My truck overheats at an idle and after getting to 251 it slowly drops 2-3mph at a time. When I start driving after it gets hot it'll drop greater. Doesn't stay at 251 for more than a few seconds. I already replaced the thermostat any ideas what could be wrong! I was told fan clutch

 

low on coolant..... or a fan clutch is busted...

 

Possiable radiator has a flow problem. Agree w/Izero coolant and check fan clutch

 

The fan clutch has small play to move back and forth. When I try to spin it it has the same resistance from cold to hot. Also it's been doing it about 6+ months. There's been plenty of coolant put into it. I've had to put more in it a few times in the last 6 months. Heater and air conditioner work 100% fine. But could it still maybe be the water pump?

 

yes it can be, how much coolant have you added? and have you busted open the pressure caps on the radiator?

 

Compression Check?

 

wouldn't hurt... but a leak down would be a better indicator if he's got something else going on, as you can leak coolant into the engine and still have good compression... (or at least acceptable compression loss).

This is kinda one of those problems that need to be looked at first hand, in person in order to diagnose correctly....

If it only happens at Idle, then I am more inclined to think that there is something going on with the thermostat and the water pump where you aren't getting any flow through the system, which is causing you to over-heat.

 

Just to add, and i know someone already said it but restriced flow through the Rad

 

Not likely a radiator issue, since it cools off when moving. Points toward the fan clutch so far. If you can afford it, an e-fan is the path to take. Otherwise, a mechanical clutch will be ~$60 or so.

 

I have to agree with the Fan theory so far, but I'd still get a leak down test to make sure you're not losing coolant somewhere else...