Engine won't maintain temperature (too cold)

[RonD]

I have issues with my temp gauge always being between 1/4 to slightly under the 1/2 mark on my 98 Mazda B4000. Since buying the truck over the summer it has never been at the dead center of the temp gauge, always a tick below it.

Well this "issue" lead me to a discovery. My mechanic (also my father in law) and I were about to swap the thermostat and purge the air and we went through several thermostats and none of them fit. We eventually discovered that my 98 Mazda B4000 has a 99 Explorer 4.0L SOHC engine instead of the OHV. So my truck has the external thermostat housing with two sensors, and both are stuck in there pretty good. We replaced the thermostat and flushed the system and the temp guage did temporarily go to the 1/2 mark. But now with the low temps it is back down to nearly 1/4 mark.

So I know the motor swap only complicates this problem. I am still not 100% sure if my truck is running off the original ECU, or the Explorer ECU, although I believe it is the ladder. My truck uses a PATS key, so that is a pretty good indication that it has the SOHC ECU. I believe one sensor sends a signal to the ECU and the other to the temp gauge. I am contemplating changing the entire housing and switching to the updated click in sensors vs the screw in sensors that often leak and fail. I am beginning to wonder if the low temps are causing the ECU to run richer on these colder days and effect my gas mileage. It hasn't been great as of late, and although it is a 4.0L I would welcome better mileage.

So is this just a "Ranger thing" or is it one (or both) of my sensors, the thermostat, air trapped in the system, or just the abnormally low temps these last few weeks? Also can the motor swap have an effect on the dummy gauge on my instrument cluster? In other words does it use a different ohm resistance to display the temp? Or is it the same between the OHV and SOHC? Any insight would be appreciated.

You should have started your own thread.

The 2 wire 'temp' sensor is for the computer, called the ECT sensor.
The 1 wire 'temp' sender is for the dash board gauge.

Symptom for bad ECT sensor would be CEL(check engine light) after about 5 minutes of warm up, computer will turn on CEL if ECT sensor is not reporting a rise in temp since startup.
If ECT sensor is reporting warm engine temp, when really cold, CEL wouldn't come on, computer just "thinks" it is a restart after driving, but if engine was really cold it would run very poorly with the "normal" warm engine fuel/air mix, until it actually warmed up.
So I doubt an ECT sensor issue.

Temp sender reads like it is working fine, reporting actual temp.

Just below 1/2 would be correct on warm or moderate temp days.

A few things happen on cold days
Yes, computer will run engine richer on cold days, that is because of the IAT(intake air temp) sensor, which is part of the MAF sensor, colder air requires more fuel, it is denser than warmer air, so cold weather will decrease MPG, no way around that.

In colder weather we run the heater at max. usually, lol, thats the point of having the heater.
So this will bleed heat from the engine.

The air flow in the engine compartment in cold weather causes much greater cooling on the block.
Some will put cardboard or similar in front of the rad in the winter to reduce this air flow.

The thermostat will try to maintain the 195degF(or 192) in cold weather and you will often see temp go up to 1/2 then down to 1/3 as thermostat opens and closes as cold coolant is added from rad circulation, this should stabilize after a few minutes, but it really depends on driving conditions, going uphill will allow engine to generate more heat, downhill less heat.

 

[Caesar]

You should have started your own thread.

I was actually thinking the same thing after posting above. I am glad that it caught your attention however, thanks for the insightful response.

The 2 wire 'temp' sensor is for the computer, called the ECT sensor.
The 1 wire 'temp' sender is for the dash board gauge.

Symptom for bad ECT sensor would be CEL(check engine light) after about 5 minutes of warm up, computer will turn on CEL if ECT sensor is not reporting a rise in temp since startup.
If ECT sensor is reporting warm engine temp, when really cold, CEL wouldn't come on, computer just "thinks" it is a restart after driving, but if engine was really cold it would run very poorly with the "normal" warm engine fuel/air mix, until it actually warmed up.
So I doubt an ECT sensor issue.

Temp sender reads like it is working fine, reporting actual temp.

Just below 1/2 would be correct on warm or moderate temp days.

I totally agree, the truck doesn't seem to exhibit any of the symptoms of a bad sensor, and it doesn't display the temporary CEL. It actually idles up after startup and then drops down to between 1000-750 rpms several mins later as it should normally. In general the truck runs great, I have just noticed a fluctuation in my mileage recently.

A few things happen on cold days
Yes, computer will run engine richer on cold days, that is because of the IAT(intake air temp) sensor, which is part of the MAF sensor, colder air requires more fuel, it is denser than warmer air, so cold weather will decrease MPG, no way around that.

In colder weather we run the heater at max. usually, lol, thats the point of having the heater.
So this will bleed heat from the engine.

I hear that, and I smoke, so I usually have the windows cracked and the heat at near max. I recently repaired the heater blend door and I learned that the blend door actuator motor is calibrated to try to push the door beyond it's max when on full heat. I believe that is why they typically break while set on full heat. So now I never set it all the way to max heat, usually a good 3/16 of in inch before the very end. But my heater does produce heat very effectively, so I doubt that it is stuck open.

The air flow in the engine compartment in cold weather causes much greater cooling on the block.
Some will put cardboard or similar in front of the rad in the winter to reduce this air flow.

The thermostat will try to maintain the 195degF(or 192) in cold weather and you will often see temp go up to 1/2 then down to 1/3 as thermostat opens and closes as cold coolant is added from rad circulation, this should stabilize after a few minutes, but it really depends on driving conditions, going uphill will allow engine to generate more heat, downhill less heat.

I am beginning to think that the thermostat we put in may not have been a 190-195. We had gone through several different brands and were shooting in the dark when we ordered one for a SOHC explorer. I can't even remember which brand it was let alone the temp threshold. I am eventually planning to change the thermostat housing and sensors to the updated click in sensors in the near future. Fortunately it can be done for right around $100 parts. I am going to grab an extra 192 degree fail safe thermostat when we do and swap it out for good measure. The threaded sensors get stuck (like mine is) and eventually tend to leak discretely.

I just wasn't 100% sure if the ranger cluster gauge (mainly the temp gauge) was calibrated differently than the explorer gauges, but they seem to be interchangeable from similar generations. From what I have read if the temp gauge sensor goes bad it usually just doesn't send any signal, so I don't think that sensor is bad either. Thanks again for your time and input!