Temp gauze indicates engine running hot (not always happening) can move up to very hot....have radiator flushed and new thermostat-(8 month ago).two weeks ago two new cooling fans...mechanic cant find the problem.. I dont think its temp.sensor(?) as temp goes up slowly when driving, and shoots up when stationary..then reduces again when driving....mechanic has added sealant to the radiator

HI Anon.
Remove your radiator cap (WHEN ENGINE IS COLD), run the motor and let it warm up a minute or two, have someone rev the engine a little while looking into the radiator. If the water goes down and/or you see the water moving, your water pump is good. If not this means that your water pump is bad and is not moving enough water through the system. Try Removing your thermostat and run the engine without it. Keep checking the radiator hoses to see if they are the same temperature. If they are not the same temperature the water pump is bad. If they are the same temperature your thermostat is more than likely the culprit. Hope this helps.

Air Temperature Control Blend Door Failure No Heat or no AC (dependes on where door sticks)

In all modern cars there is a tiny DC electric motor driven gear drive that moves a plastic door that blends heated and cooled air from the AC and the heater core, this is called (interestedly enough) the air temperature control blend door actuator. The motor that moves this door fails because the door starts sticking from warping, most common complaint is no heat but it can be no AC as well. To replace these parts (blend door and blend door actuator) you must remove the entire dash from the car. Always have this problem confirmed by a dealer or qualified repair shop as it is a very expensive repair.

Other causes of no heat in the cabin are engine thermostats that are stuck open (engine runs cold) or air bubbles in the cooling system.

Have you verified that system has been "burped" of air? That the fans kick on? Check radiator hoses for kinks. You can try running without the thermostat to see if that helps and can identify if the thermostat could be a problem. You may need to check your coolant mixture too.
Sounds like you have reduced flow or restriction in cooling (heat exchanger) as it runs fine at cooler temp but can't keep up when it gets warm.
New parts don't always equal working fine parts.

Your thermostat which closes when cold to let the engine warm up and opens when the water gets hot and lets it circulate through the radiator. If they stick closed your engine overheats in short order. When they stick open it takes forever and a day for the heater to warm sufficiently. This also makes the engine use more fuel. If your thermostat is indeed stuck open it is easy to tell. With the engine cold, take the radiator cap off and start the engine. If it is stuck open water will immediately circulate as seen at the radiator cap opening. If the thermostat were operating normally you would not see water moving for 5 or more minutes of idling. You should always drain the coolant, flush and refill every 2 years. Never ever grab that garden hose and put tap water in your radiator. They come from the factory with distilled water (about $1 a gallon at any grocery store) which has no minerals in it. The minerals in tap water promotes rust and scale build-up in the cooling system reducing it's efficiency. Don't use tap water unless you WANT to have problems.

Flushing a radiator at home is most often a waste of time. The chemicals used by a radiator shop are far stronger than anything you get at the parts supply house. What happens is that the flow you see is deceptive as the water tends to flow around a partial restriction and does nothing to remove it.
Though your garden hose has a lot of pressure it doesn't have the volume necessary. A garden hose will usually be sufficient to flush out a heater core though as it's smaller.
Have a radiator shop flush your radiator core and have a hydrocarbon test done on the cooling system to eliminate the possibility of head gasket problems. When re-filling the system, turn the engine off just before it reaches 210 degrees and let it sit for a few minutes so the thermostat can open, then continue filling. Always fill with the heat on high. Thermostat goes in with the spring end into the engine.Forget about the water pump...unless you got one made for a twin engine marine (CCW) application it has nothing to do with the problem though I've heard but never actually seen that the belt can be installed so it turns backwards.
When the engine is at operating temp cover the grille and force it to nearly go into overheat...the electric fan should come on and the mechanical fan should lock up to try to counteract that.
Don't open the system when it's hot (you found that out already). And, don't allow the engine to actually overheat.
My bet without seeing it? Likely a partially obstructed radiator. 4.0 engines make a lot of rust residue even when maintained. If that's what you have in there, or even the 4cyl. It is more than likely.

REPLACE UPPER RADIATOR HOSE IT IS TOO SOFT WORN OUT CAUSE HOSE TO CLASP.I WOULD REMOVE THERMOSTAT FLUSH ENGINE BLOCK AND RADIATOR COOLANT SYSTEM NEED FLUSHING OUT.

Replace the thermostat. I have had bad luck with aftermarket thermostats on this particular year of GM/ Chevy trucks.They tend to stick closed and then fully open, thus causing the temp to sort of surge hot and cold. A good thermostat will slowly open maintaning a good consistant temp. I would recommend spending a little more and getting a GM thermostat.


Please take a moment and rate my answer..........Thanks

Check to see if you have coolant in cylinder #3 by taking out the plug and looking at the tip.

Here are the least expensive items in order of least to greater...Check for proper coolant level in small tank next to the radiator. If low... refill to proper level. make sur eyou use the right stuff for you particular engine . Any good auto parts place can help with the "right stuff"
if OK, next...
Replace the radiator cap. if still getting hot, replace the temp sensing valve ( thermostat) at the engine side of the top radiator hose. If it looks grungy, you may still need to get the whole system flushed.

This is a very messy job that removes the foul stuff in the cooling system which may prevent coolant circulation.
If the water pump is defective and it was changed 2 months ago take it back to the place that installed it.
Check for loose belt driving the water pump first

Hello,
The thermostat was a good choice however you should go to a service station and get a complete cooling system flush.
your heater core and or radiator could be clogged.