My 97 nissan pickup v4 4x4 is overheating. I have replaced the thermostat, radiator, clutch fan, and the sending unit. What could still be there problem?

Please to use a thermometer in the radiator after the engine has run about 10 minutes to test the actual temperature of the coolant.
You may also choose to put a piece of flat cardboard over half of the front of the radiator to see if that makes it heat up more.
The temperature should be about 180 degrees, unless you have added a 160 degree thermostat to the engine, in the which case the radiator temperature should be nearly 160 degrees.
If the radiator shows the proper temperature, perhaps the temperature gauge on the dashboard or the engine block sending unit is defective.
God bless your efforts.

chek if the electric fan is turning on when the ac is turned on

Check to be sure that the fan shroud is intact in order to allow the fan to properly draw air through the radiator. Ensure that the radiator and A/C condenser cores are not clogged up with debris that can impede air flow. If these are all okay, you may need to replace the fan clutch as it may not be working correctly.

Verify the sending unit is correct. Verify it is overheating. Sometimes the sending unit and/or gauge is bad.

Bad news is the air bubbles are actually exhaust gases entering your coolant, you have a blown head gasket, and or bad head, which is also the overheating concern.

no it is not normal....have a repair shop check the coolant fan sending unit or the coolant fan relay...a bad sending unit maybe sending a false overheating signal to the fan causeing it to stay on or the on off fan relay maybe sticking on..an make sure the jeep is not actually running hot the temp guage maybe reading the wrong temp

Here are a few things to try. Check your Radiator cap pressure as a weak cap will boil over more quickly. Replace your Temperature sending unit. There may be 2, one is for the dash gauge and the other is for engine control. Unless you have other signs of overheating you may not be overheating at all, just a bad sender.

Next, the cooling fan clutch is what the fan blade is mounted on. This is checked by using a Timing light with a RPM tach made into it. The fan clutch "locks" at a variable temperature. In cold weather it slips internally. In hot weather it engages or "locks". A mechanic measures the RPM of the blade to the RPM of the engine. This tells if the fan clutch is slipping too much internally.

Before considering a new water pump try a free scan at Autozone or Oreillys. If your Catalytic converters are plugged the engine will overheat. You should have a Code for this problem in the onboard computer. About all you can try after this is to remove the Thermostat completely and see if the engine still overheats.

You will have to replace the New Thermostat again because the other engine controls will not work right without it. But without the thermostat you can test for collapsed Radiator hoses at say 2500RPM or above. Thats about all I can think of, the water pump would cost the most. The cap, sensor-sender, fan clutch, and hoses are not that expensive.

you just said it you need a new fan clutch

The thermostat is behind the upper radiator hose...remove the upper radiator hose and loosen and then remove the two bolts on the outlet (where the upper radiator hose ends) The thermostat is inside that outlet...Another thing to check is if there is water in the oil...If there is water in the oil then it could have a head gasket problem..Did you ever replace the thermostat? If it is stuck then it would also cause overheating...Excessive exhaust backpressure because of a clogged catalytic converter could also cause overheating..Also check belt tension and condition. A loose belt that slips may prevent the water pump from circulating coolant fast enough and/or the fan from turning fast for proper cooling...Another thing it could be is a faulty Water pump -- Any wobble in the pump shaft or seepage would call for replacement. In some instances, a pump can cause an engine to overheat if the impeller vanes are badly eroded due to corrosion or if the impeller has come loose from the shaft. The wrong pump may also cause an engine to overheat. Some engines with serpentine drive belts require a special water pump that turns in the opposite direction of those used on the same engine with ordinary V-belts... Also check the Fan -- With mechanical fans, most overheating problems are caused by a faulty fan clutch, though a missing fan shroud can reduce the fan's cooling effectiveness by as much as 50% (depending on the fan's distance from the radiator) which may be enough to cause the engine to overheat in hot weather or when working hard.

1 Inspect Cooling System Mix Coolant level low or flow is restricted. 2 Inspect Belt Incorrectly routed, adjusted, tensioned, missing, or worn water pump belt(s). 3 Inspect Oil Pan Gasket - Performance Ruptured, cracked or leaking radiator hose. 4 Inspect Radiator Cap Worn or damaged radiator cap 5 Inspect Thermostat Thermostat stuck closed 6 Inspect Fan Blade Broken, missing, or defective fan blade(s). 8 Inspect Water Pump Damaged, worn or leaking water pump. 9 Inspect Intake Manifold Plenum - Perform Leaking water pump gasket. 10 Inspect Cooling Fan Control Faulty cooling fan control or circuit. 11 Inspect Cooling Fan Switch - Radiator Faulty radiator cooling fan switch or circuit. 12 Inspect Engine Temperature Sensor Faulty engine temperature sensor or circuit. 13 Inspect Temperature Switch Damaged or faulty temperature switch or temperature switch circuit. 14 Inspect Fan Clutch Worn, loose or faulty fan clutch. 15 Inspect Ported Vacuum Switch Damaged, leaking, or faulty ported vacuum switch. 16 Inspect Radiator Obstructed radiator core or radiator cooling fins. 17 Inspect Head Gasket - Performance Head gasket leaking coolant into cylinders