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Coolant temp running low?


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I have a strange situation and cannot find any other posts with similar isssue... My '98 Eldo will only get up to about 180F max for the last several days, highway or city driving for well into the warmup cycle... Is the thermostat stuck open? Is there anything else that can be causing this? It seems like the heater is working fine, but hard to tell as it really hasn't been that cold here.

It usually will be running in 206-210F area most of the time once warmed up. Ocaisionally getting near the 220F area in city traffic or hotter ambient temps. I did just replace that coolant reservoir tank a while back, could something there be causing this? Coolant level seems to be fine, about 2" below filler neck on tank... I can't check temp by inserting thermometer in radiator as there isn't a cap there like older vehicles I've had... Just wondering if reading the digital display is accurate, but it does seem to correspond to analog gauge as thats below the normal straight up dial position also.

I did finally get the replacement reservoir tank for the car, (due to a defective Fluid Level Sensor) but have not swapped it out yet. Was thinking I'd perhaps replace thermostat at same time as long as I have to drop coolant level anyway... Any specific things to watch out for or suggestions on thermostat purchase?

Edited by sprucegoose

'09 Cadillac CTS-4 3.6 direct injection, 128 K mi.
'15 Chevy Tahoe LTZ, 5.3i V8, 125 K mi
'70 Firebird Formula 400, Bored+.04, RAIII heads, M21 4spd., in-process restoration!

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Make sure that it is the right part number for your car, 12TH11D (with gasket) or 131128 (without gasket), 180 degrees. The coolant comes into the block at 180 degrees or a little more, and goes out past the ECT at 195 F to 210 F. The wrong thermostat, or putting it in backward, can prevent its temperature sensing portion to be exposed to the in-block water, or force it to open against the water pump pressure instead of with it.

CTS-V_LateralGs_6-2018_tiny.jpg
-- Click Here for CaddyInfo page on "How To" Read Your OBD Codes
-- Click Here for my personal page to download my OBD code list as an Excel file, plus other Cadillac data
-- See my CaddyInfo car blogs: 2011 CTS-V, 1997 ETC
Yes, I was Jims_97_ETC before I changed cars.

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Got it... Thanks for the reco and part numbers guys!

I'm still a little concerned about the significance of this occuring right after replacing the reservoir... wonder if something dislodged in the system and got stuck in thermostat? I'm almost sure the thermostat is original anyway, so being 16 yrs old should probably be replaced.

I'll do the boiling water themometer test on it once I remove it just to check operation...

Edited by sprucegoose

'09 Cadillac CTS-4 3.6 direct injection, 128 K mi.
'15 Chevy Tahoe LTZ, 5.3i V8, 125 K mi
'70 Firebird Formula 400, Bored+.04, RAIII heads, M21 4spd., in-process restoration!

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  • 2 weeks later...

Well it was obvious once I opened this up today what the problem was. The rubber o-ring gasket that surrounds the rim of the thermostat had completely separated and most had entangled and got wedged in plunger. This prevented it from closing all the way.

No issues putting the new one in, and inserted bars tablets in to compensate for coolant lost in process.

'09 Cadillac CTS-4 3.6 direct injection, 128 K mi.
'15 Chevy Tahoe LTZ, 5.3i V8, 125 K mi
'70 Firebird Formula 400, Bored+.04, RAIII heads, M21 4spd., in-process restoration!

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Congratulations on a problem well solved.

CTS-V_LateralGs_6-2018_tiny.jpg
-- Click Here for CaddyInfo page on "How To" Read Your OBD Codes
-- Click Here for my personal page to download my OBD code list as an Excel file, plus other Cadillac data
-- See my CaddyInfo car blogs: 2011 CTS-V, 1997 ETC
Yes, I was Jims_97_ETC before I changed cars.

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