I removed the thermostat (80C)from my 1985 300SD to see if it was stuck in the open position. My car will NOT get to 80C regardless of what I do (other than run it without coolant). The T-stat was not stuck open. I even put the T-stat in a pot of boiling water to see what it would do and sure enough it opened up. After removing it from the boiling water it closed up a few minutes later. My assumption is that the T-stat is okay.

I just recently purchased this car and I lack experience with this particular model's cooling system. The PO recently replaced the radiator and water pump as well as T-stat. I am attributing the lack of heat in the cabin to the low operating temperature, duh .

My question is though if the T-stat is okay then what other "gremlin" would cause a low temperature reading and poor heat output? It doesn't matter if I am driving fast or idling at a traffic light. The heat never gets HOT, HOT and the temp never gets above 60C (one time it may have gotten to 70C).

Also, what heat I do have is only on the passenger side. Anybody have a clue?

Kevin

Kevin:

Where are you located. The reason I ask is that I can't get a couple of mine warm either due to the winter temps. Have thought about a piece of cardboard in front of the radiator, etc.

Don

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DAILY DRIVERS:
'84 300DT 298k (Aubrey's)
'99.5 Jetta TDI IV 251k (Julie's)
'97 Jetta TDI 127k (Amber's)
'97 Jetta TDI 186k (Matt's)
'96 Passat TDI 237k (Don's
'84 300D 211k Mint (Arne- Undergoing Greasecar Conversion)

SOLD:
'82 240D 229k (Matt's - Converted-300DT w/ 4 speed

South Carolina. Temps today were around the 60F but the 300Sd wouldn't get above 60C. That just ain't right!

Kevin

Take it out for a good drive and feel (Or use a thermometer) the block, head, radiator, radiator hoses, oil cooler, etc and see whats cold/warm/hot.

Let us know. It could be as simple as a bad coolant temp sender.

Your test of the thermostat may not have been effective. Check to see if yours is a ' two temp' thermostat. Have you found the pictures I have posted from the MB manual which gives the xray cross section of the thermostat area ? It is supposed to do certain things at about 80 and then something else when it gets to a higher temp...maybe one of those functions is messed up.

Hi Kevin.

I don't know if it has the same system as
the w123 but you might have a bad
aux/secondery water pump.

Louis.

I noticed the same thing on my SD when I first got it. I could not get it above around 80dC. I replaced the thermostat with one marked 80dC that I got from Advance Auto. Now it will get slightly above the 80dC mark. My thoughts are that the cooling system is more efficient than the system on the 300D and the 240D. Check the hot water circulating pump that is on the passenger side in front of the turbo area. It is in line with the heater hose going to the heater core. Put 12v to it and see if it comes on. If this pump motor is bad it can make the pushbutton unit go bad. Anyway, I would see what kind of voltage I am getting to the monovalve before I proceeded much further in the troubleshooting. Also, feel the water temperature on the hoses around the monovalve - better yet use a temp gun if you have one.

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Jim

Jim, tell us about that temp gun... does it work for all colors of things? How does it help to tell the inside temp on something which is well insulated... can't it only read the excaping heat ? Do you use it one the car engine regularly ? Do you have a suggestion in terms of ' features' if some fancy ones exist ? Which brand do you suggest ? Thanks...

This is pretty typical of what is available. Pretty good price on this one by the way. I don't remember the brand we have (at work) but they were around $100. It works best if the area that you are getting the temperature reading from is painted black. You can take these and shoot all over/around the radiator, thermostat housing, hoses, etc... and get a pretty good idea of what the cooling system is doing.

temp gun

This one was the first one that came up on a search page. The ones with the laser pointer are easier to use. We use them for troubleshooting heat exchangers and to find hot spots on rotating equipment.

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Jim

Engatwork,
I will check the aux. pump. Could it be the aux. pump even if I have the same temp heat at highway speeds AND at idle?

Leathermang,
I don't recall if there was a two temp stamped on the t-stat. I do remember seeing the brand BEHR and the MB emblem on it.

I can't work on it today, got church! I will try to check these things out tomorrow after I leave the office. I am still happy with the SD, at least that's what I am telling myself . I don't think I would have been happy with one that is perfect cause then I wouldn't get to tinker with my car and tinkering with my car is therapy for me.


Kevin

his Lord probably doesn't get into gambling. Let the arrows be thrown now!

I will replace the t-stat for now. Maybe that will fix it.

Kevin

yes, no need to post targets on our back.... so, since you saw it, I removed it...

Whatever it is you are not getting the proper coolant flow through the heater core. You will need to try to feel all the hoses at the fire wall to see which ones are hot/cold. You should be able to put your red dial on the climate control to MAX heat and it should make the auxilary water pump come on (the one under the turbo area). Keep in mind too that not long ago someone posted on here about finding a "plug" installed in one of the lines to prevent water from going through the core. Done, I'm sure, during the summer to get as much as possible out of the a/c system.

Something I do on a regular basis when I am heading south on I75. There is a long hill north of the Byron exit where I pretty much run the diesels at WOT coming up it. This allows me to watch the temperature gauge and confirm that the system is working properly. The temp will rise up to around 90dC by the time I get to the top of the hill and then will drop back down after I get back on the flat. Good test of the system, in my opinion.

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Jim

I was referring to the possible arrows that would be thrown at me for making that comment. Didn't want to sound preachy.

I don't seem to see anything that resembles a pump under the turbo area. I have an aux. pump at the firewall just to the right of the monovalve contraption. At least I think it's an aux. pump. It has several hoses connected to it and when I turn the heat system on max this little doo-dad kicks on. I assume that it is working alright. I may just order a new monovale insert to see if it helps since they are not too high priced.

I am also going to replace the t-stat tomorrow just to see if that makes a difference.

Are there not two temp sending units on the head of this 300SD?
Just thought I would ask.

Kevin