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1999 deville coolant temperature

08-29-2009, 06:34 PM
I am currently having a problem with the coolant temperature reading of my 1999 caddy Deville. I have had the car for 10 years and it has always idled at 197 F. I had to replace the coolant overflow tank and cap last week because it developed a leak and since that time the coolant temp seems to settle at various temps when at idle. after I replaced the tank I lost over a gallon of coolant in a week. found the radiator had split on one side at a seam. replaced the radiator and the thermostat. still the coolant temp goes over 197F on level ground , may go as high as 230 when I clime a hill and sometimes go to 197F on a steep decline or settle at 222-217 on a not so steep decline. it's just not acting as it did for the past 10 years. I also replaced the water pump about 2 weeks before the overflow tank. the caddy shop claims the temp is within specs but I know something is wrong. I tried a new cap today and it made no difference. the new thermostat opens at 197F just like the 10 year old one. I thought there might be air in the system but the cap is at the highest point. time for clunker to the junk shop?? aside from the coolant temp problem the car is in great shape, no dents and it still runs fine. any ideas?? reading about head gaskets the engine only has 23,000 miles as it was replaced with a new engine at 60,000 miles by GM due to a coolant leak thru the casting. it burns no oil between changes. sorry for large type i copied from other program.

08-29-2009, 07:35 PM
This may sound like a stupid question but due your rad fans come on???

08-29-2009, 09:33 PM
yes, checked that first thing. 2 fans seem to be running every time i check. just finished driving about 4 miles on level street got as high as 208F finished at 201F. the problem is if i get up to 222F on a hill it stays there when i return to level street. outside temp at 81F.

08-29-2009, 10:22 PM
I looked up your termostat and it shows 180 not 197 this is most likely your problem. you have a 4.6 eng correct?

08-29-2009, 11:25 PM
i have the northstar engine. i think it is the 4.6 but i am not sure. since day one the thermostat opened at 197F. cadillac service installed a new thermostat and it also opens at 197F. sometimes we would eat in the car with the AC on and the outside temp was 104F and after 1/2 hour the coolant temp would read 197F.

08-30-2009, 07:56 AM
If caddy installed termostat its probably correct all I can say is I went to napaonline and advance and both listed 180 for your veh. Remmember just because it opens at 180 doesn't mean it wont hoover at 190 it needs to be above 180 to stay open. I would look to see that caddy serv did in fact put the correct termostat in using there part # on the bill you got, we are just human and make mistakes? If correct I would have caddy or someone you trust flush system. You could have clogg. Does coolent move in rad when you rev eng?I know northstar engs have head problems hope you have luck with yours.

08-30-2009, 12:34 PM
i know the thermostat opens at 197F because when i start the engine say at 75F the coolant temp digital readout will clime in a steady manner till 196-197F and then plateau for maybe 10 minutes and then go to 200-203F at idle. before the water pump change done at a local garage the idle was always at 197F. i am not sure if my problem is related to the new water pump but i will have cadillac service check that next. where did you find the opening spec for the 1999 DeVille??

08-30-2009, 01:23 PM
i found this in a parts site...180 Degree; OE Temperature; Seal included . could this refer to 1/2 of 360 Degrees?? which would mean straight thru.
i did check NAPA and they showed..Temperature Range:180 Degrees i will check with cadillac parts and post again.

08-31-2009, 02:32 PM
ronaldk i found this at the forum

But in researching this I found that the thermostat STARTS to open at the rated temp, that would be 180 degrees, and it should be fully open when the temp increases 27 degrees, or at 197 degrees. That would make sense then, as to why my temp can read 192 degrees and at highway speed with cold air blowing through the radiator. Take note of the statement that a USED thermostat could be +10 degrees from this these settings.

so you are correct the opening temp is 180F and i see the plateau at 197F when it is fully open.

my car is operating within specs but not like it operated before i installed the new coolant overflow tank.
i also noted that the 2 fans behind the radiator come on as soon as i start the car and never seem to turn off. from what i read they should not turn on untill the coolant reaches about 230F.

thanks for your input.

08-31-2009, 06:36 PM
When you have your air on one fan should come but second should only come on when temp starts to climb farther then small fan can control. I can only think of restriction in coolant system. I am a firm believer in the owner knows the car he has been driving for any amt of time, so I know you see a potential problem There is one otherthing that just hit me the coolent temp sensor gives the info to brain as to when the fans turn on. Maybe that sensor is going bad just a thought.

08-31-2009, 10:13 PM
ronaldk..thanks for the input. i will start the car with the AC off and see if both fans are still on at startup. tonight the outside temp was about 80F and the AC seemed very strong set at 69F. when i turn on the recycle inside air i do not hear the air flow get lower as before.
i also thought about the sensor but what would cause it to fail at the same time i changed the overflow tank??
the temp is still within specs when i drive, just not as steady as before.

09-01-2009, 02:48 PM
started the car with the AC off and neither fan was running. did not have time to check any futher. you were correct again..thanks!!

09-01-2009, 06:26 PM
In your post 9 you have some statistics that are accurate. but when you added 27 to 180 you got 197 it would be 207 which does put your veh at correct coolant temp. I think the 10% tolerance if you applied it to your orig therm might lower your ave temp and could have caused the norm to be 197.As I see it you are not overheating in a very hot state correct?? I dont see you saying your losing coolant from over heat correct?? Then the veh is probably in specs. But with that said I know you replaced rad,pump and expan tank how about hoses? The lower hose can start to collapse which would start a restrict in flow causing an increase in mean temp.See if (when cool) the bottom hose or hoses(my 93 caddy has two) are soft and spongy if so have replaced I know your worried about a needless breakdown so thats something I would check. let me know

09-01-2009, 09:06 PM
i changed post #9 to make it a little clearer. what you see in red is a quote by someone else in the forum. i will post more latter tonight.

09-01-2009, 09:30 PM
Ok someone else added wrong but if those statistics are correct your veh is in specs. I would still like to know if you are or are not overheating with a loss of coolant. Also like to know if you changed hoses or checked as I had suggested so you can have peace of mind?

09-01-2009, 11:02 PM
This is a long shot, but check to see if the thermostat was installed upside-down (aka. backwards). The description of the problem is quite consistent with a backwards t-stat.

09-02-2009, 12:06 AM
changed all cooling system hoses when i got a new engine in 2004 or 25.000 miles loss of coolant for 5 days since i had caddy install new radiator and thermostat. will check hoses in a day or 2. drove a few miles tonight with outside temp at 76 and never got above 199F. will post in more detail tomorrow.

09-06-2009, 03:02 PM
sorry for the delay in replying but i have spent most of the week with microsoft (india of course) trying to straighten out my email. microsoft deceided not to support outlook express on sept. 1st. i tried for about 10 hours to get it to work again with my MSN account. after 5 more hours on the phone i was finally told that outlook express will no longer sync with anything let alone MSN email. now back to the car. the car is still losing coolant as of sat sept 5th. i can see a small amount on the garage floor. i went to the caddy service on sat but due to the holiday they did not have any spare mechanics. i had gone 2 days before and just asked them to confirm that the water pump belt was working correctly. i will go back wed and have them check the hoses which i changed 5 years or 25000 miles ago.
the car is not overheating it is just not acting as before. at night on a flat street it will stay at 196-201F with the AC on or off. during the day it will(outside temp 75-85F) stay at 200F for long periods or stay at 215 -222F for long periods with no change in load such as speed or hills. this is with or without AC. i am sure the thermostat is in correctly because i had it changed when they installed the new radiator and the coolant temp is acting the same.
got peace of mind by turning off the digital temp indicator. i will wait for the coolant level warning before i panic.

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