Hi,
I'm having some trouble with my cooling system since I replaced my thermostat. My truck is a 1997 Mazda B4000 with about 80,000km.

Before I replaced it, the gauge on my dash would sit at about 1/3 to 1/2 (which seems to be normal from what I've read). I replaced the thermostat after getting error code P0125. I got the code again after replacing it, and as it turned out my engine temp sensor (2 wire) was faulty, so my ECU was getting the wrong temperature. I've replaced that now and it seems to be working correctly.

Currently, my issue is that once my engine warms up, the temp gauge on the dash rises to almost the top line, then drops a bit as the thermostat opens. It never goes below about 3/5 of the way up. However, I've been monitoring the temperature with an ECU code reader and it never gets above 87 degrees C (about 188F) - it is an 87C thermostat which I realize is slightly below OEM spec.

I've tried putting the old thermostat back in, and also using a different new thermostat with no change. I've also replaced the temp sender for the dash gauge (one wire) and its wire.

Could a little air in the system explain why the temp sender and sensor are getting two different readings? Or is there another explanation? I've followed the coolant burping procedure and made sure the radiator is topped up, but I suppose there could still be some air stuck somewhere.

 

Welcome to the forum

Could be the Temp Sender, the one wire unit, red/white wire
These need the lower threads to be bare so they get a good Ground to the engine, that's why they just need the 1 wire
If you use tape on the threads then poor ground to engine

And it could just be a bad sender, new parts are not tested
Temp sender is 12volts
ECT sensor is 5volts

So while they look similar they are not interchangeable

If OBD2 reader is showing 188degF that is correct, and temp gauge on dash should be just below 1/2, 1/2 is 200-210deg on Ford temp gauge


On the 4.0l the heater core is the water pump by pass, so the heater hoses need a steady flow of coolant, if the core starts to get plugged up then temp gauge will have a random up and down movement, totally random
Just as a heads up you should swap heater hose positions at the firewall on any vehicle every 2 years, this reverses the flow thru the core so it will last longer
You won't lose any coolant as the hoses are a high spot, well very little loss

 

Thanks for the reply.

The gauge was reading the same with the old sender (one wire), so I replaced it last weekend (without tape on the threads). It's reading the exact same now, so I don't think that's the problem. One other thing to note is that the gauge needle position is quite consistent - it rises and falls in line with the reading from the OBD2 reader/two-wire sender.

I'm confused too, because the gauge only started to read high after I drained my coolant and changed the thermostat. It's hard to imagine that the sender or some other component happened to give out at that exact moment. This leads me to believe that maybe I made some mistake in filling the coolant, but I've drained/filled it and burped it 3 or 4 times now with the same result.

Thanks for the info about the heater hoses, I'll swap them next time I work on my truck.

 

When you refill the cooling system you take off one or both heater hoses and then fill the radiator
The air from the engine side of thermostat will come out the heater hose, when you have coolant coming out of the hose put it back on, the air is OUT, no burping required

 

I realize this is an old thread, and I apologize for not replying sooner. Following your advice, I tried refilling with a heater hose off with no change.

Also (unsure if it was related to this issue or not), my radiator developed a fairly sizeable crack in the plastic housing. I've since replaced the radiator, with still no change to the issue of my temp gauge running high.

Finally, I noticed that my overflow resevoir had some gunk at the bottom which was stopping coolant from flowing back into my radiator as it cooled, meaning my radiator was never quite full. I've cleaned that all up, and it appears to be working as intended. My radiator is always full to the brim now. Still no change with the temp guage though.

I've been driving for probably a month now since I last did any work on it. From cold, it takes about 5 minutes for the temp guage to rise to the top, the "Check Gage" light briefly comes on, then the needle drops to about 3/5 as the thermostat opens, before rising again to be nearly vertical or a bit below. According to my ODB2 reader, the temperature still never gets above 87 deg C (188 deg F), which is what the thermostat is set to. I'm running out of ideas, and may just get used to the gauge being high, as my cooling system generally seems to be working well.

 

I would trust the ECT sensor temp(OBD reader) over the dash gauge

Just under 1/2 is expected on Dash temp gauge after warm up

A 1996-2003 Dash/cluster from a Ranger or Mazda B-series would be plug and play, even the white face models, with or without Tach, cluster is wired for tach regardless of how it came from factory

 

I’m have the same issue with my 1997 ranger. What did you end up doing to get it fixed. I’m about to change the radiator. But that (like you said) doesn’t get hot or put there in temp to be overheating as much as the gage on the inside says it is. I’ve damn near changed everything but the radiator. I’m hoping it’s not a Head gasket. But the coolant is clean and green. I’m at a loss for what to do about getting it right. Any help would be much appreciated.