My 96 Park Ave. Ultra is fluctuating water temp. It stays roughly around 180, but will rise to around 200 and then drop to around 165...all while driving a constant 50 mph or so. The cycle is roughly a minute or so. I thought it was air in the system, but I have bled it twice now using the bleeder screw on top of the thermostat housing with very small if any result. I also noticed that the fan doesn't come on while I'm bleeding the system, and the temp gets around 215 or so, the gauge arrow almost points straight up. There are no drivability issues but I'd like to address this issue asap. Any help would be greatly appreciated.

Typically, the low speed fans will turn on around 212 and high is 226.


In all three of my vehicles, I use a 180* t-stat in the summer. It keeps the engine/engine bay a tad cooler, winter time, 195.


You can trick your PCM to turn your fans on by disconnecting the coolant temp sensor connector. The PCM will command the fans to turn on LOW. This is a built in safety feature should the PCM not see an engine temp.


On your original issue, it seems as if your cooling system is operating fine. Increase/decrease in the temp gauge is normal. Normally, you don't see this in the winter time.


And I wouldn't trust the temp gauge, they tend to be a few degrees off.

When was the last time you changed the coolant or thermostat?
Did u do recent work that may have caused air in system or this fluctuation?
I normally drain & refill the coolant every 2 years.
Is it a 180 or 195 degree thermostat in there?
Using an antifreeze tester, what is the coolant reading -40 or?
My 97 PA has a 195 thermostat and stays around 190-195 on hwy.
The cooling fans dont seem to kick in on mine til about 215 some times as well (idle)
If I engage the AC or defrost mode, the fans should automatically come on and the temp will drop 15-20 degrees in a minute or two.
Any time I see a wild swing like what u are describing, I wonder if the temp sensor is /gauge is working correctly.
Do you have a scanner that can be plugged into OBDII port & read what temp is being reported?

I'm kind of thinking it'* your thermostat......it'* opening too soon....in fact, I am surprised a P0128 is not set....

At idle, in traffic, or low speeds in the summer, temps can easily rise towards 210 or more...now get out on the highway, and natural air flow can make that thermostat drop more than usual, if it is opening too soon.....A normally opening 195 thermostat, would not allow that temp to get to 165F, that'* why I think it'* your thermostat....

Thanks for all the helpful info. I did disconnect the coolant temp sensor connector and the fans came on, along with the check engine light which I'm sure will disappear soon. I'm not sure if I have the 195 or 180 thermostat, but I like the idea of putting the 180 on in the summertime. I'm assuming I have the stock 195 in as I have never changed it. I do not have a scanner, so I guess I could be off on the exact temps. I did change the coolant about a year or so ago when I replaced the lower and upper intake manifold gaskets. I'll repost when I check/replace the thermostat. Again, thanks for all the info... really helped understand much better.