Originally Posted By: Quattro Pete
Originally Posted By: armos
If it's sweeping significantly in just a few seconds, then it's not normal.
What would you consider "significant"?
The reason I ask is that my coolant temp normally stays at about 97C, but if I floor it, it'll drop to about 80C within 20 seconds or less. Then it'll slowly get back up to 97C again after I'm done flooring it. I thought this was normal behavior.
By the way, the actual gauge does not budge while this is happening. It has a dead range in the center from 75C to about 115C.
I was thinking more along the lines of moving say 25-50F in like 3-5 seconds, or even in a sudden twitch. Whenever I've seen that there was a problem causing it.
20 seconds sounds more reasonable, and it doesn't sound like you're seeing hot spots, just a shift from normal temps to temporary overcooling.
If I did the math correctly yours is a drop from 206F -> 176F. I assume the thermostat threshold is in the 190s somewhere. Since it doesn't have high temperature excursions it doesn't sound like a problem. If the flow was poor or intermittent, you should see it get much hotter in the head (gauge reading) and then suddenly cool off when the coolant starts flowing again.
It sounds like it's having no problem staying at ~205F, so efficiency doesn't seem to be in doubt. The only thing strange is that it seems to overcool a bit at WOT.
If it's been consistent for a long time, the range of temperatures isn't widening, then maybe it is normal. I haven't noticed that happen on cars I've owned, but maybe I just wasn't watching for it under the right circumstances.
With full throttle you'll get into high RPM, and perhaps that turns the water pump fast enough to provide a lot of "excess" cooling while the thermostat is taking some time to close, so you end up well below the thermostat threshold before it closes. I've never really noticed that happen but it makes some sense.
It implies there was a differential of over 30F between the coolant in the cylinder head and the radiator, even though the thermostat was open (as it would be at 205F). But at low RPM maybe that's not unusual.
If this happens after accelerating from a stop, then the change in airflow over the radiator is also a factor, since the fan probably isn't on at 205. Between that and the high RPMs it might just be getting a surge of cooling before the thermostat can react.
Maybe the thermostat sticks open, but maybe it's nothing at all.
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I knew many cars have dumbed down oil pressure gauges, but I didn't know they were doing it with coolant gauges. I have to say I find that outrageous. The owner needs to know if the system is losing it's ability to stay in the regulated temperature range. That's the warning sign that something needs attention before it starts to seriously overheat later.
115C is 239F - I'd want to be aware of the behavior before it got to that point.
I believe that cooling malfunction is one of the most common ways older cars get ruined. So for manufacturers to deliberately reduce people's awareness of the temperature, definitely annoys me.
