First, I hope it is ok I started a new thread. I looked around and found similar problems, but nothing that matched up exactly.

I have a 1998 3.0L Auto 2WD Mazda/Ranger with about 110,000 miles. About 2 1/2 years ago the radiator, hoses, and thermostat were replaced. The entire time I have owned the truck (since new), the needle of the temp gauge has sat right in the middle. About two nights ago I noticed the indicator was around 1/4 of the way up rather than 1/2 (normal) even though I had been driving for over a half hour on the highway. It dropped a bit when I put the truck into park before shutting off the engine.

I've been trying to figure out what is going on.

I ran the engine at idle for a while (20-30 minutes) and the temp never got over 1/4. Interior heater blows hot. Top radiator hose gets plenty hot. Lower radiator hose stays pretty cool. Fan blows loud at startup but then settles down and stays on. Fan is hard to spin by hand when cold, also hard to spin (maybe slightly easier, but it definitely doesn't spin freely) after the truck is warmed up and shut off.
I did discover a smallish leak around the thermostat. I'm not sure if it comes from the thermostat joint or the hose clamp or both. No other leaks I can see.

My instinct is to replace the thermostat. It doesn't cost a ton and shouldn't be too difficult. But the symptoms I'm seeing have me confused. My assumption is that the thermostat is stuck open so the car never comes up to adequate temperature. But the heater works fine. Also, the leaks seem to suggest a closed thermostat causing pressure problems.

Thanks for the help.

 

Welcome to the forum

Just FYI
The radiator and engine always have the same pressure, which comes from the coolant heating up and expanding in volume
Thermostat just controls the circulation of coolant thru radiator when coolant in engine gets above 190degF
Lower hose is always open so pressure is always the same through out the system

It reads more like the temp SENDER for the dash board gauge is starting to fail, its not an expensive part so I would try that first

The Temp Sender will have a Red/white stripe wire connected to it, it may or may not have a 2nd wire, but for sure the RED/white wire
This is important because there is a similar device on the engine, the ECT sensor, looks the same but is NOT the same

ECT sensor always has 2 wires, it uses 5volts, and is only connected to the computer

Temp Sender, can have 1 or 2 wires, use 12volts and is only connected to dash board temp gauge

Picture here of the two: https://www.ranger-forums.com/attach...ant-sensor.jpg

Red/white wire..............

Also at the parts store they will always get you an ECT sensor first time around, say sender, then spell it S E N D E R, lol.

You can of course change the thermostat, but from your description of the heater still blowing HOT then its unlikely to be the thermostat, possible but you should notice heater blowing cooler, still hot but not as hot

1/2 on the ford temp gauge is about 200deg
1/4 is about 130-140deg and computer should set a code and turn on the CEL if temp stays that low after 5-8 minutes of driving, that would be from ECT sensor showing low temp

 

Hi Ron
Thanks for the help. I got the sender and it is installed and it sure seems like that did the trick. I ran the truck for 20 minutes or so and the needle went to mid and stopped, just like it used to. I used teflon tape on the top half of the threads and it looks like it is sealed fine. The hardest part was getting off the old one due to the awkward location and the fact that after 20 years it was stuck on with a bit more than 8-18 ft lbs of torque.
I also tightened the thermostat a bit and hopefully that fixes the leak.
Thanks again and have a great day.

 

Good work

Thanks for update and the FIX