, and the general consensus seems to be "Its not the right temperture, but it is probably fine." And that short-circuits my obsessive brain. It certainly doesnt seem to be overheating in a traditional sense.

Hey there, thanks for the info! My intuition pointed towards some kind of low-coolant flow situation. I happened to take a photo of the water pump housing internals, and also my install. Could you let me know if either of these appear to be suspect? Hopefully my pics are posting ok.



Here's the new pump installed. I used the paper gasket that came with the kit...can't imagine that it would be standing the pump off too far.



I notice that my car does not have the banjo line that goes from the pump housing to the head...is this an '85 thing? You can see the two holes with plugs in them in this pic.



Any help is appreciated!!

Thanks for the suggestions!! The more intense coolant flush seems to be becoming my next major step...I've also heard good things regarding the CRC stuff. Just dreading the process of getting out that dang block plug.

In terms of the fan clutch, I havent noticed anything notable, but I will say that it had some kind of thick greasy dirt on its front surface. Is this indicative of the fluid leaking out? Ill check its hot temp operation next time I have the car running and report back.

When the car is running should there be ANY air in the top rad hose? How about when cold? I can massage the hose and hear that there is clearly air in it. Eventually, after 10 or so massage pumps, I can get the air out and can see/hear it pull coolant in and out of the reservoir with each pump. As soon as I stop massaging, the air returns.

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.

__________________
1981 240D 4sp manual. Ivory White.

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.

OP, to rule out the possible inaccuracies use patches of electrical or masking tape on the spots you measure with the IR thermometer like Sugar Bear advised.

Also,you can check the proper function of the temp sender (measure on sender post and vehicle ground) :

https://www.peachparts.com/shopforum/attachments/diesel-discussion/156494d1585344039-foglights-coolant-gauge-not-working-resistance-values-vdo.jpg

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!


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?






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!



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.



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.




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...

Hi All, having some issues posting replies. I've responded to everyone but when I hit submit, it just says the post will be reviewed and nothing ever happens...anyways maybe this will work.



Thanks! I've heard great things about that thermocure stuff. Definitely will look into a flush with that stuff if I don't get this fixed in the next few weeks.


Here's a video of my fan clutch test. The engine was hot in this video, and I had driven about 20mins. https://www.youtube.com/watch?v=scSh-8iFM-k


Does this look normal?

Having some issues posting replies. I've responded to everyone days ago but my replies are not posting...testing testing 123.

EDIT: Okay excellent. Hilarious that this is the only reply that actually posted.

Here is a fan clutch test. The engine was at 100C here, after a 20minute drive. https://www.youtube.com/watch?v=scSh-8iFM-k
Does that seem normal?