I installed a modified cap so the system would not pressurize. I drove the car a few miles. Under a heavy load, WOT 0 to 60 several times, the car's temp gauge went from 80 to 100 in a very short time. When I cruised, at no load, it quickly dropped back to 80 as fast as it rose. When I stopped, the coolant level was where it was when I started. I tested for combustion gases in the coolant tank and found none.

When pressurized, the coolant temp never got above 80.

So:
Still no gases detected.
The temp spikes under load when not under pressure.

When the system was under pressure, under a load the pressure would spike but the temp would remain steady.

__________________
Greg
2012 S350 BlueTEC 4Matic
2007 ML 320 CDI
2007 Leisure Travel Serenity
2006 Sprinter 432k
2005 E320 CDI
1998 SLK230 (teal)
1998 SLK230 (silver)
1996 E300D 99k, 30k on WVO
Previous:
1983 240D, on WVO
1982 300D, on WVO
1983 300CD, on WVO
1986 300SDL 237k, 25k on WVO (Deerslayer)
1991 350SDL 249k, 56k on WVO - Retired to a car spa in Phoenix
1983 380 SEC w/603 diesel, 8k on WVO
1996 E300D 351k, 177k on WVO