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I have a 96’ Cougar when I got it the top Rad hose had a small leak fixed topped off. Now the temp gauge fluctuates constantly and it wants to heat up quickly at idle. I’ve changed the rad the thermostat and pump with no change but I did change the cap and that seemed to help. Even today at 35* after warmed up sits still for 5 mins creeps up till the fan kicks on then back down and cycle repeats. Any hill makes it move the gauge quite a bit. I also did a block test and it was great so is this norm for the cougars im mainly worried as summer gets here that the problem will be worse. I can’t find any leaks and did used distilled water and Preston to refill.
 

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Whoa! I had a flash back to former forum member FastFreddie when I saw your username.

Sorry, I don't have a recommendation but others might. You've covered everything that I would have recommended. If it's not the thermostat, radiator, or pump. All you're left with is the temp sensor itself.

What temp thermostat did you put in it?

BTW, welcome to TCCoA.
 

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No, that is definitely not normal. You might have air in the system still. The intake manifolds are known to crack, so that could be your issue. If the thermostat is defective or installed backwards, that could cause your issue. If the water pump is defective that could cause it as well. First thing I would do at this point is pressure test the cooling system. If that confirms no leaks, and the coolant level is staying full, then next would be get an IR temp gun and see what the temp is on the upper and lower hoses, and across the different areas of the radiator.
 

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Check the lines to the transmission with the IR reader as well; the transmission can dump a bunch of heat into the radiator when it's failing.
 

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Discussion Starter #6
Idles fine otherwise? Check engine light, or any codes?
Only code is lean bank 1-2 which could be a million things. Cleaned MAF checked vacuum so not sure but runs good.

Check the lines to the transmission with the IR reader as well; the transmission can dump a bunch of heat into the radiator when it's failing.
That could be it shifts weird to me does great once your driving. Fluid looks and smells good. I wouldn’t think it would do it at idle in park.

99
No, that is definitely not normal. You might have air in the system still. The intake manifolds are known to crack, so that could be your issue. If the thermostat is defective or installed backwards, that could cause your issue. If the water pump is defective that could cause it as well. First thing I would do at this point is pressure test the cooling system. If that confirms no leaks, and the coolant level is staying full, then next would be get an IR temp gun and see what the temp is on the upper and lower hoses, and across the different areas of the radiator.
99% sure the thermostat and pump is working correctly. I can see when the thermostat opens when you first start driving and the hoses have a lot of pressure also in 2,000 miles haven’t added coolant staying slam full. The intake has been changed at some point don’t mean it’s not cracked but no coolant loss so I don’t know. Leaning towards the trans. Thinking about installing an external cooler see what that does for the rad temp.
 

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Only code is lean bank 1-2 which could be a million things. Cleaned MAF checked vacuum so not sure but runs good.
That makes a car run hotter than it would otherwise; like everyone else by now, the evap lines are leaking.

There are 8 pieces of hose in that line, lol.

 

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I have a 96’ Cougar when I got it the top Rad hose had a small leak fixed topped off. Now the temp gauge fluctuates constantly and it wants to heat up quickly at idle. I’ve changed the rad the thermostat and pump with no change but I did change the cap and that seemed to help. Even today at 35* after warmed up sits still for 5 mins creeps up till the fan kicks on then back down and cycle repeats. Any hill makes it move the gauge quite a bit. I also did a block test and it was great so is this norm for the cougars im mainly worried as summer gets here that the problem will be worse. I can’t find any leaks and did used distilled water and Preston to refill.
Coming into the summer here in Phoenix. Started having the same problem... When I bought my 95 Cougar changed all the fluids, including coolant. Flushed with Motorcraft flush. Changed thermostat and refilled with Motorcraft coolant.
I bought the Tstat at Oreillys, and they recommended changing the O-ring. I found out that the MotoRad thermostat is designed to NOT use the o-ring !! Siliconed the housing surface and BINGO !! Runs normal and no fluctuations... Maybe this is your issue ?
 
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