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

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1988 420 SEL
1991 420 SEL with 560 motor
1993 Miata

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.

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1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra

Wrong thermostat, or defective part?

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1988 California version 260E (W124)
Anthracite Grey/Palomino
Owned since new and still going strong and smooth
MBCA member

Past Mercedes-Benz:
1986 190E Baby Benz
1967 230 Inherited from mom when she downsized
1959 220S Introduced me to the joys of keepin' 'em goin'

There are only 10 kinds of people in the world--those who understand binary and those who don't

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?

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Ron
2015 Porsche Cayman - Elizabeth
2011 Porsche Cayman - Bond,James Bond
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14 38HP John Deere 3038E Tractor -Mean Green
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71 220 - W115-Libby ( my first love) -SOLD
73 280 - W114 "Organspende" Rest in Peace
81 380 SL - Rest in Peace

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.

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1988 420 SEL
1991 420 SEL with 560 motor
1993 Miata

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

__________________
Ron
2015 Porsche Cayman - Elizabeth
2011 Porsche Cayman - Bond,James Bond
Sadly MERCEDESLESS - ALways LOOKING !
99 E320 THE Queen Mary - SOLD
62 220b - Dolly - Finally my Finny! Sadly SOLD
72 450SL, Pearl-SOLD
16 F350 6.7 Diesel -THOR
19 BMW X5 - Heaven on Wheels
14 38HP John Deere 3038E Tractor -Mean Green
84 300SD, Benjamin -SOLD
71 220 - W115-Libby ( my first love) -SOLD
73 280 - W114 "Organspende" Rest in Peace
81 380 SL - Rest in Peace

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

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1988 420 SEL
1991 420 SEL with 560 motor
1993 Miata

remove (slowly) the overflow tank cap

__________________
Ron
2015 Porsche Cayman - Elizabeth
2011 Porsche Cayman - Bond,James Bond
Sadly MERCEDESLESS - ALways LOOKING !
99 E320 THE Queen Mary - SOLD
62 220b - Dolly - Finally my Finny! Sadly SOLD
72 450SL, Pearl-SOLD
16 F350 6.7 Diesel -THOR
19 BMW X5 - Heaven on Wheels
14 38HP John Deere 3038E Tractor -Mean Green
84 300SD, Benjamin -SOLD
71 220 - W115-Libby ( my first love) -SOLD
73 280 - W114 "Organspende" Rest in Peace
81 380 SL - Rest in Peace

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

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86 560SL
With homebrew first gear start!
85 380SL
Daily Driver Project

http://juliepalooza.8m.com/sl/mercedes.htm

So, maybe it opened once, then stuck open?

__________________
1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra

Did he put it in facing the right way?

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A.S.E Tech A1,A6,A7,A8 & MVAC 609 + EPA 608

Unless stated otherwise, any question I ask is about my greymarket 1985 380SEL.

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.

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1988 420 SEL
1991 420 SEL with 560 motor
1993 Miata

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

__________________
1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra

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.

__________________
1988 420 SEL
1991 420 SEL with 560 motor
1993 Miata

Tell him to put it back in!!

__________________
1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra