Hi Everyone,

2002 Ford Ranger Edge 3.0L 120K. I posted last week about an issue I was having with my thermostat and temperature. I was noticing the truck was going a quarter the way up past the halfway temperature marker and then going back to level when running cold. I figured the thermostat was on its way out and for whatever reason was sticking closed to long. I replaced the thermostat and coolant sensor as I noticed that was broke while in there.

Now I have a new issue.... My truck reaches normal temp like it always had, but now when slowing down or idling the temp gauge is going from the normal zone and dropping to nearly the cold marker on the temp gauge? Once I get going again the temperature on the gauge goes right back to normal. I have full heat/AC/ and have ensured the system is burped. I even replaced the thermostat thinking it might have been defective, with no luck.

Any ideas? I am completely stumped!

Did you replace the SENSOR or SENDER. They are 2 different things, the sender controls the guage and the sensor gives data to the ecu. The easiest thing to do is use a scan tool to read the temp from the sensor to see if it matches the guage. If it doesn't then the sender is bad. If both the guage and scanner fluctuate the same then you know there is a physical issue with the cooling system.

I think you have an air bubble in the coolant system.

__________________
1998 ranger extended cab, 2wd, 4.0 ohv, 5 speed, sony xploid 52 4 way watt radio, 2 pair of pioneer 240 watt speakers, Cobra 29 LX LCD CB radio pushing 100 watts, k&n air filter, 15 inch exploder wheels, dual exhaust with 4 inch stainless tips, 2 12 inch subs with a 1000 watt amp just for the hell of it!

new project: dirt track modified racing at the local saturday night track.

Interesting, I did not replace the sender. Maybe the sender is what was crapping the bed and not the thermostat. I pressure tested the cooling system before replacing the thermostat and there was no leak, etc detected. What could physically cause this that wouldn't show up on a pressure test if not the sender?

----------

Hmmm I bled it twice, wouldn't a bubble result in a higher temp reading not lower? I have been bleeding it by just letting it idle with the rad cap off, would you suggest doing something different?

The only physical issues would be air in the system. Air can get stuck in one spot or move around making the gauge do all sorts of weird stuff. The wrong temp thermostat can also cause issues. Opening too soon or too late. Like I said though, I would verify the guage is functioning correctly before chasing down those issues.

P.s. on the bleeding, depending on the engine there is often small bleeder ports on the manifold, t-state housing, or heads. But usually running up to temp with the rad cap, and overflow cap off works fine unless something is clogged. Jacking the front of the truck up often helps a bit too.

Thanks! Really appreciate the help. I bought the 195 temp, I believe it is aftermarket, I bought both from Advanced Auto and that was the exact match. I will try the sender and then if that doesn't work try to re burp again.