Ny wife’s car is overheating. If the coolant is fine, how can one tell if it is the thermostat? And, does the thermostat tell the fan when to kick on? (I haven’t had a chance to look to make sure the fan is coming on. If not, it could be a faulty switch or relay, I gather.) What can you tell me about diagnosing the thermostat and undertstanding its role? Thanks!
You might as well provide some make, model, and year info for the experts to work with.
The most common problem is that it sticks shut causing overheating.
You don’t want the engine to run too hot, but you don’t want it to run too cold either. A cold engine doesn’t burn fuel as efficiently. The way the thermostat works is that it opens up at a specific temperature (typically somewhere around 190 deg F or so). So it should stay closed until the engine gets up to its operating temperature, then it should open, allowing coolant to flow through the engine. When the coolant cools the engine down below its operating temperature the thermostat closes again, allowing the engine to heat back up to its operating temperature. The thermostat keeps opening and closing as needed to regulate the engine’s temperature.
The easy way to check the thermostat is to remove it from the car. If it’s open when you remove it, then it’s stuck open and needs replaced. If it is closed (like it should be) stick it in a pan of boiling water on the stove. If it doesn’t open up, it’s shot. Replace it.
Important Note: Some people (like Mrs. Geek) get rather upset when you put car parts in a cooking pot on the stove.
The fan is controlled by a temperature switch. You can check the voltage coming off of the switch to see if it turns on, then check it on the other side of the relay to see if the relay is turning on. Then check the fan. If there’s never any voltage coming off of the switch, then the switch is shot. If the relay gets voltage but doesn’t turn on the voltage to the fan, the relay is shot. If the fan itself is getting voltage and not spinning, then it’s the fan that’s shot.
If the fan is spinning but you are still getting overheating problems, then you might have something like a blockage in the radiator or a bad water pump.
Easier said than done on some cars. On my two (older) Toyotas and my (older) VW Golf, once you’ve gotten the thermostat out there’s no point in testing it–just replace the Goll-danged $20 part.
There are a series of easy tests that you can do that will tell you exactly what the problem is. If you have a temp guage it is done slightly different than if you don’t have a temp guage. So do you have a guage?
You start the car and let it idle till it warms up, if your upper radiator hos suddenly gets hot it is because the thermostat just opened. The fan should come on anywahere from 5 to 10 degrees after your thermostat opens. Just continue to let it run and you should feel your fan kick on, it will be spinning regardless but you can feel when it kick on. You may have to run it a while to do this but proably not more than 20 min or so. Thermostats don't go bad that often, fans go bad all the time. I don't know if you fan is electric or viscous drive, if electric it may have a sensor that is bad.Well you already understand the first decision… does the FAN run or not ?
The fan system has its OWN thermostat to switch it on and off.
The other thermostat is a valve on the water flow. It holds the water in the engine to ensure the engine can warm up, and only lets hot water out. (otherwise it could be like ‘cold’ all the time )
When does it overheat? Stop and go driving or on the highway? If overheating on the highway, there can be a different set of problems compared to stop and go overheating.
How do you know it’s overheating? The temp gauge or idiot light is in the red or it’s actually blowing steam under the hood? I don’t trust gauges all that much; however, a engine boiling over is hard to dismiss.
Assuming it is actually overheating at lower speeds and it has an electric cooling fan. Turn on the air-conditioner on the car (I’m assuming that it has AC). The fan under the hood should kick on. If not it could be the fan is bad. It there are two fans under the hood, one could be for the AC and the other engine cooling. Most cars just have one fan for both though. With the engine turned off; keys out of the ignition; AND the engine is cold, can you turn the electric fan with you finger pretty easily? If not the fan could be going bad.
How’s the exhaust system? Are there any bent pipes?
Is there a check engine / service engine light on? Some of what turns on the light could cause overheating such as a bad catalytic converter.
Without knowing the make model and engine variant it is impossible to give an exact answer.
The thermostat regulates the coolant temp in the engine to allow it to warm up but not overheat.
Thermostats do fail. Often in the open position but sometimes in the closed position.
The temp gauge on the dash is NOT always a valid indicator of engine temp.
A non contact infrared thermometer is excellent for diagnosing overheating issues
There are many causes of overheating many/ most more common than a bad t-stat.
To name a few:
Bad radiator
Air trapped in the system
Bad hoses
Bad water pump
Radiator plugged with debris (dirt, sand, pet fur)
Engine running issues
Bad electric fans
Bad sensors (the engine might not be o/heating but a bad sensor thinks it is)
Plugged coolant passages in the block
How about a few details?
Two extra little points about the fan. One, if the fan isn’t coming on but the car is still overheating, that doesn’t necessarily mean the fan is the problem. The fan switch is usually in the radiator, but if a stuck thermostat isn’t letting coolant flow from the engine to the radiator it won’t get hot enough to come on. The other thing is that the fan is sort of a last resort measure and on many cars it will almost never come on under normal driving conditions.
So, with that in mind, if the car is overheating consistently (not just when stopped in traffic) and the fan isn’t coming on, that’s actually a pretty good indication of a stuck thermostat, not a bad fan.
Or a blown head gasket, or an air lock in the engine, or a plugged radiator, want me to go on?
Yeah that’s true. I probably should have just said that was a better indicator of a bad T-stat than a bad fan, but it could be other stuff too.