That does not make sense to me, looking at the diagram on pg 3

http://rbk3.com/mbz/tsm/MBZ20-005%20Coolant%20circuit.pdf

a t-stat installed backwards should keep all hot water circulating within the engine block... unless the pressure from the pump is enought to defeat the spring of the thermostat.

Either way, this site rocks, I keep learning new things every time I drop in here (almost every day).

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Cheers,
Robert

I don't know how/why my backward stat caused the engine to run cool. Maybe there is enough pressure to force it open, or maybe it was stuck open in addition to being backwards. The first time I drove mine in cold weather I noticed it was never getting over 60C, so I brought it to my Indy to replace the stat. He replaced it and told me it had been backwards. He didn't test the old one. With the correct installation, I have driven at highway speeds at -5F and the temp stays at 80C with plenty of cabin heat.

My 81 240D is too cool .... I have done everything but pull the fan and shroud ( that's next). I cannot get this car up to temp no matter what.
I put a block heater in last night and it makes the startup easy and a faint bit of warmth can be felt very soon after takeoff. Glo plugs seem to be OK as I asked in another post.
Even in the summer I had difficulty hitting full 80*c on a hot day.
What I have done...
New bosch water pump ( weep hole was leaking)
Bosch 80* thermostat and in correctly
I have cardboard and taped off the entire front of the radiator and this made very little change.
Head temps with Infared thermomoter reads 130 at best in these cold temps ( 20*) I am at 4500' elevation and low 20s at night and 30s in daytime.
no restrictions in the vents etc.. fan blows awsome.
Dash Temp gauge reads 50*c and goes no higher even with cardboard.
Should I pull the fan and shroud?.... will this really help?

Something is not right, my 240D was running at 80C today within 5 minutes (with about 20-25F outside temperature). When the engine is as warm as it gets, can you feel hot/warm coolant going to the radiator? The radiator and the upper hose should stay cold until the thermostat opens at 80C. If you are getting flow to the radiator at less than 80C it pretty much has to be a problem with the thermostat opening too soon. If the thermostat is working correctly, blocking the radiator, removing the fan, etc. should no make any difference anyway. I would be tempted to replace the thermostat again (with an 80C OEM) just to make sure. I just can't think of anything else that would cause this. Anyone?

have the cooling system and rad. cap pressure tested. if the system cant pressurize it wont heat up right

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Are the German motorad thermostats the "fail open" type?

when doing the block heater last night I took the thermostat out and it looked fine in the closed position. I figured if it failed it would fail open causing teh motor to run too cool....

Is there a possibility that your temerature gauge or sender is out of calabration? Do you have a warm heater? Can you get your hands on a infared temerature probe for a few minutes to check block temp when warm? Probably above does not apply as you mention little cabin heat. To find out if constant circulation is a problem carfully clamp one of the rad hoses using a couple of pieces of wood and a wood clamp to eliminate damage and see if it heats up. Of couirse remove or really loosen up the waterpump belt as you now have no coolant bypass circuit. If it does heat up than somehow or other the coolant is getting around or through the thermostat when it should not be. I do not think any other possibilities exist really other than above. Remember some mention that should be in archives about corrosion in thermostat neck area but thought problem caused was overheating. Would not hurt to pull themostat and put in a pan of water with a thermometer to see what temperature it opens at. It is possible to get a bad new one. If car is the same though it is a long shot but should be checked .Have a look for any major corrosion in area perhaps letting coolant get around the thermostat when your in there. A really long shot and impossible on most waterpump designs anyways is the impellor on the pump backwards. The effects produced if this where so are unknown to me but just verify flow direction is right as you are probably getting exasperated by now mind the cold air from a poor heater.

As I posted above I did use a infared thermometer on the head after a 30 minute drive last night... 130* F ... interior gauge read just over 50*C.
No my heater is not warm.. it is semi/barley warm... just enough to knock the chill off the cabin. The motor is not getting warm and thus I have very little heat.
I will try a new t-stat today.
No corrosion in the t-stat housing either.. I checked it out during the block heater install the other evening. I can rule out the impellor being on backwards too.. The previous water pump gave me the same temps as well... gotta be thermostat related.

I still appreciate all suggestions...

small update...

I pinched off both the upper and lower hoses tonight and let the car idle for 1 hr. Outside temps 25*f
results=

block and head temp 134*f
Temp gauge on cluster 55*c

This car will never get warm


This rules out the thermostat so I don't know what to do now. I was thinking I could haul a generator around in my trunk and run my block heater all the time

seriously... anyone have any suggestions?

idling it wont make any heat. the engine has to be working to make heat. i am with craig. get another stat. installed with disc down?

good luck.

tom w

__________________
[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

Disc down?.. not sure of the meaning ... my t-stat goes in horizontal from the front ( see pic)... when I took it out the t-stat the disc was toward engine?

is this correct?
I appreciate you guys hanging in with me on this... I am determined to find the cause.

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i must have forgotten the correct way that it sits. in your case i suspect it is disc toward the engine. if it is in backwards it wont work, but i think it will overheat. but that may have been in the summer. it definately wont work correctly if wrong. i checked my chilton. disc toward the engine looks correct.

again sorry to confuse the issue.

tom w

__________________
[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

This manual shows the direction of the thermostat:

http://skinnerbox.steaky.org/Service/W123/w123CD2/Program/Engine/615/20-110.pdf

I don't have any better ideas.

[QUOTE= in your case i suspect it is disc toward the engine. if it is in backwards it wont work, but i think it will overheat.
tom w[/QUOTE]

I have it in right guys.. thanks ... funny thing.. as I said this motor won't overheat for anything... after clamping off the lower and upper radiator hoses completely and ildle for 1 hour.. the temp gauge did not go over 135*f

a hill with the hoses clamped off?

tom w

__________________
[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.