Engine overheats when the airconditioning is running

[Rexflex]

Hi all

First post from me!

I have an issue with the engine overheating when the air conditioning is running. When the A/c is off the car is absolutely fine. But when you turn the a/c on the engine temp starts to rise sometimes after 30mins or so. If I turn the cabin temp to high and let all the hot air into the cabin it cools down to normal very quickly.

Thank you

 

[LostKiwi]

Air conditioning is a heat pump. It uses the natural properties of the change of state of a liquid to a gas to provide cooling in one place but when compressing the gas back a liquid it generates heat in another. This happens at the condenser behind the grille in front of the radiator. As we then have a heat source in front of the radiator the cooling efficiency of the engine cooling radiator is reduced so the engine may run hotter as a result.
You can see this effect of pumping heat with a domestic fridge the inside is cold but the back of the fridge is hot.

Air (and ground) source heat pumps work on the exact same principle.

 

[Blobcat]

Is your engine fan working?

If you car suffered from overheating because the fan had failed the simple way to get you home was to put the heater on full. It doesn’t make for a pleasant drive home but it stops the car overheating.

I don’t think you’ve an AC issue, I think you’ve an overheating problem. Blocked radiator or fan not working would be where I’d be looking. Having a Land Rover that I wade through thick mud with I know about blocked radiator and failed fan..

 

[Rexflex]

Yes the fan is doing overtime at the moment. Main Dealer is looking at it and thinks they could disconnect the heater booster.

 

[mioba]

disconnecting the heater booster is not solving the issue but a sidestep. Suprising that from a main dealer and expensive

 

[peternutkins]

In the olden days that sort of problem could be caused by a partially blocked radiator but with modern cooling systems and year round antifreeze it shouldn't, unless you have had a leak and it has been frequently topped up.

Peter

 

[Flyinspanner]

Or someone has dumped a load of radweld in it?

 

[Rexflex]

I had a leak previously and so the water pump was replaced last year or the previous year. Otherwise only Mercedes or SN Autohaus have worked on it. SN Autohaus replaced the water pump.

 

[Bogus]

Air conditioning is a heat pump. It uses the natural properties of the change of state of a liquid to a gas to provide cooling in one place but when compressing the gas back a liquid it generates heat in another. This happens at the condenser behind the grille in front of the radiator. As we then have a heat source in front of the radiator the cooling efficiency of the engine cooling radiator is reduced so the engine may run hotter as a result.
You can see this effect of pumping heat with a domestic fridge the inside is cold but the back of the fridge is hot.

Air (and ground) source heat pumps work on the exact same principle.

He said it was overheating, that suggests its a bit beyond just running hotter. If everything is as it should be any hotter running would be a few degrees that barely register on the temp scale. He definitely appears to have a cooling issue.

 

[LostKiwi]

He said it was overheating, that suggests its a bit beyond just running hotter. If everything is as it should be any hotter running would be a few degrees that barely register on the temp scale. He definitely appears to have a cooling issue.

Overheating can mean different things to different people. For me it's more than 10 degrees above normal. For some it's more and for some less. With no quantitative values it's hard to be sure.
That said my r129 was running 10-15 degrees hotter than normal during the recent hot spell yet I wasn't concerned as it's done it before on very hot days. Aircon off and it happily dropped a few degrees (almost back to normal).

 

[Bogus]

In the olden days that sort of problem could be caused by a partially blocked radiator but with modern cooling systems and year round antifreeze it shouldn't, unless you have had a leak and it has been frequently topped up.

Peter

Radiators can become blocked on the air side as well.

 

[Rexflex]

The thermometer was showing 100 degrees and once it got to 110 before I intervened and turned the A/C off. I was too nervous to see how hot it was going to get.

 

[LostKiwi]

The thermometer was showing 100 degrees and once it got to 110 before I intervened and turned the A/C off. I was too nervous to see how hot it was going to get.

And it normally runs at?

 

[LostKiwi]

Radiators can become blocked on the air side as well.

On a 9 year old car it's a definite possibility as are deteriorated cooling fins.
It's also a CDi so not only is the AC condenser in front of the radiator so is the intercooler I believe.

 

[Rexflex]

Normal operating temperature is about 90 degrees. Anything above that causes the fans to come on. I have done about 100,000 miles myself total 126,000 and perhaps it’s all got a bit dirty. I did have the bonnet catches cleaned out as the sensors had stopped working due to the accumulated dirt.

 

[harrye500]

Plus 1 for blobcats suggestion, possibly do a cooling system flush, relatively cheap to do and never a bad thing anyway. Sticking thermostat possibly under strain with AC on? Are they electric on W212 though? Some aerosol condenser cleaner on the fins may help - spray on liberally then brush off with a fine bristled brush. They can clog up with flies and muck over the years and a garage AC service inevitably forgets this basic clean!!