Is it safe driving with 100% grill blocking under 80 degrees outside?

DKTVAV - A FWIW FYI extracted from my "Ten Tips..." website:

"As an additional aid to accelerating engine compartment warm-up and keeping and running the ICE in the"Sweet Spot" temperature range that delivers Max-MPG-FE, some drivers operating in frigid Northern Temperate Zone environments (≤ 41°F/5°C Ambient Outside Air Temperatures [OAT]) partially block some of the space in part of the Prius front grille air intake louvers with cylindrical foam insulation like that used to insulate hot water pipes in your home.

NOTE OF CAUTION: Operating the ICE with the front air intake grille louvers partially or completely blocked off with foam insulation when Ambient Outside Air Temperatures (O.A.T.) exceed 41°F/5°C may result in harmful and damaging overheating of the ICE and propulsion components. Grille Louver Blocking should only be done when outside ambient temperatures will remain at or below 41°F/5°C throughout the day. Leaving engine compartment ventilation louver blocking material in the grille when O.A.T. temperatures rise above 41°F/5°C may result in damage to the engine, inverter and transmission.

People who venture to use grille louver blocking above 41°F/5°C usually have after-market ScanGauge II or similar add-on instrumentation installed so they can monitor the temperatures of the ICE and key propulsion components to keep them from overheating and being damaged."

 

This. Get a Scangauge, Ultragauge, ELM327 Bluetooth reader with Torque app for Android, or other device to keep an eye on the temps. I have the lower grille completely blocked on my Gen2 and its been fine up to about 70F on the freeway. It hasn't been any warmer since I've installed it. In all likelihood, the fans will probably kick on and be able to draw enough air to keep it cool, but its better to have enough flow already to keep it from having to use the fans too much.

 

If you have to ask (i.e. you don't have an OBD engine monitor to watch the temperature yourself), then it isn't safe.

I have left mine in (on a Liftback, not 'c') at that temperatures without ill effect, but wouldn't even dream of doing so without the ScanGauge. Just because it can work in gentle conditions doesn't mean it will be safe for long hill climbs or other conditions where the engine is working hard. And as the car ages and radiator fins get flattened and blocked, the safe ambient temperature for grill blocking may fall.

If you use the common pipe insulation stuffing method, it takes only a few seconds to yank out two or more strips. And a few seconds to re-install them later.

 

Always have a way to monitor the engine temp, as stated above. I am presently running a ScanGauge. Temp rises very little on warmer days. I would not consider blocking with no gauge.
Dan