Does grill block reduce engine oxygen intake?

[2009Prius]

It's getting cooler so I just blocked the upper grill for the first time. I noticed the fuel economy of the first 5 minutes seems have dropped from before the grill block. (Correction: This may be just noise.) My question is that whether the grill block would reduce the oxygen going into the engine, especially at low speeds.

[jayman]

Odd how blocking the grille reduced your fuel economy. With my 2004 Prius, it generally helped, especially in temps <0 F

Like most modern cars, the Prius doesn't draw any intake air from the grille area, but from the fender

It could be a coincidence, but remove the grille block and see if that returns fuel economy to normal. There could also be a mouse nest in the intake piping somewhere, and be sure to check the air filter housing by removing the air filter

[efusco]

How many times on your regular route have you looked at this? It really doesn't make sense. I think for most of us, except in the coldest weather, the grill block has little or no impact during the first 5 minutes. It does seem to improve engine warm-up time, but most importantly it keeps it from cooling down and allows it to stay warm and efficient.

[Rangerdavid]

doc is correct. It should not reduce oxygen supply to the motor, but it should assist with engine warm up time. It keeps the cold air from blowing directly in on the engine compartment. I seriously doubt that is what is affecting your first 5 min. bar. Did you block upper and lower grill? I have blocked mine ever since I purchased the Prius, and have never had any trouble whatsoever.

[JimN]

Originally Posted by 2009Prius

It's getting cooler so I just blocked the upper grill for the first time. I noticed the fuel economy of the first 5 minutes seems have dropped from before the grill block. My question is that whether the grill block would reduce the oxygen going into the engine, especially at low speeds.

IMO the blocked grill isn't the cause. You said it's colder now. That's the cause. In moderate temps I don't see much of a difference & expect 25mpg for the 1st 5 minutes & 50mpg for the next 5. Even with the grill blocked some winter mornings I wouldn't be surprised to see minus mpg. After the 1st 10 minutes you should see some improvement & green bars more often. Over the winter you may find that the lifetime average was maintained instead of decreased but this is very dependant on the OAT.

[David Beale]

Wow, where is it colder now? Here in the "great white north" we're having a hot "Indian Summer". It's supposed to get up to 30C today (about 80F), about as hot as it gets even in summer!
Grill block is the furthest thing from -my- mind.

But yes, as the above posters have said, the upper grill block will not decrease the engine's air supply, nor will the lower grill block. The engine breaths from just in front of the passenger front wheel. And I've also never seen a decrease in fuel economy from grill blocking. It -should- improve it slightly as it would lower drag a bit. But probably not enough to see.

[patsparks]

I tried an upper grill block for the first time and had no improvement in fuel consumption over earlier in the winter. I removed the grill block and my consumption improved by 0.1L/100km. I don't know why.

[2009Prius]

Thanks to everyone for sharing your knowledge and experience. The "reduced first 5 minute MPG" may be just noise. Sorry about that. I will edit the original post.

[jayman]

Originally Posted by David Beale

It's supposed to get up to 30C today (about 80F), about as hot as it gets even in summer!
Grill block is the furthest thing from -my- mind.

Unusually hot here too. This time of the year, I'd have the A/C turned off and covered up. We're getting our July weather a bit late, but at least we're getting it

Originally Posted by patsparks

I removed the grill block and my consumption improved by 0.1L/100km. I don't know why.

That's odd. I would usually start grille blocking once the temps were around -10 C consistently. I do know that at -30 C and colder, the grille block makes a big difference in comfort

[tom1l21]

I have all top grills blocked and 3/5 from top to bottom of the bottom grill blocked. It is about 40F in the morning and gets up to about 70F highest in afternoon. I drive about 45-60 mins on rolling hills at around 40mph. I don't have any temperature monitor, is this going to get my car too hot or can I go ahead and block another bottom grill?