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Matt
Matt, Engineer
Category: Mercedes
Satisfied Customers: 20602
Experience:  Mechanical Engineer with 23 years experience in the auto industry, 8 yrs in formula 1 engine testing
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Check coolant warning come on, but coolant is full, car temp

Customer Question

Check coolant warning come on, but coolant is full, car temp gage fluxiates....when I turn on A.C. Fan does not come on right away.....sometimes it comes on and car is fine....but most time I can drive about 30 minutes before I get coolant warning
Submitted: 5 months ago.
Category: Mercedes
Expert:  Matt replied 5 months ago.

Hello

it could be a problem with the coolant level switch / float in the header tank so I'd replace that sensor as the next step as they do corrode internally and then stop working

the fluctuating temperature sounds like either a failing thermostat or trapped air in the system

so I'd consider replacing the thermostat as the next step and its possible the belt is slipping so its worth replacing its especially if the rubber ribs are cracked or it looks shiny on flat side of the belt - while its off check that the tensioner and other pulleys all spin freely the exception to this is the AC pump ( if fitted)

I'd also suggest ensuring that you've got all the air out of the system as this sort of episode can leave pockets of air trapped and prevent the coolant from circulating properly.

So check that the small hoses that run from the top of the engine and radiator that run back to the header / expansion tank are free flowing and not blocked

If these are OK then remove the tank pressure cap and run the engine from cold with the heater set to full hot

As the engine starts to warm, squeeze the large hoses running to the radiator to help displace any air - all the time keeping an eye on the level in the header tank, making sure it doesn't drop below the minimum and open any bleed screws built into the top cooling hoses - closing them off as soon as they start to gush water

Once the engine is up to temperature the tank should be quite full. Switch the engine off and leave it to cool

Then top up to the mark once its all cold and replace the cap

Expert:  Matt replied 5 months ago.

Hi

do you still need help?

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