'93 Civic Cdn CX with an original computer (P08) and a non-original engine (D16Z6 - the D15 engine died a while ago) with a new radiator (installed with the Z6 engine a little over a year ago) - this problem showed up only in the past month.
Engine comes up to just-below-midpoint temperature reading normally. After long drives (i.e. engine warmed up) when left idling (red traffic light, stuck in traffic, etc) temperature reading starts climbing. I've seen it go almost to the red line. If the engine RPMs are brought up the temperature will come back to normal - either through normal driving, or by putting the car in neutral (with brakes on, of course) and gently touching the gas.
Any sort of engine load (RPMs up) and the temperature regulates properly.
Coolant level looks OK - there's fluid in the overflow tank and the inside rubber hose is 'wet'. Color is green, looks clear.
When the engine is warm, the 'low idle' seems to be quite low - sometimes the engine seems to want to stall. I wonder if this very low idle mode isn't conducive to proper coolant flow. Any other theories? Water pump, maybe?
Second question: my mechanic is fairly insistent that older Honda engines (> 200000 km) tend to start consuming oil, and that this isn't a big concern. Agree/disagree, anyone?
I'm about 1500 km into an oil change and noticed that the dipstick level has come down from 'full' to halfway between the dots. I also see some oil under the engine on the driver's side where the plastic of the bumper comes in under the engine compartment, so I suspect a slow leak more than actual oil burning (tailpipe doesn't show any signs of oil burning).