Temperature gauge needle dropped to the cold side for no reason. all fluids are good. then the check engine light went on.

Reasons the Temperature Gauge Reads High

If the temperature gauge reads high, it could mean your engine is overheating. Another reason your reading might be high is you could be losing coolant. A small leak or evaporation may cause your radiator to slowly lose coolant. A third reason your temperature gauge reads high could be because the thermostat is broken. If this is the case, you may need a coolant temperature switch replacement. The last reason the temperature gauge could read high is because of a water pump, or water pump gasket failure. If the water pump is malfunctioning, it may need to be replaced by a professional.

What to Do If Your Temperature Gauge is High

If your temperature gauge is reading high, it means your car is overheating. This is a very serious matter and you should never drive an overheating car. If your car starts to overheat, shut off the air conditioner and open the windows immediately. If this does not reduce the overheating, turn on the heater as high as it can go. If this still doesn't work, pull over on the side of the road, turn off the engine, open the hood carefully, and wait until the vehicle cools down. Never open the radiator cap while the engine is hot - coolant can spray and burn you. Once the vehicle has cooled, take the car to a mechanic right away so they can diagnose the problem. Cars are especially susceptible to overheating in hot climates, like what's common in cities like Los Angeles, Phoenix, Las Vegas, or Atlanta.
The temperature gauge is an important tool in your vehicle that shows the temperature of your engine's coolant. Contact YourMechanic and have your car inspected for overheating if it reads too high, as this can cause serious problems...

Reasons the Temperature Gauge Reads Cold

On most vehicles, the temperature gauge reads cold until the engine has run for a few minutes. If the temperature gauge still reads cold after the engine has warmed up, the gauge may simply be broken. Another reason the temperature gauge could read cold is if the thermostat in the vehicle stays open. With the thermostat stuck open, the engine can be overcooled, causing a low temperature reading. If this is the case, the thermostat may need to be replaced.

Engine noises don't leave fault codes unless the knock is so bad the engine is misfiring. When you say it "cuts off right away" do you mean the engine stalls or the knock goes away? If the answer is the knock goes away right away then you have a crank or rod bearing that is loose from wear, the oil pressure fills the excess bearing clearance gap and the knock stops.

Most common reason is low engine oil level, so check your oil level first. and if that's ok then check the following:

1. Oil pressure sensor maybe faulty
2. Check oil pump pressure with a gauge fitted where the oil sensor is test oil pressure when cold and at operating temperature.
3. Loose wiring connection or faulty wiring to the sensor /gauge. Good luck, let me know how you get on.
   

jtg

Is the heater nice and hot when the needle shows low temp? If the cooling fans turn on and the heater is hot, that leads me to believe the coolant temperature sensor is bad. You should try replacing that sensor. They are only $6 usd here. As long as your check engine light is not on and your fans are running, this will not hurt your car... but this is something you should fix soon if you can. Make sure the engine is cold when you replace the sensor

Check the obvious first!! Check the wires to the sensor on the engine are clean and price up replacing the actual sensor as well. If it's running a fan belt it'll be spinning as soon as you start the engine so won't affect the engine. It's only the more modern belt-less electric ones that depend on a sensor to work.
On a car that old it could be as simple as the gauge on the dash has died. Price up a replacement set of second hand dashboard dials and swap them yourself. It's a fiddly job but not hard.

either a faulty temp sending unit or a thermastat stuck open assuming all wire connections are good. if heater blows hot id lean towards the temp sending unit. please rate me trying to make a buck

It sounds to me like you're low on coolant. When you checked the coolant level, did you actually open the radiator cap and make sure the radiator is full? The plastic tank you generally add coolant too is actually an 'overflow' or 'reserve' tank. There is a hose that runs from that tank to the top of the radiator so if the radiator gets low on coolant, more coolant -should- run in from the overflow tank. However in practice this does not always work. I have a Jeep that leaks coolant and experiences this exact same problem. Coolant will not run from the reserve tank into the radiator because the hose is plugged up, and whenever the radiator gets low on coolant the temp gauge will go real high and then drop, and the heater doesn't work well. This is because the cooling system of your car is supposed to be a closed system, full of coolant and no air. When coolant leaks out, the space it used to occupy is now occupied by air, which does not transfer heat well. When 'air' is passing through your cooling system, no heat can be transferred from your engine to the heater and radiator, resulting in a hot engine and no heat at the heater. Then when a pocket of water passes through the system, the temperature gauge quickly falls as the water absorbs the heat from the engine. The hot water that cools the engine is where the heater gets it's heat from as well, so when water passes through the heater core, the heater works, but when it's filled with air, it doesn't.

Ok, judging from what you've said it sounds like a vacuum issue. The temperature controls are activated by vacuum which is produced by the engine. Look for small hoses that lead from the intake manifold to the firewall, check them and make sure they are not broken, disconnected, or leaking.  The vacuum directly controls the doors inside your dashboard which push the air to the heater core. Also make sure that the coolant hoses going to and from the firewall are hot.  If all that checks out, then the controls inside the car may have gone bad, or may have become disconnected from the vacuum supply, or the actuator door inside the dash is stuck or broken.