How can I be sure coolant temp sensor is accurate?

[TorontoGuy]

Got our first really warm day of the year today and expected to see the temp gauge go a little higher than usual (since I bought the Ex in February I really haven't been able to test it in all conditions). Typically it's just at the N of NORMAL or a hair above or below. Got it up to operating temp today and it was at the same spot as always. I changed the thermostat and coolant a few weeks back just as routine maintenance on a used vehicle without maintenance history. Noticed hotter heat coming through the vents but no real differences on the temp gauge.

I've had a few older Fords and know that these gauges aren't perfectly accurate but using the gauge as a scale, I'd expect to see a bit of an increase in temp. After I got home I just poked my head under the hood and the upper rad hose and thermostat housing are way too hot to touch which has me questioning the relatively low needle position on the gauge.

This is really just a case of "How can I be sure the engine isn't running hotter than it says?"

Thanks guys! Amazing forum here, done a ton of reading and will do a ton more.

Cheers!

BTW it's a 1991 2 door, 5-speed, 4x4

 

[Anime]

If the gauge indicates a temperature reading, it should be working. It would be unusual for a gauge to work but be stuck at a low reading or to read a low temperature when it's actually at a higher temperature unless the needle is mechanically stuck somehow.

The coolant temperature sensor, on the other hand, can get gunked up or just go out, though that is also rare for an Explorer, at least for the factory sensors. Cheap aftermarket brands are not always as reliable and I've replaced several sensors over and over on other vehicles that only have cheap aftermarket sensors available.

One of the easiest ways to check the actual engine temperature is to use a basic digital thermometer, either a wired one that uses a contact probe, or even better, a laser temperature sensor that you just point at the surface and get a reading without touching anything. Either way, you take a bunch of measurements at different points and average them out to find out if the engine is running around 200 degrees F.

If the gauge runs low, it can sometimes be that the thermostat was replaced with a 'colder' 180 degree model rather than the 190-195 degree models that are for normal operating temps. Be sure the model you used is a 190 degree (Motorcraft) or 195 degree (Stant, other aftermarket) to get the correct operating temperature thermostat. If the engine still runs cold with a 195 degree thermostat, be sure the fan clutch isn't seized, as that can cause it to keep sucking air in full force and cooling the system more than it needs, constantly keeping the coolant lower than operating temperature. This is usually indicated by poor fuel mileage, and the fan being rather noisy all the time, like an airplane taking off.

 

[TorontoGuy]

Thanks very much for the reply!

I've got a laser temp probe around somewhere, I'll definitely see what I can find with that. What are the best places to check? I'm assuming the upper rad hose, lower rad hose and the thermostat housing would be a good place to start.

While I was letting the coolant drain I took out the coolant temp sensor just to give to give it a wipe down (I didn't use any chemicals, just a dry rag) and it wasn't that dirty, but I figured I was there so why not.

Thermostat was replaced with a 198* MotoRad unit. It was all the local parts place had but thinking back now I should've just got with the Motorcraft part. Today I'll be going by the dealership for a new radiator cap as I'm pretty sure mine is original, so may as well get it out of there.

Cheers!

 

[Anime]

The hoses, thermostat housing tube, and even the water pump exterior can be good places to check the coolant temperature, though you can of course also take temp readings on either side of the block itself from above and below at operating temp, and average those out seperately to see if it's really running cold.

It might be the temp sensor, but it could be the gauge, since they are electronic devices after all. It's rare for them to have a problem but that's not the same as it never happening.

I think the 198 degree thermostats are too hot, but it might be what you want in the winter of colder climates up North. There's no need to spend the $$$ on the Motorcraft thermostats, I used them for a long time but switched to Stant for the price, they make a stainless steel one called the Superstat that is rather nice and much less than the Motorcraft version. It might be worth a shot though, and the new radiator cap should help if the old one is causing the pressure to be too low. The Motorcraft radiator cap is definitely the best choice, and it's pretty inexpensive, too.

Also be sure to use a thick rubber gasket around the thermostat, this might seem obvious, but sometimes people do put in thermostats without the gaskets and of course the engine runs cold because the thermostat is just fluttering around in the housing with coolant flowing all around it.

 

[TorontoGuy]

Thanks very much for the tips!

Thermostat came with the thick gasket and I also didn't use any RTV on the housing based on the reading I've done around here. I'll be picking up the temp probe tomorrow and I'll report back with what I find.

Cheers!

 

[TorontoGuy]

So I'm just digging up an old thread to say that since the cooler weather has returned my temperature gauge is back to just below the N of NORMAL and doesn't ever really climb much higher than that. Heat coming out of the vents really isn't that hot. It's warm, but I don't feel it's as hot as it should be, especially since we're barely into October.

After driving for half an hour the upper rad hose and thermostat housing are hot, as hot as they should be, but the lower rad hose didn't feel nearly as hot. I'd bet that it was only warm because of it's surroundings.

So what's the verdict on a hot upper rad hose, a sort-of warm lower rad hose, and a coolant temp that won't go past the N of NORMAL?

Pressure builds fine in both rad hoses, there's no leaks and the amount in the overflow tank fluctuates exactly as it should.

Any ideas?