If your stat is working, once the reading has gone past 79 degrees on the secret menu it shouldn't ever drop much below that (maybe 78, but only briefly). When the coolant drops below 79, the thermostat should be fully shut and the engine should warm it back up relatively quickly. If you're seeing mid-70s repeatedly, more than likely you have a bad (or going bad) thermostat.
I happened to be out today and watched my engine temps via OBC. 49-50F outside temps. My car toggled between 78-79C the entire time on the highway as 60+ MPH. Drove about 16 miles directly on the highway, engine never went over 79C. This is on a 1 year old stat and temp sender. Temp gauge is at about 11 o'clock position.
Once I pulled off highway into a few stops, temp made it to 82C. Might have gone up more if I was really in stop and go traffic, but I only went a few miles after the highway run.
Anything from 79 to upwards of 100 degrees C is fine (I haven't seen a confirmed upward limit on the board anywhere). My car runs up to 96 degrees C when the outside temp is 105 degrees F and I'm climbing into the hills with the A/C on. The needle stays dead center.
Anything below 79 consistently and the thermostat is likely not closing all the way and the engine can't maintain the temp of the coolant. This can lead to the car running rich (the computer thinks the engine isn't fully warm) and will eventually damage your cats. It should probably be changed sooner than later.
Edgy, Not sure if your car is running "hot", but it appears to be running a bit higher in temp than mine today and we are in the same area. Could be you were in "real" stop and go traffic, I only had a few traffic lights after I pulled of the highway.
BUT, this being said, there are some common issues on these cars were they can get leaves and debris between the AC condensing coil and radiator causing them to run a bit warmer than most.
As for your lower temps, they should pull just up to 77-78C and then peak to 79C, at least may car does not stay pegged at 79C, it toggles between 78-79C. Note the difference between 78 and 79 C is only 1.8 F, so its a very minor change, factor in rounding, it would be in the 2.5 F area? Temps will rise on 50F days in traffic, but I would probably need to drive over 1 hour at highway temps for the coolant to stay at 79 or 80C?? This would be more to the oil temps creeping up more than anything.
Not sure how much you drive in to and from work, but on a day like today, around 50F, you probably need to drive at least 5-7+ miles before the oil is even close to getting near 79C. I find the engine temp cannot really stabilize until the oil temp is up close to 80C.
Usually the oil takes 2-3 times longer to getup to temp as compared to the coolant.
As for my top temps, this summer @ 106 F outside temps at 80 MPH (with AC on of course!), my oil temp was not even a needle beyond center (210 F) maybe 212 F?? and I believe the tops my coolant temp hit was 96C.
My simple test was wet/oily tail pipes and a really dirty rear bumper and license plate, tiny black suet on the rear of the car, if you have that, probably time for a stat change. Once the temps drop to 40 F or below, a questionable stat should no longer be questionable from my experience. At 30-40F you will have no question in your mind about a bad stat, it becomes far more obvious in these temps. Oil temps will also take a LONG time to get even to the first dot as I recall?
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