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Q: Why is temperature gauge fluctuating so much, and running hot? on 1992 Toyota Pickup

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In May 2012 I had the thermostat changed to one that can handle hot, high desert conditions because my engine was running hot. The mechanics in Arizona replaced the thermostat, a cracked radiator cap, flushed the radiator and added a "special" radiator fluid. It diffused the problem, although it ran hot when climbing hills and gaining elevation (mountains and passes). They also added this purple stuff into the radiator and checked for a blown head gasket, and found it was fine.

Now, in January in Northern California, the engine is almost immediately running hot - at the beginning of the red line. The temperature gauge is fluctuating all over the place and randomly. I live in an agriculturally flat area. When the radiator starts running hot I kick on the defrost full blast and the highest heat. Some times it helps, other times not. During stop starts I hear a swish and gurgle of water flush through under my dash. I haven't heard that sound lately. It is getting worse. Last night it blew cold air for the first time. Today it ran in the red. It has never steamed over or blew the cap off.

I'm beginning to think the erratic temperatures are partially symptomatic of my high altitude / hot climate SW thermostat not liking the cold low altitude. But now there's the new symptom of blowing cold air, which makes me think it's the heater core.

My budget is not allowing me to take my truck into a shop, so your insights could help me to help myself fix my truck, which I need to get to work. Thank you for reading.
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First I want to say it might be better if you take it to a good repair shop. In the long run you might save money with a good cooling system diagosis. All you can get here are a few guesses from folks that cannot touch and feel your truck. Thowing parts at it can cost a lot more. But here are some of the possibilities...Due to the age of your truck the first thing I would be checking is the condition of the radiator, next if it has a fan clutch that would be suspect and needs to be checkedb, even though you had a thermostat replaced it still could be stuck open and not allowing the coolant to stay in radiator long enough to cool down and the water pump impella could also cause your symtoms. And the blue stuff you used was checking for combustion gases getting into cooling system but it does not tell you if the head gasket is blown between 2 cooling jackets and that can cause coolant circlelation problems. Do not keep driving and overheating that engine. Get it fixed before you are shoping for a engine. Good Luck!
Milesauto is right.
The noise in the dash you described tells me that the vehicle is low on coolant. Because you ran the vehicle hot the coolant may have boiled out when you ran it into the red mark on your gauge. The cooling system is presureized and when it runs hot the radiator cap allows the coolant to boile out. On GM vehicles the cooling system can withstand pressures up to 15 psi then the radiator cap opens to release the pressure. If the coolant did in fact boiled out of the radiator that explains your noise.
Now you need to find out where the coolant went. If it boiled out, why did it boile out? Is the cooling fan clutch not operating properly? Does the radiator have proper air flow? Is the thermostat opening? Is there a clog in the cooling system? If it did not boile out is there a coolant leak from the engine or heater core? If there is no leak is the engine burnig coolant (leaky intake, warped head, bad head gasket)?
Start by filling the coolant then run the truck and get it hot to verify if the fan is pulling air across the radiator. If the clutch is ok check the coolant hoses to verify the thermostat is opening. After it gets to operating temp. the hoses should feel about the same temp. As you check the temp of the hoses feel for coolant flowing through the top hose. This will verify if the waterpump is moving fluid through the engine.
Good luck and always BE CAREFUL around moving engine components while you check for your problem
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