Engine running cold with good thermostat, what gives?

[otto huber]

I just installed a factory rebuilt engine with 16K on it in my 240D, previous car was totaled. Engine starts up great on the first bump, cruises well on the highway, and hardly any blow-by. The problem is that it's not reaching operating temp. It stays at about 150 F, which isn't enough to open the thermostat. I tested the thermostat in a pot of water on the stove top and it opened up at 180 F and then closed as the water cooled. The lower radiator hose is cold compared to all the other hoses. I installed a new water pump. Everything seems to be fine besides this temp issue. Any ideas? Thanks,

Otto
'79 240D

[t walgamuth]

Some time the tstat will do that and still not function properly in the car. I would try a different stat.

Tom W

[otto huber]

The thermostat was working fine in the other engine just last week. I took the thermostat out last night and ran the car for about 20 minutes, and it never got over 100 F. If the thermostat isn't opening up, then the car should overheat after 15 minutes on the highway, right? What baffles me is why the engine is staying at 150 F with no coolant getting pumped through it.

[tangofox007]

Originally Posted by otto huber

What baffles me is why the engine is staying at 150 F with no coolant getting pumped through it.

Maybe you should also think about testing your temp gauge/sender, not just the thermostat. (Or maybe just feel the upper radiator hose; it shoud be too hot to hold onto when the engine is at normal operating temp.)

You also might want to study up on the operation of a bypass thermostat. I doesn't prevent coolant from being pumped through the engine. It just controls how much gets pumped through the radiator.

Originally Posted by tangofox007

You also might want to study up on the operation of a bypass thermostat. I doesn't prevent coolant from being pumped through the engine. It just controls how much gets pumped through the radiator.

It is a mixing valve, it controls how much coolant is pumped back into the engine and how much goes into the radiator.

[kerry]

Originally Posted by otto huber

The thermostat was working fine in the other engine just last week. I took the thermostat out last night and ran the car for about 20 minutes, and it never got over 100 F. If the thermostat isn't opening up, then the car should overheat after 15 minutes on the highway, right? What baffles me is why the engine is staying at 150 F with no coolant getting pumped through it.

That would apply to the typical GM engine but MB uses a bypass thermostat which allows the coolant to circulate through the engine all the time and forces the flow through the radiator when warm. I'd second Tom's view. Try another thermostat.

[nhdoc]

Before you get too excited I would also second checking the temp sensor. You can buy a infrared thermometer and shoot the block around the sensor or just replace it for about $20 and see if that doesn't change the reading.

[pawoSD]

As the temperature senders get old they read low most of the time, so I agree with replacing it. They're only like $14...and easy to replace.

[otto huber]

It was the temp sensor. I replaced it with one from the old engine. Thanks for all of your input.

[t walgamuth]

cool

tom

[charmalu]

I have also had temp indicating 90 to 100 on the gauge. have new engine, radiator, water pump, thermostat etc... just changed the temp sender. couple days ago I bought an infrared thermometer at Sears, $80 plus $.
I haven`t had the car out on the road yet, but by the week end we will get her out and heated up. then I can see where the temps are at.
Iam leaning towards the thermostat running hot. I have read that around 75 or 78 SL thermostats run around 175 deg. might go that route.

Charlie