[thepolymath]

Hello everyone,

I was hoping that there may be someone here that can help shed some light on a stubborn overheating issue I have been trying to troubleshoot. I recently changed the water pump and thermostat, and flushed the coolant as part of maintenance. The old coolant was starting to get discolored and I already had everything apart to take care of the timing chain rattle which gave me a check engine light.

After all of that work, I found that there is one situation where the car will start to overheat - which is when I have the car at a long stop light, or a long idle ONLY WITH THE AC ON. Without the AC on, or with the heater on, no issues - I can go up hills and all over town without running into an overheating problem. The second I turn on the AC, all hope is lost. I verified the temperature through my Autel scantool and I can see the coolant quickly go from 215F to 240+F within a few minutes.

The entire cooling system has effectively been replaced, brand new radiator, brand new water pump, thermostat and all. I can see the radiator fan working and it speeds up and slows down according to vehicle speed and coolant temperature. I can't for the life of me figure out why it only overheats when the AC is on. I have checked the AC system to see if it was over or under charged and the pressure was within specification and the AC does blow cold.

Please help with any ideas, I've run out of just about every idea.

Thanks everyone in advance!

 

[peterjmdr]

Is the fan speed kicking up to full speed (loud) when you have the AC running?

 

[Tymac03]

Faulty t-stat?

 

[thepolymath]

Is the fan speed kicking up to full speed (loud) when you have the AC running?

Yes, it maxes out at full speed, I can see it spinning up on the scan tool and I can audibly hear it as well.

 

[thepolymath]

Faulty t-stat?

I don't think so, I have replaced the thermostat twice with the Mercedes Benz original part (first time, then replaced again). I don't think I would have gotten a defective thermostat twice in a row. I also can see the thermostat signal on the scan tool with the voltage going up and down depending on the temperature of the coolant, so it does appear to be responsive. I can't get it to overheat with the AC off, and this is in 90-100 degree weather with the radiator fan on full speed.

 

[peterjmdr]

Is the fan speed kicking up to full speed (loud) when you have the AC running?

Are any of the hoses collapsed?

 

[peterjmdr]

Is the fan speed kicking up to full speed (loud) when you have the AC running?

Possible air lock in the system preventing proper coolant flow?

 

[peterjmdr]

Is the coolant level correct?

 

[thepolymath]

Are any of the hoses collapsed?

It does not appear that way, I vacuum filled the system and the system held air and did not leak, and it drew in an appropriate amount of coolant. The level is correct, no leaks are seen in the system. From the hoses I can visually follow and inspect, they all look like they are in the correct shape and condition.

 

[peterjmdr]

Does the serpentine belt have proper tension? Do all pulley’s spin normally/ no wobble, no bearing noise?

 

[thepolymath]

Does the serpentine belt have proper tension? Do all pulley’s spin normally/ no wobble, no bearing noise?

Indeed, serpentine belt was replaced and the belt tensioner was replaced with Mercedes Benz original parts.

 

[peterjmdr]

Lastly, can you confirm good airflow through the new radiator. No clear shrink wrap left over the fins that might obstruct airflow?

 

[thepolymath]

Lastly, can you confirm good airflow through the new radiator. No clear shrink wrap left over the fins that might obstruct airflow?

Nothing that I could see or feel, when the fans kick in, I can feel the air flow through the front grille into the engine bay.

 

[peterjmdr]

Lastly, can you confirm good airflow through the new radiator. No clear shrink wrap left over the fins that might obstruct airflow? Is the ac condenser clean and allowing good air flow.

“Although not as common, it's also possible for a heater core to become obstructed enough to restrict coolant flow and cause overheating.
The most common cause of an overheating engine when idling with the AC on is a broken (in some way) compressor unit. If your AC is blowing hot air when at idle, the compressor or pulley is likely to be failing but not yet completely broken”

 

[thepolymath]

“Although not as common, it's also possible for a heater core to become obstructed enough to restrict coolant flow and cause overheating.
The most common cause of an overheating engine when idling with the AC on is a broken (in some way) compressor unit. If your AC is blowing hot air when at idle, the compressor or pulley is likely to be failing but not yet completely broken”

I was thinking it may have had something to do with the AC system; however, the AC is very responsive, and when I set it to "LO", the temperature drops to about 59F at the vents (measured with the thermostat). I also checked the heater core by having high heat, and I can see that the heater gets exceptionally hot, and I can see the coolant temperature plummet when I do so as well.

It is really confusing to me as it seems the AC system works as one would expect.

 

[cgrutt]

Is it actually overheating or just seeing temperature go up? Possible faulty and/or overly sensitive temp sensor? Can you confirm actual temp with external thermometer (believe your scan tool uses same sensor as gauge). All else equal it would seem normal for engine temp to increase at idle with AC on.

 

[Tymac03]

I don't think so, I have replaced the thermostat twice with the Mercedes Benz original part (first time, then replaced again). I don't think I would have gotten a defective thermostat twice in a row. I also can see the thermostat signal on the scan tool with the voltage going up and down depending on the temperature of the coolant, so it does appear to be responsive. I can't get it to overheat with the AC off, and this is in 90-100 degree weather with the radiator fan on full speed.

Chances of two bad units is unlikely. Just a thought. Good luck.

 

[thepolymath]

Is it actually overheating or just seeing temperature go up? Possible faulty and/or overly sensitive temp sensor? Can you confirm actual temp with external thermometer (believe your scan tool uses same sensor as gauge). All else equal it would seem normal for engine temp to increase at idle with AC on.

I used the external thermometer on the radiator and the upper and lower radiator hoses and the temperature tracks. There is something I did notice, once the coolant started to creep up to 240F, I started hearing the sound of bubbling, like a kettle going off in the radiator and upper radiator hose.

 

[cgrutt]

Definitely sounds like it's overheating don't think you should hear gurgling. Are you sure system is filled to correct capacity and there are no air locks? You're not losing coolant (e.g., through leak or blown headgasket)? Not sure if thermostat is one piece with housing (the style on my 2006 E350) but could it be installed upside down? Possible bad expansion tank cap not allowing system to pressurize? Bad water pump (I know you said it is new but could it not be circulating properly)? Blockage somewhere in system? Very odd I hope you sort it out good luck.

 

[thepolymath]

Definitely sounds like it's overheating don't think you should hear gurgling. Are you sure system is filled to correct capacity and there are no air locks? You're not losing coolant (e.g., through leak or blown headgasket)? Not sure if thermostat is one piece with housing (the style on my 2006 E350) but could it be installed upside down? Possible bad expansion tank cap not allowing system to pressurize? Bad water pump (I know you said it is new but could it not be circulating properly)? Blockage somewhere in system? Very odd I hope you sort it out good luck.

The coolant is correct in capacity, I have double checked after the engine cooled from the overheating incidents - I doubt there would have been an air lock since I used the vacuum method to fill the system. I have not lost coolant, I don't see the white smoke indicative of head gasket failure, nor do I see contamination of oil in coolant. I replaced the water pump twice, and I've replaced the expansion tank cap twice as well.

I checked the thermostat as well, it is the one piece with housing and I don't think it could be installed upside down. Thanks for all the help so far with brainstorming. I'm so confused by this one, maybe I have to take it in to a Mercedes specialist shop to figure out.