: 208 at 40+ 212 at <40

09-23-09, 11:49 PM
I'm confused. And I'll admit it.

I've got just shy of 1,000 miles on my head gasket repair and the temperature has been holding steady at 208 deg. F going down the road at 67 mph. The outside temperature has been above 40 deg. F. Today, the outside temperature was around 35 deg. F and going down the road at 67 mph the coolant was holding steady at 212 deg. F. What the heck is going on?

And this, going up a hill (not a steep one, but a hill nonetheless) the temperature actually went down to 208.

Can anybody explain this? I've scoured my FSM and cannot come up with an answer.


Palmer, AK
1998 Deville

09-24-09, 09:07 AM
Probably not enough of a temperature variation to get worried about, but curious nonetheless. It could be a result of heating mode, temperature (HVAC) settings, load, speed - anything that would affect the amount of heat generated by the drivetrain vs. the amount removed by the radiator and heater core. If you have a stock thermostat in the engine it is set to begin opening at 188 and to be fully open at 208 +/-.

Your car seems to run around at 208 while mine runs around at 198, all year. Dollars to doughnuts that GM or the thermostat manufacturer (Stant ?) would say both are "Within normal limits.".

Reach down under the throttlebody area and wiggle the ECT connector. Loose connectors change electrical resistances which is what the gauge works on - it's actually a damped voltmeter.

09-24-09, 10:04 AM
I'll check that out tonight.

Also, forgot to say that last night on my way home, outside temp around 50 deg. F and coolant nailed at 208 deg. F.


09-24-09, 10:42 AM
Those temps are certainly in the normal range. Chalk it up to normal fluctuation caused by any number of factors as Sub mentioned and don't worry about it.

09-24-09, 02:54 PM
208 sounds high. My thermostat seems to be tuned in for exactly 196 degrees - that's the temp the engines seems to stabilize at when cruising on the highway, AC off, outside temp not crazy high (like it gets far too often here in Phoenix!). Not 197, not 195 - it says at 196 exactly, unless something changes like stop and go traffic, a steep hill, I step on it hard - then it could go up, but otherwise it's 196 when on the highway, or put another way, when warming up, the engine temperature goes up to 196 and then stops rising.

09-24-09, 08:44 PM
208 is not high at all Tateos. My '97 (digidash) ran a consistent 205 (Summer) and 215 (Winter).

09-24-09, 10:13 PM
OK - then let me say 208 sounds high TO ME - that is, compared to my car, which maybe runs low.

It depends on the particular thermostat, maybe? Maybe with my thermostat in your engine, it would run 196 - with your thermostat in my car, maybe 205-215?

All I know is, once it's warmed up, my car runs exactly 196 on the highway, pretty much year round - not when it's 110-115 here, which is common in the summer in PHX - then I of course have the AC on and it might be running 207 - 215 on the highway - 220 - 235 in stop and go traffic.

09-24-09, 10:31 PM
My car runs on the highway at about 199. Idling in traffic, it will go up to 224 and bounce in between 214 and 224 until I go on the highway again.

09-24-09, 11:13 PM
Thanks for all the input guys.

I wasn't really worried, I've got over a 1,000 miles on my stud kit head gasket repair and no problems. I can get on it pretty hard and the temp spikes a little, but nothing too major. My oil light flicker problem seems to have gotten solved as well, it's been five days without a flicker.

My digidash display says 25.8 mpg but when I do the math at fill up, I get 23.3 mpg.

All in all, well worth the effort and I am loving my commute with this car. Heated seats would be a plus, maybe next time.


09-28-09, 07:57 PM
I was thinking that, not only can there be calibration differences between thermostat, I suppose sending units differ also - a 208 reading on one car could actually be the same temperature as 196 on my car...or are these things more accurate than that?

09-28-09, 09:17 PM
:sneaky: Never reset the long-term gas mileage calculator. For the most accurate readout of what your car really does, day in and day out, leave it alone and watch it once or twice a week for long-term trends. Instantaneous is fun for playing with the gas pedal, and that's about it. Tank-to-tank, use the ol' pocket calculator, but that lies also; it's long trip/long term mileage that's most accurate.

I can go out in the garage right now and look at long-term mileage and it will read about 20.3 - since November of 2005 ! ........but on the run from here to Gainesville, FL next month it will ratchet up to the mid 24's, actual will be 25.3 gas used for that trip only. After we get back to MD the long-term will slowly ratchet back down to mid-20's.

13,14,15,16 in town, 25 on the road. It all averages out and to try to keep track of mileage from day to day will put you in the looney bin.

Actually, thermostats are remarkably close to specs. It's the electric gauge sending units that are suspect. For real gauges always use mechanical units; electrics are for convenience only.