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No heat when stopped/comes back when driving 2012 F-150 5.0L

Not sure what generation truck my 2012 F-150 5.0L 4x4 crew cab is so if I'm in the wrong place for this question my apologies.

So The wife is telling me that when she's driving the truck she gets good heat but as soon as she stops at a light the heat goes cold and comes back when she starts to drive again.

It's a 2012 F-150 5.0L 4x4 crew cab with tow pkge.

The rad doesn't have a filler cap so I checked the reservoir and it was way below the full cold mark but still has coolant in it so I am thinking one of two things. Low on coolant or maybe the coolant reservoir cap(which I think is the rad cap in this case as it has the markings of a rad cap)needs to be replaced.

I started the truck and looked for leaks which I didn't find but when I shut the truck off I noticed that there was a sudden sucking sound and kind of a short pop from the coolant reservoir cap so I'm thinking maybe it needs to be replaced.

Truck doesn't seem to be overheating otherwise.

What do you think?

Sent from my Yamaha Grizzly 700 EPS somewhere in the Great White North.


Hitech hillbilly
Staff member
Your truck is a Gen 12
I would guess it is just low on coolant.
At higher rpms water pump is flowing the coolant but at idle the coolant level is to low for it produce heat.
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Added five litres of coolant today(yikes)to the full cold top line on the reservoir and heat is back 100% so I'll keep an eye on it now.

Sent from my Yamaha Grizzly 700 EPS somewhere in the Great White North.