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1996 Ford Explorer: temperature gauge..steady..indicator..keeps going

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I have a 1996 Ford Explorer and the temperature gauge on the dash keeps fluxuating between hot and normal. This is a steady slow rise to about the hot indicator on the gauge and then a very quick drop to the normal range and then it starts the rise again. It never reaches the danger point on the hot side but keeps going up and down consistantly. I am almost sure that the temperature of the engine is going up because I can feel the difference in the heat from the vents when the gauge goes up and down. I have done this with 100% accuracy with my son driving and him watching the gauge while I tell him when the heat is getting hotter or cooler. There is no sign of coolant in the oil and no exhaust smoke. What can cause this?
Hi this is Al, What you are describing sounds like you have air in the cooling system. Try removing the radiator cap and run the engine until the top radiator hose gets hot, that is how you will know the thermostat is open. The engine will most likely push coolant out while warming up, this is the air making it's way out. Now slowly fill the cooling system and allow the coolant to settle before putting the cap on. If it is air in the system this should cure your issue, Al
Customer: replied 4 years ago.
Air in the coolant was one of my first considerations when the problem started and I tried to solve it by running the car with the radiator cap off. I also put a flush valve kit on the car and power flushed it before I refilled it. However on your advice I tried it again. Coolant spewed out of the radiator and I continued to run it for a few minutes after the spewing stopped. I then refilled the radiator and took it for a lengthy test ride and the problem still exists with no change. I'm pretty much at my wits end.
These engines did have issues with thermostats if a top quality thermostat was not installed it could be sticking. There could also still be an air pocket which the best and absolute way to remove air is with an AirLift which is a tool that puts a vacuum on the cooling system and draws the coolant and displaces any air, Al
Customer: replied 4 years ago.
I changed the thermostat again and the problem persists. I talked it over with another local mechanic and while he wasn't sure of anything suggested that the heater core could be clogged. The vehicle gets heat just fine. What is your opinion about a heater core problem and how would I check it without changing it out? It is the last piece of the cooling system that I haven't touched. Otherwise I will have to go back to the air pocket theory.
I doubt a plugged heater core would cause your grief, but I have had air pockets in heater cores, To check if the core is causing the issue disconnect both hoses at the core and connect them together. Does the temp rise when you accelerate and come down when you let off? Al
Customer: replied 4 years ago.
The temperature tends to jump up after I let off of the accelerator and then it cools down quickly. When I was revving the motor with the radiator cap off, nothing came out of the radiator until I decelerated and then it spewed out the openning hitting the under side of the hood. It does rise when accelerating but tends to jump faster after letting off the gas.

That almost sounds like the water pump is rotating backwards. Here is the belt routing verify it is proper. Do you have the part number for the water pump that was put on? I have seen aftermarket pumps that have had the wrong impellers before, Al


Click here for diagram

Customer: replied 4 years ago.
I might think that but it was doing all of this before I changed the water pump. I will check the belt routing but the car has the diagram on it and I am confident it is correct. I will have to do that in the morning.
Let me know, Is it possible the thermostat is upside down? The pointed side should be up, Al
Customer: replied 4 years ago.
When I changed it today I was hoping I had done something dumb like install it the wrong way but no such luck. I am certain it is in correctly.
Has the coolant temp sensor been replaced?
Customer: replied 4 years ago.
No but once again I can feel the difference in heat coming from the vents when it is reading hot and warm. It is obvious so I can only assume that the gauge is reading accurately.
I,m sorry this just makes no sense, I will keep digging, Al
Customer: replied 4 years ago.

Could an inefficient water pump possibly cause the problems I am having? I have replaced the water pump but I guess its possible that it was not remanufactured correctly and not be pumping effectively enough. Here is the thinking...If coolant is not being circulated good enough, the engine would get too hot. When the coolant got hot enough it would naturally circulate like a closed radiator system in a house. Then the water that had been in the radiator longer than normal is cooler than normal and it is forced into the engine cooling it dramatically. I'm grasping at straws but I think I am going to replace the water pump again and see what happens.

That is very possible, I have had reman pumps with the impellers on backwards and flowing the wrong direction, Al
AlvinC, Ford Technician
Category: Ford
Satisfied Customers: 1241
Experience: With Ford 20+years as Dealership Technician Ford Senior Master Technician Gas and Diesel
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Customer: replied 4 years ago.
I'll let you know how it turns out. I won't be able to get to that project till next week. Thank you for all of your help.
Sounds good look forward to hearing back from you, Al
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AlvinC, Ford Technician
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