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FIXITMAN
FIXITMAN, Master Technician
Category: Mercedes
Satisfied Customers: 17198
Experience:  ASE MASTER CERTIFIED, AA DEGREE DIESEL/AUTOMOTIVE
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Im having problem with an w140 S500,its overheat and barely

Customer Question

hi,im having problem with an w140 S500,its overheat and barely its cooling down, no matter if i drive it down the highway or city,more like on the highway the temp is increasing rather then cooling down....leaving it on idle the temp will start to decrease but not where it supposed
Submitted: 1 year ago.
Category: Mercedes
Expert:  FIXITMAN replied 1 year ago.

Hello, my name is Bobby. Thank you for the question. Can you give me the year of the vehicle?

Customer: replied 1 year ago.
95 w140 s500
Expert:  FIXITMAN replied 1 year ago.

Thank you for the response. If it is running hotter on the highway than it is when it is stopped then you typically have a failing thermostat, a clogged radiator or you have a failing head gasket. You can check for a head gasket leak with a block tester from most auto parts stores. If the coolant is very dirty then suspect a radiator issue. One thing to keep in mind is that the thermostat is the cheapest place to start. If you were going to do anything first it would be this. If you are not sure I would replace this first to see if it will help. Please let me know if you need further assistance.

Customer: replied 1 year ago.
Well so far changed thermostat,water pump,radiator,fan clutch, belt, 2 aux fans new coolant Mercedes specs and problem still there. Did a block test which was good,there are no leaks,coolant level is the same even after a month of driving.Most of the time its overheating during highway speeds more increase speed more temp rise
Expert:  FIXITMAN replied 1 year ago.

Did you happen to also install a cooling system pressure cap?

Customer: replied 1 year ago.
i forgot to mention about the the tank cap,that was changed to
Expert:  FIXITMAN replied 1 year ago.

The only other things that I would assume would be the issue would be a very small head gasket leak that will allow combustion pressure into the cooling system and cause this to occur and will not show up during a block test. I would also check the front of the vehicle and make sure that the a/c condenser that sits in front of the radiator is not clogged with any debris. This can cause air flow issues.

Customer: replied 1 year ago.
That was clean when the radiator was changed also in case even if a small gasket leak i should loose coolant ?! I made around 5000 miles and level its still there....doesnt make sense...no matter how small a gasket issue i would have i should be losing cooland and level drop after so many miles
Expert:  FIXITMAN replied 1 year ago.

I have had this engine fail in a way that will not burn coolant, but instead will only allow combustion pressure to enter the cooling system.

Customer: replied 1 year ago.
youre assuming i have a cracked head gasket even if the engine its not boiling just go over normal temperature?! And why is it cooling down when i let it on idle ?
Expert:  FIXITMAN replied 1 year ago.

From the symptoms you are having that is what it sounds like. When you are at idle there is not as much internal combustion pressure building up so the pressure cannot bypass wherever the failure is. There are not many other possibilities there unless there is simply a lot of build up in the cooling passages which is not too common.

Customer: replied 1 year ago.
it shouldnt rise over and over and start boiling? It rise over normal temp and stops there and without AC its running at normal temp
Expert:  FIXITMAN replied 1 year ago.

If it is running at normal temperature with the a/c turned off then It may not be the issue. It should be happening even with the a/c off. Were the upper and lower radiator hoses replaced? Do you have a infrared heat gun for diagnosing further?

Customer: replied 1 year ago.
i changed them when i change the radiator abd yes i did measure the temps of in and out,difference is temps is 5 deg celsius so whats coming out the radiator is 5 deg cooler than whats going in like with AC on is 105 at the entry and whats coming out its around 100 deg celsius with out AC whats going in its 100 and its coming out at 95 deg celsius
Expert:  FIXITMAN replied 1 year ago.

Do you have a scan tool to see if the computer is reading the same temperature as the gauge?

Customer: replied 1 year ago.
i did that too dash gauge it reads same as computer....before changing the radiator& cats the temps were a bit higher that whats reads now like on the highway reached almost the red line
Expert:  FIXITMAN replied 1 year ago.

If the ignition timing is too advanced this can happen also.

Customer: replied 1 year ago.
how do i know if the timing is right or not?
Expert:  FIXITMAN replied 1 year ago.

You will either need to see what your scan tool says that the timing is or use a timing light.