05 Yukon xl engine temp rises at lower speeds, then drops at higher speeds, and intermittent interior heat

[Surface Dr]

Hi my 05 Yukon XL with 135k mi is having some overheating challenges when driving under 50mph. The heat can be intermittent , The E fans seem to be howling when at idle. Cranking the heat can help, but mostly at high speeds. I am just not sure where to start with the diagnosis. Any help is appreciated.
Thanks

 

[swathdiver]

Hi my 05 Yukon XL with 135k mi is having some overheating challenges when driving under 50mph. The heat can be intermittent , The E fans seem to be howling when at idle. Cranking the heat can help, but mostly at high speeds. I am just not sure where to start with the diagnosis. Any help is appreciated.
Thanks

Start with checking the coolant level and then do a pressure test of the system and cap. If the level is low look for leaks of course. Make sure both fans are running at high and low speed settings to ensure one is not burnt out or a relay has gone bad.

 

[Fless]

^^ This. Also check the engine coolant temperature (ECT) sensor to be sure it's reading correctly at cold start, and following the engine warmup. Put a scanner on it to read live data.

Any work done lately on it?

 

[SnowDrifter]

Check coolant level as others have said

If that's good, then check for belt slip / thermostat operation

If those are good, take a realllllll good look at your coolant. May have gone sour and plugged stuff up.