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Solution to my fluctuating gauge problem was two issues associated with the thermostat:

Aftermarket thermostat was opening too soon and too much thereby causing the engine to not maintain sufficient core temperature (normal operating zone).

Doing a boiling water test I noted that the Motorcraft was rated the highest (197 degrees) and opened spot on that temp. I was given two aftermarket thermostats supposedly the OEM temp rating, but nothing could have been further from the truth. The 192 degree Prestone model opened at 180 degrees and the 195 (noname) opened at about 190. Both aftermarkets all appear (eyeballing opening) to have a significantly larger water opening (valve body opens more) which allows more coolant flow out of the engine and causing rapid temperature drop.

Now the aftermarkets may work really great in summer as they allow more coolant flow, but in winter the OEM Motorcraft keeps the engine at the right temperature maintaing maximum heating and more consistent running temperatures.

I hope this clarifies the key issue between aftermarkets and the Motorcraft although I would always check the opening temperature of any thermostat because I don't trust any of them. At least the Motorcraft is designed to work with this engine and obviously the aftermarkets are leaning toward keeping the engine cool rather than maintaining the best operating temperature of the Taurus Vulcan engine at least.

Thanks for the great feedback as it kept me working in the right area to figure this out! I hope it helps somone else too! :icon_cool:
 

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Got an updiate on this thread with a couple of more ideas. After my thermostat work I found that the improvement in gauge fluctuation was marginal. After that work I found a leaking timing cover which was causing a loss of pressure and antifreeze. After it was repaired the gauge fluctuation actually dimished significantly. I would have to say that the thermostat was a minor issue and the timing cover leak a major contributor to gauge fluctuation.

One more piece of speculation is the fact that I have such a dirty coolant system with major debris floating around and heater core blockages that require a back flush about every 2 months to regain proper flow. This added problem I believe contributes to heater core blockage which clears up under higher rpms (a bit better) and sometimes releases a blast of cooler hearter core antifreeze which since it immediately contacts the coolant sensor switch near the engine, causes a huge temperature fluctuation until the heater core system stablizes in temperature. This I believe is the biggest cause of temperature gauge fluctuation when you have no other issues, except the bouncing needle. For what it is worth this explanation may help someone sort out their system, by considering that this can happen. Now that I have a much cleaner system after about 6 heater core backflushed the gauge is much more stable and the heat is also stable. My future modification is coolant filter to catch the debris and maybe save me a bit core flushing. :rolleyes2:
 

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All speculation is over! I am posting the final solution to this issue and hopefully it will help someone else.

The cause of the fluctuating temperature gauge is "hands down" without a doubt, a blown head gasket. This summer I did leakdown test and found cylinder 1 & 3 (on dome upper side) leaking compression into the water jacket. So this fall I tore the engine down and replaced the head gasket, lapped the valves back in, replace seals, etc. and my engine runs perfectly now with NO Fluctuation in the gauge.

Yesssss.................! :D
 

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7 Years later, what I have learned!

While the blown head gasket (2013) definitely caused some temperature fluctuation back then, it was not primarily responsible for the fluctuating temperature gauge. During the car's early life the antifreeze was not changed and I had extensive rust flaking throughout the water jacket. This was going to be a problem especially in winter when you needed heat and the heater core became plugged with rust flakes.

Here is what I have done to get another 7 years and almost 200,000 more km (326,000km so far):

I would typically pull the inlet and outlet hoses off the motor (left and right side of engine) and hook up my water hose and flush the heater core only. There was no sense to also flush the block or head (as it was already done), since the antifreeze was clean and rust is still going to flake off for the rest of it's natural life.

When I first hooked up the hose flow was reduced by at least 70% flushing from right to left side. Flushing in both directions cleaned it up easily and flow was restored. After every flush the temp gauge would remain steady with no fluctuations. As time went on (a few months of driving) the gauge would start to fluctuate slightly again and the heater was growing cooler in the cabin. These fluctuations were more pronounced in winter because of the thermostat closing and the flow of engine coolant backing up in the engine because of heater core obstruction. When the thermostat opened up it would allow the trapped coolant to escape and a colder flow to immediately rush into the engine. This became the fluctuating heat gauge phenomenon. Interestingly, it did not fluctuate (at least significantly) in the summer because the thermostat was mostly open allowing good coolant circulation.

I am still driving this amazing car and typically flush my heater core about twice a year depending on how much driving I do. I made up some hose adaptors to attach to my water hose and it takes me about 30 minutes to do the job. I use a pail to catch the water so I know when the rust flakes have stopped coming out. Turned out to be a little bit of an inconvenience but an easy fix!

Hope this helps someone else who stills drives a Vulcan!
 

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When i did my block vulcan flush, i took the water pump off and used the hose and nozzle directly into
the block where the water pump was. Seemed to get a lot of crud out of the block. did the heater core with
bi directional flush too which seemed to help. When the core was clear, it had blistering heat in the car!
 

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Most people don't flush period! That looks like a good product VC-9, should have been used about 20 years ago. Once water jacket is filled with flakes it is much too late. Best to just leave it alone and flush heater core as needed.
 

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Most people don't flush period! That looks like a good product VC-9, should have been used about 20 years ago. Once water jacket is filled with flakes it is much too late. Best to just leave it alone and flush heater core as needed.
This stuff removes the flakes and renders the surface somewhat inert, so it won't rust as easily.
 

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I just replaced my reservoir on my 96 Taurus (72,000 miles) because of a massive leak. My old one had a crack in it, which Speed Test Scrabble Word Finder Solitaire drained all my coolant fluid. So I filled it back up. I have been checking afterwards for any other possible leaks and fluid loss and have not found one since. However, now my temperature gauge is acting up. Here's a video that I took while I was driving:


Sometimes it even goes higher to about 80% then back down to around 20% despite the cold weather here; it ranges from 40 to 50 degrees farenheit here in VA now. It did this before when I haven't changed my reservoir yet but mostly it stayed on the high side. Now I don't know what's doing this. Hopefully ya'll could help me out.

Thanks
I will give that a try. I replaced the Radiator over the weekend and it helped slightly. I think I will create my own thread with my findings to help others that might have been in the same shoes as me.
 

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You need to use a OBD2 scan tool that reads temperature to confirm the temperature the PCM
is seeing is the same as the dash. Dash uses a separate sensor line. If its really fluctuating,
you need to do a cooling system pressure test to see if its holding pressure. If not, the system
will not pressurize properly and overheat. On my 97, it did this because of a leaking frost
plug on the firewall side where it was hard to see.
 
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Solution to my fluctuating gauge problem was two issues associated with the thermostat:

Aftermarket thermostat was opening too soon and too much thereby causing the engine to not maintain sufficient core temperature (normal operating zone).

Doing a boiling water test I noted that the Motorcraft was rated the highest (197 degrees) and opened spot on that temp. I was given two aftermarket thermostats supposedly the OEM temp rating, but nothing could have been further from the truth. The 192 degree Prestone model opened at 180 degrees and the 195 (noname) opened at about 190. Both aftermarkets all appear (eyeballing opening) to have a significantly larger water opening (valve body opens more) which allows more coolant flow out of the engine and causing rapid temperature drop.

Now the aftermarkets may work really great in summer as they allow more coolant flow, but in winter the OEM Motorcraft keeps the engine at the word counter right temperature maintaing maximum heating and more consistent running temperatures.

I hope this clarifies the key issue between aftermarkets and the Motorcraft although I would always check the opening temperature of any thermostat because I don't trust any of them. At least the Motorcraft is designed to work with this engine and obviously the aftermarkets are leaning toward keeping the engine cool rather than maintaining the best operating temperature of the Taurus Vulcan engine at least.

Thanks for the great feedback as it kept me working in the right area to figure this out! I hope it helps somone else too! :icon_cool:
I agree with your comments, it is really useful for users. Thank you!
 
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