Temp fluctuation

[Firestar]

I have a '94 Ranger, 4.0, 5-speed, 4x4. My problem is hard to explain but i'll try. In the morning, after driving about 3-4 miles the temperature gauge will quickly climb to about the "L" in the word normal on the gauge. Then it will quickly drop down to below the "N", and usually stay there. Occasionally it will climb back up to around the "O", then drop back down again. It doesn't seem to matter what the air temperature is. It usually only does this in the morning, after the truck has sat all night (about 14 hours), hardly ever when i go home from work after it's sat for around 9-10 hours. The air coming out of the heater never gets very warm. The heads and thermostat were replaced about 5000 miles ago, the radiator and hoses shortly thereafter, and the thermostat was replaced a second time a few hundred miles later. Seeing the gauge go that high, even for only a couple of seconds makes me nervous. Any thoughts on the reason for its erratic behavior?

 

[cp2295]

Could be the temp sending gauge. Could also be trapped air, are you sure the cooling system has been totally purged?

Another thing that comes to mind is the possibility of the water pump going bad. The only other thing in the cooling system is the heater core. I don't see how this has any connection to your issue. There's no coolant loss right?


Sent from my iPhone using Tapatalk

 

[RonD]

On my '94 4.0l it was the heater core, partially blocked.

The heater core is the water pump by-pass system on this model, coolant flows thru the core regardless of heater temp setting in cab.
If core isn't flowing well then you get random engine temp ups and downs, just like you describe, no pattern just random.

Goods news is the core is not expensive, $20-$30, and you only need to remove 2 hose clamps, at firewall, and 4 screws under glove box in cab to swap it out

 

[Firestar]

Thanks for the info. I would've never guessed that it could be the heater core. I'll look into it.

 

[RonD]

Yes, surprised me as well, after replacing thermostat and fan clutch, not to mention a good flush of radiator.

Tried back flushing heater core, almost no flow in either direction, tried CLR and vinegar, then called for core price and found it was only $26, lol, so wasting time on that flush, quick easy swap, under 30minutes, first time, could do it 15min now.

problem solved

Put a towel down in cab, when you tilt core to get it out it will spill a bit.
And have some thin(3/8") self adhesive weather strip on hand the stuff on the old core will most likely be unuseable

 

[cp2295]

Wow good info Ron didn't know it had any effect..!


Sent from my iPhone using Tapatalk

 

[jeremysdad]

Thought I would bump this old thread, being winter and all.

Had same symptoms: heat not great, temp gauge fluctuation/hovering just above the first | in normal range.

Replaced thermostat. Heat slightly better, temp gauge the same.

Back flushed heater core today. Gauge now stays just under 'N' might get into 'O' after long idle. Heat will roast a turkey. lol

Will be replacing rad, heater core, and all hoses when funds are available...

Couldn't believe the trash that came out. Also couldn't believe the simple (free) fix to an issue I've had since buying the truck in Oct of 2014.