Overheating troubleshooting

2008 Mazda5. Est. 90K miles or so. Wife called today that the temp gauge was "all the way to H". Mostly stayed "in the middle" during the 45 minute drive. When accelerating, creeps up toward the H. When slowing down, it cools down, or when maintaining speed.

Checked the overflow, it's at the correct level. When she pulled in from driving 45 minutes, I threw some towels over the radiator cap and popped it loose. Coolant was cold, and just enough pressure to make it trickle overflow. Not like the blast of hot coolant you would expect. I started the engine back up with the rad cap off and the coolant level went down. I'm hoping this means the pump is pumping.

Radiator Cap, thermostat, water pump. The first two are an easy replacement. The latter not so much.

Thoughts? I'm thinking thermostat is stuck closed.

That was my initial guess. Thermostat stuck closed.

I think so too. Let us know what it turns out to be.

Radiator Cap, thermostat the two I'd replace fist and see how it goes, sounds like water not circulating through the engine.
Cheap enough to throw a few parts at it before spending any more serious time diagnostic.

When my T-stat was bad, it stuck open and threw a CEL. Its a bit of a bugger to get to. IRC, I thought I could do it all from above, but it jacking it up and pulling the skirt gives better access. Its $40 and incl the housing.

No CEL. Replaced the thermostat and radiator cap. We'll see what it does.

Run the heater on HIGH inside and see what kind of output you are getting. Also, this is a great way of keeping the engine from overheating if the water pump is still functioning properly as the heater core is not t-stat controlled.

is the problem sorted now