Z28 95 running hot

follow the radiator hose to the top of the engine where it goes in that housing that the hose goes into probably only has two to four bolts holding it on remove that housing and there is your thermostat take it out put the new one in pretty easy fix just remember make sure the motor is not hot and scrape off the old gasket and put a new one on or it will leak

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.

Have you checked the thermostat and/or the coolant level? SAFETY: DO NOT DO ANY OF THIS WHILE THE ENGINE IS HOT!

To check the coolant level, open the hood and look for a plastic cap with the thermometer symbol on it. (Do not confuse this with the windshield washer reservior, which has a windshield symbol on it). With the engine cold, look through the translucent plastic and compare the coolant level to the raised markings on the reservior. Fill as needed with a 50/50 mixture of coolant and distilled water.

To find the thermostat housing, follow the radiator hoses back to where they enter the engine block. You should see a hemispherical housing held on with two bolts. That is the thermostat housing. Remove the bolts and the thermostat, being careful to note how everything was positioned. Replace the housing and refill any lost coolant. Realize that we are still in the diagnostic phase. Run the engine until you are satisfied that it is not over-heating. It will not hurt the engine to run without a thermostat, but your heater will not work. To install a new thermostat, remove the housing and clean off all the old gasket and install the new one that comes with a new thermostat (if you do not clean off all the old gasket before installing the new one it will probably leak). Put the new thermostat in place in the same position as the old one, replace the housing and refill any lost coolant.

The thermostat housing is located in the upper radiator hose. You will see a metal elbow secured by two bolts into what I remember is the upper intake manifold...don't quote me on that but you will see it regardless of whatever I call it. Anyways, GM decided it was a good idea to put stainless steel bolts in there but that makes it easy to strip the threads in the bolt hole so I think it's best when you put your wrench on the bolt to just tap on the handle till you vibrate it loose, then turn it.
When you pull off the housing you'll lose a little coolant and it will create an air pocket which we'll address. It's a 50/50 chance that the thermostat housing gasket will either stick to the housing or the engine...it may be torn, cracked or flaking in either case clean both contact surfaces before you replace the gasket. I recommend an oil impregnated poly material gasket.
Note the position of the thermostat in the hole and place the new one in as such.
I recommend a 180 degree thermostat...stock is 195 but the GTP runs better with the 180.
Put on gasket and reinstall the hose/housing assembly, alternate from one bolt to the other so the housing mates flat and cannot cause a leak.
Next, fill the coolant overflow bottle to the "HOT" mark. If the bottle is dirty and hard to see just put a light on top of the bottle...GM didn't mark it very well so if it's not squeaky clean you can hardly see a damn thing.
*IMPORTANT** if your car still has Dexcool in it, DUMP IT. It will eventually destroy the motor.
If not...continue on. Start the car. Go back to the thermostat housing and place a rag on top of, and twist the peacock valve on top of it to let the air out until it starts to get coolant on the rag. Done.

The cap is not the culprit. More than likely you have a bad thermostat. Since you are going to have to remove the thermostat housing, now is a good time to get a flush kit from an auto parts store and also check the radiator hoses to make sure they are in good condition. If not, this is an excellent time to replace them.

Remember, when you take apart the thermostat housing, you must be sure to scrape all the old gasket off the housing and block as well. Be sure to put the new thermostat in right side up. You will have a gasket, and you will need gasket cement.

You either have a head gasket problem or the
coolant system wants bleeding as air is being trapped in it .
Take out thermostat and try bleeding and driving without it in if ok then try the old one or a lower temp one

Let me know how you get on Ron.

drain anti freeze follow upper radiator to the t-stat housing take off hose then 2 -13mm. bolts pull off housing there will be the t-stat pull out then clean housing put in new t-stat and gasket.then hose fill up slowly with antifreeze let run at 2000 rpms for 3 minutes with heater on hot air once you get heat your done.please rate this answer thanks.

It's not hard, just a bit messy. Follow the top hose that runs from the radiator to the engine. Where it stops is the location of the thermostat. You will need to drain the coolant out of the radiator before you begin taking things apart. Be prepared to replace the coolant with new coolant mixed 50-50 with water when you are done.
Go to the parts store and get a new thermostat for that year/engine/model combination. Remove the hose from the thermostat housing, remove the housing and pull the thermostat out. Note how the spring on the thermostat faces. Clear out any gasket material that remains with a soft scraper -don't damage the area where the housing goes or it will leak. Put the gasket and thermostat in, put the housing back on and remember not to over-torque the bolts or when the engine gets hot bad things will happen. Replace the hose (is it time for a new one anyway?) and then refill the cooling system with antifreeze. Start the car and check for leaks.

if car doesn't run hot with no thermostat...you may be air-locking system when installing thermostat. I always fill the engine block up to the thermostat housing to make sure that engine is full of fluid ..then install thermostat...spring facing down..thenfill radiator making sure car is running so as not to air lock system ...run until top hose gets warm ,showing that your thermostat opened

look for bleeder valve on thermostat housing or other coolant hoses or lines. possible traped air in cooling system,will cause car to run hot.