Engine running toooo cold
 

Car: 1991 560 SEL 103K miles
Problem: temp gauge never go above the tick between 40 and 80 C even after 30 minutes of stop and go.
1st Diagnostic: Thermostat stuck open.

Spend three hours to get that ^&*^%$ thrid screw to get the T-stat out. Put the unit in a pot and cooked it. The 1st stage open at 82 C and 2nd stage open at 95 C, working perfectly.

Now what? Please advise how to check the temp sending unit?

Low engine temp seems to contribute to my car running high idle. Engine keep wanting to go to a higher RPM to get warm.

You could verify the engine temp if you have access to an infrared thermometer.

It's just my opinion, but the cooking-on-the stove thing is not proof conclusive.

Buy a new t-stat and make sure it's a Behr.

I'm sure that your temperature gauge does not show the correct temperature. Engine combustion produces high temperatures which have to be cool. Just drive the car for 10 minutes, open the hood and touch the upper hose and if you can not hold it the temperature of the coolent is higher than 40-80C. I will look for the problem in the sending temperature sensor or wiring from the sensor to the gauge.

Roman
87 300SDL 152K
86 300SDL 212K
86 420SEL 226K
81 300D 139K

I did exactly that when I got home Thursday night (30 minutes drive). I could put my hand on the radiator feeling hot but not burn.

I have a behr T-stat coming Monday and we will see what happens after put it in.

air lock run the engine without the rad cap on so all the air gets out and watch for the cooling fan to cut in then all is ok

also ensure the heating is set to hot in the car for a full system air bleed

Thank U teky!!!

I was worried about how to bleed all teh air out. So what you are saying is to connect everything back with gaskets and everything. Then fill the expansion tank with MB coolant (44%). And just let the car run with heat at Max?

How could I know if all the air is out? Five minutes or 20 minutes?

If you had an air lock, you would be running hotter; not colder.

Good idea to bleed anyway.

If the upper radiator hose isn't too hot to touch, chances are the thermostat is opening too early. Otherwise, the thermostat housing and hose should be at 80 C or more, much to hot to touch.

To bleed, try squeezing the upper radiator hose flat, then crimp the hose to the expansion tank shut, the relase the upper radiator hose. This will pull the air out of the radiator, where most if it sits (no way to vent except to the tank). Repeat and fill tank until it won't take any more, then expect to have to refill it a couple times. If you have the engine running with the ACC set to full heat (defrost with the temp wheel at the red clickstop), you will get most of the air right away.

Peter

Hello Mike, I was referring to bleeding after I put the new T-stat on. There will be so much air in there and I was worried. So, you also agree that teh correct way to bleed is to let the car run while leaving the cap off?

BTW, why did you not like the "boil in a pot" test? I don't really know how T-stat works but it is surely amazing. Especially these MB two stage T-stat.

all hoses have to be hot when running this test, any cold pipe means an air lock

and its the lower hose you need to compress to get the air out, if it remains cold the stat wont cut in it sees the engine as cold

not as bad as the BMW 525i but its a sod

Hello Teky,

I would like to think that air lock = poorer efficience = higher temp. Since my engine running cold, I think it is either T-stat or sensor. I will put in the new T-stat and drive it on highway for 20 minues. If it is still at 60C, I will replace the sensor.