Help! Replaced Water Pump and Themostat Now Engine Runs Cold
 

My mechanic just replaced my water pump and thermostat on my 91 Mercedes 420 SEL with a 560 motor. Now the engine barely warms up. It used to run at just over 80C. Now it is barely getting to 50C and the heat is no longer very hot in 25 degree Fahrenheit weather. The thermostat was bought at Mercedes. Any ideas would be appreciated.

Thanks

How good is your mechanic? Maybe he forgot to put the thermostat in? When you start the car from cold, the thermostat is supposed to stay closed until the engine warms up. What this means is nothing should flow in the radiator (and radiator hoses) until it is warmed up. What this means is that the hose to the radiator should stay cold until the temp guage gets up a ways. Soooooo, if you feel that hose getting warm right away, then something is wrong there, but you already knew that. Maybe someone else has another idea.

Wrong thermostat, or defective part?

thermostat needs to be oriented properly and the system needs to be burped properly ,sounds like a vapor lock or bad T stat or bad orientation or all of the above, assume mono valve is working?

Thank you for your replies. My mechanic is not a Mercedes specialist but he does work on a number of 126s. That is why I am posting here. I will relay your suggestions to him. How do you "burp" the system? What is the correct way to orient the thermostat? I will assume that the mono valve is working because there was no temperature issue until the new pump and thermostat were installed.

Thanks again.

there is a small arrow on the tstat that must point upward. elevating the front of the car ( on ramps) , get her up to temp, allow to cool a little and then slowly release pressure, do it a few times. Milk the hoses.
When I flushed my SD it took days to burp

Thanks I will ask the mechanic if he made sure to put the arrow upward. How do I release the pressure?

remove (slowly) the overflow tank cap

Brand spankin new thermostats have been known to be DOA, generally because of mechanical damage.

So, maybe it opened once, then stuck open?

Did he put it in facing the right way?

I drove the car today and was able to monitor the temp. It seem to heat up quickly to 50C as though the thermostat is operating correctly. The temp did creep up slowly even as high a 80C but went back down on it's own when not under as much load. It almost seems as though the thermostat is calibrated for the wrong temperature. My mechanic is going to look at it on Saturday. He said that he did put the thermostat in the right way and that he burped the car. So we will see what he finds on Saturday.

That's just how mine acted, WHEN THE THERMOSTAT WAS BAD! Maybe he put the old one back in. Also, some people here are sensitive to thermostat brand. The concensus seems to be: Wahler = bad, Behr = good

He bought the part form a Mercedes dealer so I am not sure who the manufacturer is. The old thermostat worked perfectly. He changed it "to be safe." I think it is a bad part too at this point. I will post when we figure it out. Thanks.

Tell him to put it back in!!