Low Engine Temp at High Speed

[Fixxxer]

The other day I got my 2004 996 Targa up to ~120 mph at a track day event. After my first run, I glanced down at the coolant temp gauge, and I noticed that it had dropped to ~140 (it's normally around 185 when warmed up) and was slowly coming back up to normal. On my next run it happened again, but only after I was over ~100 mph. When it happens the needle quickly drops to 140 - within 5 to 10 seconds. On one hand, I'm thinking that the volume of air at that speed is cooling the coolant down very quickly. But that's an awful big drop in temp... I'm also wondering if I have a thermostat that's stuck open. However, the motor doesn't take a long time to warm up in the mornings - maybe 5 minutes at normal city speeds (40 - 60 mph), and it stays constant at 185 in everyday driving conditions. Has anyone else experienced this? Is it normal?

Thanks!

-Lee

[JFP in PA]

I would not describe that as "normal". Usually, it takes more power to go faster, which means the engine is pumping out more heat. Even with the increased air flow at speed, both the coolant and the oil tend to get hotter with sustained speed on the track. It is also interesting that the temperature drop is rather sudden when it occurs, which is also counter intuitive. I run a 160 degree stat in my cars, and they do not get cooler the faster they go on the track.

So then the question becomes why is it happening? Your thermostat sounds like it is functioning normally. It may be the temp sensor itself, or perhaps a voltage fluctuation causing the gauge to read this way. You might want to try running a data log on the car while at the track to collect the actual system voltage, oil, and coolant temps at the sensors to see if you get different values more inline with expectations.

[logray]

The coolant temp sensor is also an inexpensive part and fairly easy to replace (aside from having to drain block of coolant and refill):

99660641000 about $25

[Fixxxer]

Thanks for the replies so far, guys. I was able to pull some data from Torque that shows this happening. I only made it up to 111 mph on this quick run, so the temp only dropped from 179 F to 150 F, but it's still noticeable in the data. Attached you'll find a screenshot of the raw data, a graph, and a ZIP file containing the whole spreadsheet of data. In the graph, the green line is coolant temp; the blue line is GPS speed; and the tall, flat purple line in the center is throttle position.

There are no unusual spikes (+/-) in the coolant temp's data (as seen in the overall zoomed-out view at the bottom of the graph), other than the one discussed here at high speed, so I find it hard to believe that it's a temperature sensor issue. Could it be the water pump, or maybe air in the coolant system?

Thanks again!

-Lee

2013-05-14 2004 911 Targa Torque Log.zip

[JFP in PA]

While I have no experience with "Torque", the data is mystifying, the engine simply should not do this. That said, I note that all your coolant temps seem quite low for a car equipped with the OEM stat, which does not even begin to open until 186F, and usually is not fully open until around 205-210F. What kinds of temps do you get with this software during normal street driving? If they are as low ( or lower), there is something wrong with the thermostat.

[wvicary]

I would suspect the thermostat, it could be bypassing coolant and the water pump is a positive displacement type so the increased volume at rpm might explain what's going on plus the high speed run isn't sustained for very long.