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post #1 of 10 (permalink) Old 08-24-2011, 03:39 PM Thread Starter
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Thermostats Q

I feel really stupid for asking this, but it's one of those "second opinion" things.

Okay, background information. I bought the Cougar about 13 months ago; when I was driving it in the warmer summer weather, the temperature gauge typically remained either over the "N" or with the needle just touching it from the left. As the weather cooled off, the needle naturally settled a little lower on the gauge, about 3/4 from the inside of the white bracket past the blue zone to the "N". It's hard for me to remember for certain, but I seem to recall the needle typically hovering anywhere from 1/2-way to "N" (from the inside of the white bracket after the blue zone) to 3/4 to "N".

Now this is where trouble started. Driving home one night while on the freeways, the temperature gauge barely made it past the blue zone. The heater wasn't getting very warm; barely warm enough to keep the car at a comfortable temperature driving down the freeway in low teens. The needle was fluctuating a little, moving about a needle thicknesses up or down.

I got the car home and replaced the thermostat with a Stant 195 (economy) unit from AutoZone, and to be safe I also replaced the gauge sender. I tested the thermostat by running the engine at about 2000 RPM until the fan kicked on, which happened by the time the needle was between "N" and "O". The gauge seemed to run in the "cold weather" range I described above (1/2-3/4 up to N past blue), for a while. Then it started to only get about 1/4 up to the N. Again, I assumed the thermostat had croaked. So I went to NAPA this time and got their replacement thermostat and took out this "new" thermostat.

I put both into a pot of hot water and watched as the temperature increased, the "old" Stant opened noticeably faster than the new NAPA thermostat. So the NAPA thermostat went in for the remainder of the winter, until I started driving the T-bird again. However, the gauge started only making it up the gauge about 1/4-way to N again, so I now assume that thermostat has died.

I have a new "premium" Stant here ready to go in. I'm assuming all these thermostats have somehow just gone bad as a stroke of poor luck. Is there something I might be missing that is killing thermostats?

I wish I had a way to get actual temperature readings while on the road. I might pick up a cheap probe thermometer if this keeps up to see what's going on.

-Brandon
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post #2 of 10 (permalink) Old 08-24-2011, 04:15 PM
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Before you panic - there's a TSB on bad sending units that cause the sending unit to not properly decrease in resistance, thereby keeping the gauge at a low setting (!!!)

Here's a link to a PDF of it:

http://www.philent.biz/TSB-BAD_TEMP_SENDER.PDF

So before you pull that thermostat AGAIN, I'd swap the sending unit.

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post #3 of 10 (permalink) Old 08-24-2011, 04:24 PM
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I would say so from what you described they could have; depends what noticeably faster is if they start opening around the same time but one just opens a tiny bit faster they are both probably ok, but if they are rated the same and one opens a long while before the new one (assuming the new one is good) it would explain why it is running somewhat colder than normal also. The ford thermostats seem a bit beefier to me than the others from what I have seen not that it's a huge deciding factor or anything they should both work fine. Also, check your radiator cap is holding pressure as it can have an effect.

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post #4 of 10 (permalink) Old 08-24-2011, 05:12 PM Thread Starter
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Ralph, I didn't know about that TSB... thanks for the link. The temp sending unit on the car now is new however. From what I was able to tell both the old and new sending units read the same gauge position for a given temp (both senders had the gauge needle between "N" and "O" when the fan kicked on).

Thanks for the tip about the radiator cap 91CougarLS, that totally escaped my mind. The one on the car now is the 17-year-old original.

In any case, I'll put the new thermostat in the car, replace the radiator cap and go from there. I'll grab a cheap thermometer and try to figure out at what temp it opens up at and maintains, that way I have a reference to know when something is wrong later.

-Brandon
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post #5 of 10 (permalink) Old 08-24-2011, 10:20 PM
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Quote:
Originally Posted by theterminator93 View Post
Ralph, I didn't know about that TSB... thanks for the link. The temp sending unit on the car now is new however. From what I was able to tell both the old and new sending units read the same gauge position for a given temp (both senders had the gauge needle between "N" and "O" when the fan kicked on).

Thanks for the tip about the radiator cap 91CougarLS, that totally escaped my mind. The one on the car now is the 17-year-old original.

In any case, I'll put the new thermostat in the car, replace the radiator cap and go from there. I'll grab a cheap thermometer and try to figure out at what temp it opens up at and maintains, that way I have a reference to know when something is wrong later.
I have a very similiar problem on my 96 bird. In April this year I had the same thing happen to me, except I ended up blowing my head gaskets. After replacing the head gaskets and new thermo the car warms up to just touching the N and I can watch the needle drop to halfway to C when the thermo opens. After it closes it raises back to the N and sits there with no movement for about 25 min (I have a 30 min drive to work). At about 25 min into my drive the needle starts moving into Norm and as soon as it hits the M I can hear and feel the fan kick on. At this point it drops to the N and will go back and forth between N and M when the fan cycles on and off. I have replaced the thermo, sending unit and cap twice with the same results both times. I even switched out the gauge from one at a bone yard and same thing.

Here is the weird thing. With the AC running, (fan runs constantly) after the thermo opens and the needle comes back to the N, it stays there and does not move period.

I almost wonder if I have a huge air bubble somewhere, but I have burrped the system using the procedure in this forum so much that I have gone through an entire gallon of antifreeze burrping it.

Anyone have any ideas?

1996 3.8 Thuderbird
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post #6 of 10 (permalink) Old 08-25-2011, 07:25 AM Thread Starter
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I admit I also had at one point suspected head gaskets, but I feel (strongly) that someone has already replaced them on the car. Why? My incident a year ago when one of the rocker arm bolts worked its way loose, and all of the gaskets I saw as I worked my way down to the valve covers were new-ish Fel-Pro (decidedly not Ford standard issue on this motor). I suppose it is possible that they are on their way out, but I have a hard time believing a 3.8 that was babied along for the first 40,000 miles by an old man then the next 70,000 miles by an old(er) woman could go through two sets of head gaskets in that time. (Well, now that I think of it... if the person who did the head gasket job was stupid enough to screw up torquing the rocker arm bolts, what else did he screw up)

According to my dad, when the head gaskets were toast on his '90 some years back, his temp gauge had very large, sweeping motions across the entire gauge face. My variations are very minute.

-Brandon
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post #7 of 10 (permalink) Old 09-01-2011, 03:58 PM
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I have the same problem with my 3.8L, not sure what it is. In the 2 years I have owned it, the gauge has barely risen above "cold." Now I'm not sure if the "coolant sending unit" is the same as the "coolant temperature sensor" but I switched the sensors with one from a 1990 Cougar that had a reliable gauge and my sensor worked just fine in the Cougar, but the Cougar's working sensor didnt make my gauge work. I've been considering replacing the Thermostat, what degree thermostat should I get? What else could be causing the issue, my cluster?

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post #8 of 10 (permalink) Old 09-01-2011, 04:53 PM Thread Starter
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I would stick with 195. From what I was able to tell with my thermometer (haven't done any "real" testing with it yet), the thermostat starts to open at about the right temperature, but how it reacts as the coolant temperature decreases is yet to be seen. The car has a new radiator now with the stock cap. At some point I'm going to try and match gauge positions with coolant temperature to see if the gauge is providing constant readings, and go from there.

As far as gauges themselves, there is always the possibility that the gauge itself doesn't respond like the one in your Cougar. Run the engine at a moderate RPM in park and watch the gauge when the fan kicks on. That will give you an idea where "operating temperature" is at on your gauge.

-Brandon
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post #9 of 10 (permalink) Old 09-01-2011, 08:06 PM
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Quote:
Originally Posted by theterminator93 View Post
I would stick with 195. From what I was able to tell with my thermometer (haven't done any "real" testing with it yet), the thermostat starts to open at about the right temperature, but how it reacts as the coolant temperature decreases is yet to be seen. The car has a new radiator now with the stock cap. At some point I'm going to try and match gauge positions with coolant temperature to see if the gauge is providing constant readings, and go from there.

As far as gauges themselves, there is always the possibility that the gauge itself doesn't respond like the one in your Cougar. Run the engine at a moderate RPM in park and watch the gauge when the fan kicks on. That will give you an idea where "operating temperature" is at on your gauge.
And that will tell me if its reaching operating temperature?

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post #10 of 10 (permalink) Old 09-01-2011, 11:04 PM Thread Starter
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Basically, and as long as the fan disengages (IIRC pre-94s have a mechanical fan/clutch on the water pump pulley) shortly after it engages, that will tell you that the thermostat is opening by the time the EEC commands the fan to kick on, and that the radiator and fan are doing their job. Also you'll know that the fan engages when the coolant temp is around 210F (disengages at 200), so you can relate gauge position with a ballpark ECT.

-Brandon
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