2006 Prius runs poorly only in hot weather when first started

Maybe the HV fan needs cleaning. If the fan is clogged and the battery is hot then it won't be able to help with acceleration.

 

It might be a traction battery problem except I would expect a warning light or warning icon to appear.

Check the fuel line pressure. Maybe it is marginal and vapor lock is occurring in high ambient temps. It should be 43 psi or more at engine idle speed.

 

Parking in the sun is very hard on the HV battery. If parked with the battery showing all green bars, even worse, due to exothermic reactions in the NiMH battery.Being in NM, I would think this might be a problem.

Does the engine sound like it is faltering? Probably just revving higher than normal.

If the HV battery is actually too hot, the HV ECU will limit the amount of current in or out of the battery, and that will cause acceleration from a stop to be sluggish. It is surprising how badly the car can be degraded and still not show any error codes, based on various posts here.

Use the tonneau cover in the back to help keep the rear of the car cooler. Use reflective window shades. Use AC, and make sure the battery is down to blue bars before parking. And for sure check that the battery cooling fan is clean, but if the fan is audible from the start, then it is not the root of the problem. If there is any way to park in the shade, it is worth while in the long run.

 

I had a very similar problem for the first time this week. On Tuesday we hit 100 degrees in Richmond, VA. My car was acting normally until I went to drive it home around 7pm. It's a 2007 Prius, 102k miles. I started normally, but then hesitated whenever I gave it gas. Felt very similar to driving in a lower gear when it was hesitating. Did this for a few seconds and then I would get full power. I avoided the highway and took a slightly longer way home that was on 35-45 MPH roads. took about 15 minutes to get home and the problem persisted the whole way. Like you Johnhnhn, the fan to the right of the rear seat was running the entire way home (I assume this is the fan that keeps the HV battery cool. No warning lights came on. I turned the AC up to 80 in case it was an issue with the engine. It's been running fine ever since, but I've noticed that my HV battery doesn't hold a charge when parked, even for an hour or two. 90% of the parking I use is exposed to the hot sun, so that is interesting to know how bad that is on the HV battery. I use sunshades only in the front windshield in the summer.

I have an appointment at the Toyota dealer on Friday (earliest I could get), so if they find anything i'll update it here. I was already planning to purchase a new Prius in the near future, so i'll be looking at the 2014's while I'm there!

 

To the OP your letting the cabin get extremely hot and its cooking the Hybrid battery.

You don't have:

Window tint
Windshield heat shield shade
Pull out package cover in the back

With no tint the sun beats down directly on the rear hatch area and cooks the Hybrid battery. Your shortening its life. Greatly.

With no windshield heatshield shade the sun is cooking the entire cabin.

With no package pull out cover there's nothing blocking direct focus of the sun on the Hybrid battery area.

The fan coming on the second you get in the car tells it all. Look at the battery on the Energy screen. See how fast the charge level dissipates.

I made a insulator for the Hybrid area. I cut a piece of foam sheet the kind you see at Home Depot that goes in walls 4x8 sheet.
Then covered it in foil. Cut to fit over the pull out fabric package cover. Keeps the Hybrid battery area very cool.
I use the heatshield in the window religiously (bought here at PriusShop) Heatshield is the best shade out there. Its insulated and custom cut to fit in the Prius window.
And have the best and darkest tint money can buy. In 7 years only heard the Hybrid fan come on once when I left the car on top of a parking garage all day on cloudless day and forgot to put the shade in. Car was like 120 degree's inside. Fan ran like crazy the minute I started it and car ran like crap till the ac cooled the cabin down.

 

Just blast the AC, not 80 degrees, like 70-75 degrees. Don't worry about the few cents you will save on gas. I thought everyone knew this. My HV battery has gone down to the teens--got stuck in a car wash on a hot, humid SoCal day. And I am still on my original HV battery. Car has 270k on it. I am a pro when driving on hot days. I often go to Vegas during the summer and have to climb a 17-20 mile hill in 110F plus. The key to not draining the HV battery is to keep it cool. Also, don't drive too fast up the hill. Again, don't worry about saving gas by not using the AC. Your HV battery will never last 150k with that style of driving. Peace.

 

Yep been preaching that for years. The insulator board I made for the back package rack really helps haven't heard the fan come on since.
I keep the car very cold and battery is like new 7 years later. But.....my car has never seen a hill.

Hill work is very hard on the mg's as you know.

 

I went to Toyota today to have them check this out today (I posted reply a few days ago) and they found nothing wrong. I too don't have window tint, don't use the cover in the back, and don't have a heat shield shade (although I do use a generic folding shade that fits decently). I will definitely start using the back cover and will look into making the insulator edthefox5 talks about. I'm looking into buying a new Prius, just can't decide on 2014 or 2015 model. Will wait to buy the heat shield for that version.

I'm curious, those of you with dark tint, did you get the factory to do it or was it done locally? I'm hesitant to do it locally...it used to be that they didn't last. I still see bubbly blue tints out there! . But maybe the technology has changed. I do like the tinted look on a Prius!

 

Locally. Tint has really changed in the last 20 years. Much higher quality. Ask around at work where people got there's done.
Try to get the back seat windows and the hatch the darkest they allow.

Expect to pay $200 for a good tint job. Without tint the car in the sun is a pizza oven.

And huge thermal difference between cheap windshield shade and a Heatshield. Heat shield is custom cut and is very insulated. They also make a hatch shield too:

Heatshield: Sun Shades for Cars, Trucks, and other Automobiles

I had my tint done 7 years ago and now just starting to notice tiny little bubbles in just the drivers window. Probably get that re-done and they didn't put that strip along the top of the windshield low enough. May have them put a bigger strip in there also.

 

Your car is probably just less efficient when it's hot. All the electrical wires are hot and is not working to full potential. It's like if your computer or phone gets hot and you start losing speed from the Internet. I think I notice less power when I'm driving in 90 degree heat

Since your car can go to 80mph, I don't think there's a fuel issue.

 

I still think you should have the fuel line pressure checked as suggested in #3 above, when the power train is cold. If the pressure is low, the issue could be the fuel pump (within the fuel tank) or the Circuit Opening relay (which is part of the Unit C relay housing that contains four relays, within the main relay/fuse box.)

You also might try cleaning the throttle body interior and throttle plate to make sure the throttle plate is not binding.

This is not normal behavior, my 2004 with 178K miles bakes all day in the summer sun, high temps approaching 110 degrees F, and the car has immediate acceleration response when I am ready to go.