Hey everybody, i bought a 2000 ford ranger 4x4 3.0 recently. I have a question about the coolant temperature. When turning it over on a cold start, obviously the temperature is cold but after warming up and driving the gauge will start to go high to 3/4 of the gauge for about 10-15 seconds but goes down without having to decelerate or slow down. Engine is not under any load whatsoever. Thank you in advance for answers and/or tips.

 

You have a Sensor for the computer and a SENDER for the gauge. The Sender may be going out or just sensitive. As it warms up the coolant in the block and heads warms up first and as it gets to operating temp the thermostat opens up. The rise you seeing is the initial warming of the coolant and then when the t-stat opens, it gets that cooler water going in and it brings the temp back down to normal.
My 2.5L did this. I hooked up my OBDII reader and the computer was seeing a more normal increase and decrease of temp which told me the Sensor was more accurate.So I replaced the SENDER and the that made it follow more closely to what the computer was seeing from the sensor.

 

Could be a couple of things

Thermostat may be sticking, so not opening and closing smoothly so it sticks until coolant gets hot enough to unstick it

So I would get a new thermostat with jiggle valve, 190-195degF
(jiggle valve is a hole in the t-stat plate that allows air out when refilling, should be placed at 12:00 position when installing)
(you can drill your own hole, 1/8" is fine)

Then after draining coolant to install new thermostat, flush heater core with garden hose
Unhook the two heater hoses that go to firewall from the by pass valve in engine bay
Point one down at the ground and put garden hose on the other hose to flush
Then repeat using other direction, flush BOTH ways
If flow seem low then you could have a clogging heater core, which is hard to change in a 2000 Ranger

The heater core is the water pump by-pass so if heater core gets clogged up then temp will go up and down randomly after warm up
Flushing might help this

To make any heater core on any vehicle last longer, you should reverse the hoses at the firewall every year or so, this back flushes the core while you drive, and its FREE to do, lol

 

Installed a new thermostat and flushed the coolant system. Problem solved. Thanks a ton guys!

 

Good work

Thanks for the update and the FIX