'00 Montana says overheating but its not

I drive the van for couple minutes it says it overheating, open the hood and no steam, no boiling in the over flow, return hose hot but graspable. I replaced the temperature sensor and the needle still pins over the 260 degrees and light goes on and bogs down, if you keep trying to drive it the temp needle will drop back to normal for awhile. :banghead: ... any help would be appreciated... thank you

is the water pump still working? take cap off and run engine. see if fluid moves.

have taken the cap off with the engine running and the fluid started flowing out why I assumed it wasnt the pump

"have taken the cap off with the engine running and the fluid started flowing out why I assumed it wasnt the pump "......that just means pressure is building up in the system....

Does your gage start at 100, and gradually go up, or does it jump up quickly? Is your cooling fan working?

When this thing overheats, via the temp gage, do you get any heat out of the vents with the temp set all the way up? If no, there is an air pocket being formed, sounds like a possible head gasket...

both cooling fans are running... blows cold when the temp gauge is pegged and it is gradual till about halfway then speeds up and pins off the scale

also there is no cloud of white exhaust or coolant in the oil

Still doesn't mean there isn't an air pocket formed......could have formed over time....I said possible head gasket......I forgot to mention possible lower intake gaskets as well....once air is in there, it doesn't burp out by itself.....can block fluid flow, and cause the vehicle to overheat quickly....

Another possibility is a blocked heater core....

You haven't noticed any loss of coolant in the reservoir or radiator?

nope coolant level has not changed since we had it from February ... and I wasn't trying to sound brash just first 3 things I checked for was the water pump,head gasket, and thermostat

air in the system for sure. sound like the heater core is full of air. bleed the air out using bleeders on top of thermostat and on top of waterpump housing.

Me thinks air in the system from intake or head gasket leak. When these items leak "sometimes", the fluid remains the same level. The gas inside the engine leaks (is pushed) into the ATF liquid cooling system. Thus, consistant air locks - even after the air locks are flushed out.

Years ago, I had a GM i4 - 2.2L engine. It kept over heating, then suddenly cooling - like a stuck thermostate. To test for head gasket leak, I removed the rad's cap (on cold engine), started the engine and at idle RPM and with a very bright flash light, I looked down - inside the rad cap opening. Sure enough, there was the odd "bubbles" - bubbling up in the ATF fluid. It was a bad head gasket - that allowed air to be pushed into the ATF liquid cooling system. Traded that vehicle in - since fixing it for $1,200 wasn't worth it.

Good luck...

Does sound like air in the system somewhere.

I found the easiest way to get the air out was to park the van in the driveway (we had a 3% grade slope) with front facing up hill. Then I opened the bleeders (one near the water pump, the other on the driver's side near the rear of the left, or front cylinder head * (* depending on how you are viewing the engine). With the engine on high idle, heater(s) on full hot - (we have rear heating / AC) watch for clear fluid coming out and close the bleeders. May have to do it more than once to get any air out.