SDL won't heat up, suggestions? - PeachParts Mercedes-Benz Forum

muleears · #1 01-27-2013, 08:26 AM

My son's new (to him) 86 SDL won't heat above 60* at its warmest. Usually around only 50*. When we got the car it acted like this so I changed the coolant and thermostat thinking the 'stat was stuck open. Didn't change. Next I was thinking gauge/sending unit because son tells me the heater works ok. Yesterday he tells me its taking a long time to heat compared to his 123. Says he doesn't get any heat till at least two miles (which I didn't think was so bad). So now I have a sending unit that may not be needed, since it appears the gauge may be accurate after all. What else could cause it to run too cool? I don't want it too hot, it is a 603 with a #14 head. We bought the car back in the fall before it got cold so this isn't a cold weather only thing. I know the car has a pump that maintains coolant circulation at idle (or is that the 617?), could this be stuck "on" and over cooling? Is there a bypass somewhere that is stuck? I obviously am at a loss here, any suggestions would be appreciated as I don't think it's good for the car to run this cool.

pawoSD · #2 01-27-2013, 10:36 AM

Did you use an OEM Behr 80C t-stat when you replaced it?

My dad's 420 was doing the same thing, replacing the t-stat with a nice german-made new one led to proper 82C operating temps (and way better heat)

barry12345 · #3 01-27-2013, 12:06 PM

All of our own current modern cars seem to have to go a couple of miles to get heat as well. The colder it is the longer the distance of course. You may improve it slightly by changing or pulling the existing thermostat and replacing it. It may not improve it either.

I always bring thermostats up to opening temperature with them immersed in a pot of water and a thermometer on the stove. Even new ones before installation. In fact always as I seldom would re install an old thermostat even if it checked good.

I preffer not to use chinese thermostats but it is getting to the point that is all there may be all too soon. I am also getting to the point of checking the oil cooler to see if it has any temperature after only a couple of miles with a thermal reader. Just in case it's circuit has an open thermostat.

We have not been checking this but it is possible for the oil cooler thermostat control to fail as well even if rare. We have no way to know yet how rare it might be as I feel it has been generally ignored. If we find even one or two have failed open with site members maybe things will change.

muleears · #4 01-27-2013, 12:17 PM

Quote:

Originally Posted by pawoSD

Did you use an OEM Behr 80C t-stat when you replaced it?

My dad's 420 was doing the same thing, replacing the t-stat with a nice german-made new one led to proper 82C operating temps (and way better heat)

Yes, I double checked it was a Behr 80*. I used a new gasket too. I' also sure I put it in facing the right direction. Having said that... I put it in facing the same direction as the one that came out. What if that one was in wrong? Would the 'stat being in backwards cause this problem? IIRC it was somewhat difficult to get in place, it would only go in oriented in one direction. Thoughts please?

Partscarver · #5 01-27-2013, 01:44 PM

1985 300SD thermostat replacement | Mercedes-Benz Club of America
post #3

muleears · #6 01-27-2013, 02:16 PM

Quote:

Originally Posted by Partscarver

1985 300SD thermostat replacement | Mercedes-Benz Club of America
post #3

Thanks for the info. Partscarver. I appreciate the input but my car has 603 not a 617. The 'stat is in a different location on the 603. But the way it fits in the housing is probably the same. Can you verify this? Wouldn't the car overheat if the 'stat were in backwards?

Thanks again.