These multifuels typically run cooler on idle compared to normal vehicles, the coolant system is quiet effective to say the least, lol. No bubbles, that's good and as long as the coolant level is full, check it after a few drives to make sure it's still full, if so, you're good. Hopefully that new temp gauge gets you squared away on that issue. Out of all the gauges, imo, the temp and air gauge(s) are the important ones. I can do without a speedo/rpm, generator and fuel, but air does the brakes and in some cases, steering, and we know that's a needed function. The temp gauge is the only sure way to know if something is really wrong without physical proof like steam, smoke, ect. It's also more accurate in the sense that if those generator/water pump belts come off or break, you'll know as the temp begins to get into unfamiliar territory. My belts have shredded in the fan on me three times and never heard it happen once, but the temp gauge was always the clear indicator as it was above normal. A quick inspection under the hood proved the gauge was not faulting out, the belts were now shredded pieces of rubber.