Either an air bubble, the thermostat or the water pump.
- Air bubble (pocket) - Just because the radiator is full, doesn't mean the engine is. When an air bubble circulates through an engine it will cause the temp gauge to swing from hot (when water is in contact with the sensor) to cold ( when air is in contact). The less water, the hotter the engine. Easiest fix - add water.
- Thermostat - Keeps water circulating in the engine separate from the water in the radiator until it gets to a certain temperature. At that point, it opens and allows the hot engine water and the cooler radiator water to cycle. Hot to radiator, cool to engine. As the cooler water flows over the thermostat, it closes, allowing the cycle to repeat. If it opens at the wrong temp, or not at all, your engine will overheat. Water will begin to boil. Steam will make the temp gauge swing erratically.
- Water pump - If the water pump is failing (sometimes the impeller will come of the shaft), the water will not circulate. Same results.
Let me know how it goes. If you have any other problems, click the ASK button.
- First and foremost: BE SAFE!!! Wear safety glasses. Start this with the engine cool. When you look in the radiator, do it from an angle and as far away as possible.
- Have a hose or a couple two liter bottles full of water.
- Take off the radiator cap. (Do not take off if engine is hot)
- Is the water level low? If so, top it off.
- Make sure the transmission is in either PARK or Neutral and set the parking brake.
- Start the engine.
- Observe the water level in the radiator. If it drops, top it off again.
- At first it should do nothing. The water in the engine is still cool. The thermostat is closed. As the water in the engine heats up, the thermostat will begin to open. When it does, you will notice the water level begin to surge up and down.
- Check your temp gauge. If the gauge getting close to the 'Red Zone', the thermostat should have begun cycling before now. Go back to the radiator and check it.
- When (if) the thermostat finally opens all the way, the water will start moving as it cycles through the system. You will be able to tell the difference.
- If you see the water flowing, the water pump is ok.
- If the water does not start to move and the gauge is getting close to 'Red', shut the engine down and let it cool. Do not put your face near the radiator. If the thermostat suddenly let go, The hot water, driven by steam, could erupt out of the filler hole. Replace thermostat. Rerun this test. If results are the same, replace water pump.
- Does the water level drop at this point (air bubble)? If so, top it off while it's circulating. Adding cold water will close the thermostat. Be prepared to wait through the cycling two or three times.
- Check the temp gauge again. Has it leveled off?
- When you don't need to add water any more, put the cap back on. Observe the temp gauge for a few more minutes. has it stopped the erratic behavior? If so, it was an air bubble and it's gone. If not, replace the thermostat.
Remember: Good thing come to those who Rate.