Chrysler Repair: Engine temp fluctuates as seen on gauge:2.5L 4 cyl., engine temp, hi john

Question
QUESTION: Temp goes up too hot and down to norm. with in 10-20 sec. Does this constantly while driving. Replaced water pump and therm., elec.fan is working and heat in the cabin. Circulation is exellent with out thermostat. I am baffled. Maybe a sensor l`m missing that has failed? Thanks John.
ANSWER: Hi John,
There is always going to be variation due to thermostat opening variations, but the very rapid changes may be due to something other than that or a sensor (there is only one for the gauge). I wonder if you might have air trapped in the upper level of the cooling system and that will cause the temp experienced by the thermostat and the sensor to vary as coolant displaces the air and then is displaced itself by another bubble. You didn't say which engine or year you have but there indeed may be a procedure that you need to follow to bleed out the air. Let me know the specifics and I may be able to pass along those techniques.
Roland

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QUESTION: Hi Roland. WOW! Could not believe the short responce time,tyvm. Anyways, the engine is a 2.5 1992, plain jane. When gauge reads hot you can take the cap off with no pressure behind it while it is running. However, after shut down for a couple min. it has pressure when you start to loosen cap. Thanks again , John.

Answer
Hi John,
You are welcome. Indeed, I have the same engine in my '89 LeBaron and it does have a special procedure for draining and refilling the system. You might want to partially drain the system as described below, and then follow the refill procedure. Here is how I have described it in the past:
"Then to properly avoid air getting in the thermostat housing you have to drain and refill the system following a specific procedure. Before I tell you about that though, you might try very slowly removing the plug on the top of the thermostat housing (8 mm Allen head wrench) and observing whether when you gradually lift it out of its hole the coolant is right up to the top of the opening. If not, then add coolant through the opening until it fills to the top of the housing and then replace the plug. Put some anti-seize compound on the threads of the plug.
If you have drained the radiator in the recent past, you may have air in the thermostat housing or the radiator core circuit. The best thing to do would be to partially drain the system and follow the instructions for refilling:
There is a plug in the top of the thermostat
housing (8mm allen head) that has to be considered when you are
dealing with the cooling system or you will trap air in the housing or
the heater. Here is the way to do drain and refill:
Start the engine and put the heater control slider in the Heat
position (this opens the water line to the heater). (A/C should be off
of course)
Shut off the engine
Open the radiator drain valve and watch to see that the water drains
first from the coolant recovery bottle. Then remove the radiator cap
and let about another quart or so drain out. (You can collect the
coolant in a pan for reuse if you choose).
Then remove the plug from the top of the thermostat housing and let
the water drain out of that housing, use a flashlight to observe level
(you don't have to drain any further if you are just trying to remove
trapped air, otherwise complete draining).
Close the drain.
Start refilling thru the radiator cap opening
When the coolant level rises into the thermostat housing and up to the
top of the plug hole, stop filling and put in the plug (put some
never-seize compound on the plug threads).
Continue filling thru the radiator cap opening until radiator is full.
Put on radiator cap.
Fill the coolant recovery bottle to the "max" line on the bottle.
In theory you will have now an air-free cooling system. At least if
there is any trapped air it will work its way out more quickly if you
follow this procedure exactly. This may lower your operating
temperature and also eliminate the initial rapid rise of the gauge."
Roland