Outside Temp Rises Quickly?
Thread Starter
Super Member
Joined: Jun 2015
Posts: 643
Likes: 92
From: Houston, TX
'15 C63S
Outside Temp Rises Quickly?
I'll preface this by saying I understand how a thermistor functions and have a pretty good understanding of heat transfer (BSME here).
As I stated in another topic I started yesterday, I've been trying to quickly get my OBD Monitors to switch to Ready so I can get my state inspection done. I was outside yesterday morning trying one of the many driving cycles I've looked up online. This one begins with letting the car cool completely overnight, starting it up, and then idling it with the trans in Drive for a few minutes. Outside temperature (not the gauge reading, but actual temperature) was 78F-79F degrees when I did this.
As the car is idling, the outside temp in the dash showed 80F degrees, but partway through the test the outside temp began to slowly creep up every few seconds. It topped out at 91F before I drove away and then eventually came back down to 79F degrees after half a mile or so of driving.
Didn't think much of it seeing as it could easily be reading trapped heat from the engine bay that had nowhere to go until the car starting moving. No big deal. It just heightened my attention a bit that it rose so quickly from a cold start with only an idling engine.
Fast forward later in the day. It's easily 96F or 97F degrees outside, engine is completely warmed up, and while driving on the freeway the outside temp gauge is reading accurately. I have to slow down for construction in the area, slowing from 75-80 mph to a steady 15-20 mph. In the span of maybe 30 seconds, the temp gauge had jumped up to 104F. Again, the temperature eventually came back down once I got back up to speed.
All this can easily be explained as normal, but I was just wondering if this mimics what others are seeing with their cars. I'm one of the unfortunate people that has debris take out the front radiator and had to have it replaced. Because of that, the little voice in the back of my head is now paranoid that something wasn't installed correctly and now is not cooling properly. I've noticed a few things that didn't seem correct (one being my trans temp seems to spike a lot easier than it did before), but my mechanic has looked it over and said everything looked to be working properly.
As I stated in another topic I started yesterday, I've been trying to quickly get my OBD Monitors to switch to Ready so I can get my state inspection done. I was outside yesterday morning trying one of the many driving cycles I've looked up online. This one begins with letting the car cool completely overnight, starting it up, and then idling it with the trans in Drive for a few minutes. Outside temperature (not the gauge reading, but actual temperature) was 78F-79F degrees when I did this.
As the car is idling, the outside temp in the dash showed 80F degrees, but partway through the test the outside temp began to slowly creep up every few seconds. It topped out at 91F before I drove away and then eventually came back down to 79F degrees after half a mile or so of driving.
Didn't think much of it seeing as it could easily be reading trapped heat from the engine bay that had nowhere to go until the car starting moving. No big deal. It just heightened my attention a bit that it rose so quickly from a cold start with only an idling engine.
Fast forward later in the day. It's easily 96F or 97F degrees outside, engine is completely warmed up, and while driving on the freeway the outside temp gauge is reading accurately. I have to slow down for construction in the area, slowing from 75-80 mph to a steady 15-20 mph. In the span of maybe 30 seconds, the temp gauge had jumped up to 104F. Again, the temperature eventually came back down once I got back up to speed.
All this can easily be explained as normal, but I was just wondering if this mimics what others are seeing with their cars. I'm one of the unfortunate people that has debris take out the front radiator and had to have it replaced. Because of that, the little voice in the back of my head is now paranoid that something wasn't installed correctly and now is not cooling properly. I've noticed a few things that didn't seem correct (one being my trans temp seems to spike a lot easier than it did before), but my mechanic has looked it over and said everything looked to be working properly.
MBWorld Fanatic!
Joined: Jun 2014
Posts: 1,589
Likes: 683
From: IL
19 E63s, 23 M3 compX, B9 SQ5
I believe it's because of the location of the thermostat, when you're not moving ie engine idling or at a stop light, the outside ambient temps due to engine/exhaust heat will cause the readout to rise. Once you're moving it's measuring accurately outside temps not affected by engine/drive train heat.
Thread Starter
Super Member
Joined: Jun 2015
Posts: 643
Likes: 92
From: Houston, TX
'15 C63S
I'm sure that had an effect when I noticed it on the highway, but the first instance I mention happened maybe 15 minutes after the sunrise while parked in my driveway. Doubt there was any radiant heat to affect anything.
