I have an 02 911T and live in Vegas. In the summer at 115 degrees at full throttle my boost gauge reads up to 0.8bar (about 12psi) boost. Recently in colder weather (45 degrees) I was only getting a max of 0.6 to 0.7bar (about 7-10psi) boost.

My understanding is that with the higher air temp comes two things. 1. Thinner air and 2. higher induction temps - both which require either (or a combination of):
more fuel,
less advance
less boost
to stop the engine from detonating/pinging.

Further, my understanding was that with colder temps (denser air and lower intake temps) you could drive MORE boost with the same fuel and not have the engine ping.

So my question: why does the boost gauge show LOWER boost in cold temps, rather than higher boost - which is what I expected ....... ?

 

In theory you can get more boost in cold air. Sounds like you have a vacuum leak somwhere, or a bad diverter valve.

 

"Diverter Valve" - ok guess you mean "wastegate controller" ?

Anyway did a little more testing and I noticed a weird thing. Even after driving for like 20mins - the temp gauge was under "180" instead of just over as per "normal". Obviously this would effect things. So not sure why the engine wasn't warmed up yet (or if that is normal in cold weather).

So I ensured the temp was over 180 this time, and then hit the gas, and it went up to 0.7. Bottom line not a big deal. But I did notice this: I got on the gas and built some speed, after I eased off and got back down to a normal speed, my temp gauge went back under 180 again?

Is this normal or do I have a problem in the cooling system - maybe a bad 'thermostat' (or the equivalent in these cars)....?

 

Nope, i meant diverter valve....as in one of the two sitting on top of your engine. The stock units have a rubber diaphragm in them that will rip over time and not operate correctly. Most replace them with billet units that have a billet plunger and do not go bad as easy. Temp will fluctuate depending on driving....a small amount. When you say below the 180 mark, how far below the 180 mark? And how far over the 180 mark? It is possible you have an air bubble in your cooling system, have you had any work done on it recently?

 

My guess would be, that you get larger effect in cold weather, so the engine dont have to produce higher boost to get more power, unlike in warm weather.