Inconsistent Coolant Temperature

[BWDuty]

Not sure if the old signature I made a couple years ago will show, so I'll start with the specs.

2000 Ranger
2.5L I4 Lima Engine

I replaced the thermostat and thermostat housing (that includes temp sensors) about a year ago with OEM Motorcraft parts.

I noticed the winter after that, that the coolant temperate is sometimes inconsistent. It seems to be mostly when it's cold outside, but that may be irrelevant.

It never overheats. 90%+ of the time the gauge shows right in the middle which I'm assuming is approximately 190F which is the rating on the thermostat.

Sometimes, however, the gauge will drop. Sometimes just a bit, sometimes it nearly bottoms out. There is perhaps a bit of a difference in the heat, but it might be my imagination. It still feels pretty hot.

My first impression would be a thermostat that intermittently sticks open, but even then would it drop *that* low? Not to mention it's an OEM thermostat that's a year old -- although I know defective products aren't unheard of. Any ideas for me to toy around with? Maybe something in the cluster? I did replace the cluster from an '02 explorer sport track because I wanted a tachometer (why Ford put them in the automatics and not the manuals is beyond me) and the white just looks different/cool, and I hadn't noticed this before that. Everything else on the cluster is perfect, it's a "plug and play" swap. It was around the same time I changed the thermostat and housing though. I hate electrical problems! I'd just change the thermostat since it's a cheap part but it's kind of a bitch on this engine. Thanks!

 

[RonD]

Yes, unfortunately could be defective thermostat, they are suppose to "failsafe" which is OPEN.

The Limas do not generate a lot of heat, so radiator rarely warms up on colder days, just not enough extra heat to dissipate via radiator.
If thermostat did stick open then temp gauge would drop quite a bit until it closed.

Gauge does use a separate Temp Sender, not an ECT Sensor which is only used by the computer, it is possible that is failing but if Heater temp in cab is also going down with the gauge then temp drop is real not a sender gauge issue.

EDIT
If you were to get a Bluetooth OBD2 reader(under $30) you could use a smartphone to monitor ECT sensor engine temp in real time, see if it corresponds with gauge.
These are also nice to have for vehicle issues or codes, and they work with ANY vehicle sold in North America after 1996, so not a Ford thing.
Free APPs or $5 for full range APPs
If you do get one get the $4 extension cable with it, some OBD ports have narrow access covers so unit won't fit without removing the whole panel, extension cable comes in handy then, plug n' play

 

[BWDuty]

I have a bluetooth OBD2 scanner, as well as Forscan on my laptop and Torque on my smartphone so I'll give that a try so that I can pinpoint the problem. Thanks! I had considered trying that just out of curiosity anyway, but I didn't realize that the gauge and the computer get their readings from different sensors.