I was driving this morning and noticed that the temp guage was really high, and that the battery light was on. I pulled over and check the rad and oil, which were both fine. I let the car cool down for an hour and drove it around the block and within 30 seconds the needle had hit the top again. I continued to drive and the needle began the drop.
Had a similar problem with the temp. When driving the temp was fine, but if the car was idle or moving slowly the temp would just slowly keep climbing. Turned out a hose was cracked and needed to be replaced.
If you had a cracked hose, you would be losing coolant, and you said your coolant levels were fine. So it’s probably not that.
In addition to any of those generic things Brewers mentioned:
The battery light means that your alternator is malfunctioning. Now… This can mean that in addition to the overheating problem, your alternator is also going bad. Or, these two problems may be related. All cars have a belt of some sort that transfers rotation from the crankshaft to various pulleys that drive the fan, alternator, water pump, oil pump, power steering pump, a/c compressor, etc. A slipping belt could lead to both an alternator problem and overheating. If you have a serpentine belt, there’s a spring loaded pulley that’s supposed to keep tension on the serpentine belt, and maybe this pulley isn’t working properly. Or the belt has really stretched out over the years.
You might also need a radiator flush if your car is older than about 9 years. Over time, the inside of your radiator rusts, and these flakes of rust clog up the passages that the coolant flows through.
Take your car in to the garage to get serviced. Describe the problem to them fully. If the temp starts to climb up again on your way there, turn on your heater to full blast and roll down the windows. The heater core is also a heat exchanger, so running it at full blast will help you avoid turning your engine into molten slag on your way to the garage.
Yeah, this isn’t something that will fix itself. Thankfully, overheating problems are pretty cheap to fix unless there’s component damage (busted water pump, etc)
How old was the car when you got the new radiator? Replacing a radiator will not unclog the passageways inside the engine itself. Only a flush will do that. If it wasn’t flushed when you replaced the radiator, you still have whatever rust was inside the coolant passageways in your engine at the time.
I don’t think it’s the cause of your current problem, but depending on the age of your car, getting that rust flushed out would increase the efficiency of your cooling system. It’s something worth doing if you expect to keep this car until the day the junkyard tows it away.
My old 88 Bonniville started doing this sort of thing last year. The temp gauge pegs out as hot as it can get and just when I am sure it is goign to blow, it drops back down again. In a few minutes, it does the same thing all over again. I took it to the shop and they just want to replace the entire cooling system because it’s old. I say the hell with that. If it ain’t broke. I notice that if the dummy light for an overheat condition comes on, then the water level is most likely down, and I fill it, and everything goes back to normal (the temp gauge pegging itself but then dropping back down). I have just learned to live with the temp gauge and not bother to even look unless I get the dummy light and the temp gauge, which means add water. So far it is working for me, but I try never to drive it more than 3 or 4 miles at a pop (local trips to work or the store, etc).
This exact thing (the gauge slowly creeping up, way past red, then suddenly going back to normal) was happening to my 1999 car recently. What makes it worse is that it started right after I had an alternator/battery die on me and I thought it may somehow have been related (a mechanic bumped a hose, the thermostat died, etc). I finally got fed up with it and checked it out. Turns out I was about 20% low in the radiator. When the car was cool I took a hose to it and filled it up, and the temperature-jumping hasn’t happened since (about three months).
Topping off your radiator with tap water should be fine. Coolant doesn’t boil off, the water does, so you’re just replacing the lost water.
Obviously, though, if you’re losing actual coolant through a leak, you’re eventually going to mess up the coolant/water ratio.
If you’re in a warm climate where it doesn’t freeze, this is just fine. But don’t do this if you live somewhere cold. In places that actually have winter, you need to add a proper mix of half coolant/half water.
A jumpy needle is tell-tale sign sign that you’re low on coolant. The liquid is sloshing around, and touching the temp gauge probe intermittently, causing it to spike. When there is a proper amount of coolant, the probe is entirely submerged, giving a constant reading.