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Discussion Starter #1
I have noticed a significant difference in how my new 2010 HL SE AWD behaves driving downhill at highway speeds as compared to my Avalon.

With my Avalon when I crest a hill doing, say 65 mph, I can let up on the gas and the transmission will do whatever it does to allow gravity to take over and the Avalon will speed up perhaps reaching 80 mph if it's a good size hill.

What I notice with my HL is that the transmission seems to stay engaged and seem to use "engine braking" to hold my speed to the cruise control set point. I have not tried it without cruise engaged.

I did shift into neutral on one hill and the HL did speed up, taking advantage of gravity on the hill.

  • Is this a cruise control issue on the HL?
  • Is this an AWD "feature" on the HL?
  • Anyone else notice this?
  • Any thoughts on shifting into neutral on the downhills?
Thanks
Ken
 

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Perhaps summerwind can dig into his bag of documents and come up with one that describes the HL's engine braking, but in lieu of that I remember reading in the past that that's exactly what the HL does when in cruise control mode. I've noticed the same thing you mentioned, that if I drive down a steep hill with cruise control engaged (for example the Cajon Pass in California) and a starting speed at 70 MPH my HL will maintain 70 MPH, but if at the same speed CC is not engaged my speed goes up to 80 MPH every time. I wouldn't shift into Neutral because I don't think it's necessary, that's something for hypermilers. I don't look on it as any kind of problem, just a very effective CC. My Avalon always speeds up when going downhill and CC engaged, and I hate it.
 

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Discussion Starter #3
Thanks TD!

So you see the same as me on both vehicles.

I kind of like what the Avalon does as it allows me to take advantage of the hill and improve my mileage some. Since I am never starting out as the fastest guy on the road anyway, allowing for some acceleration on the downslope is good for me.

I'll try it the next time out with the HL and I'll just pop off the cruise and see what happens.

Have the HL for a bit over two weeks and I love it!

Ken
 

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I bet it's just that Toyota decided how much engine braking it should use based on the targeted use of the vehicle. While your Avalon is set to run like a lot of the american made vehicles where they "coast" (it is more of a highway cruising car), the HL seems to have a little engine braking but nowhere near the Tacoma! Now if you drive a Tacoma, you'll really feel it. The Tacoma cuts ALL fuel off from the engine when you let up on the gas pedal, so it uses full engine braking (and of course, if you're at the right speed and hit the brakes, it then downshifts with the fuel cut off and adds even more engine braking). Maybe the HL cuts off all fuel too but just has a lower compression ratio? Not sure, but my bet is on the design being based on the targeted use of the vehicle.
 

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Isn't the whole point of cruise control to control the speed you are going whether it is up hill or down?

You said you let off the gas in your Avalon, which tells me you weren't using cruise control. Yet you say you have not tried it in your HL without cruise control yet. Comparing apples to oranges? I'm confused.
 

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Thanks TD!

So you see the same as me on both vehicles.

I kind of like what the Avalon does as it allows me to take advantage of the hill and improve my mileage some. Since I am never starting out as the fastest guy on the road anyway, allowing for some acceleration on the downslope is good for me.

I'll try it the next time out with the HL and I'll just pop off the cruise and see what happens.

Have the HL for a bit over two weeks and I love it!

Ken
Yup, I see exactly the same thing. Oh, and for reference I own a 2008 Base Highlander and a 2002 Avalon XL. My Jimi Hendrix sig for September has taken place of my usual sig that lists my vehicles....:D
 

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One of the features of the fuel injection logic is below

SFI System
The system uses a fuel cutoff in which the injection of fuel may be stopped temporarily in accordance with the driving conditions.
If you are going down hill the injectors most likely are cycling on and off creating a engine braking effect.

A description of how the system works can be found here.


SFI System

The right up is focused on answering the why does the engine sound like a diesel when cold but also mentions the above quote.
 
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Discussion Starter #8
Isn't the whole point of cruise control to control the speed you are going whether it is up hill or down?

You said you let off the gas in your Avalon, which tells me you weren't using cruise control. Yet you say you have not tried it in your HL without cruise control yet. Comparing apples to oranges? I'm confused.
Right, this is a bit of apples and oranges. But to clarify, the Avy will increase speed down hill even with cruise engaged. The HL seems not to when cruise is engaged. I will try the HL w/o cruise next time I am on a big hill.

Ken
 

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Right, this is a bit of apples and oranges. But to clarify, the Avy will increase speed down hill even with cruise engaged. The HL seems not to when cruise is engaged. I will try the HL w/o cruise next time I am on a big hill.

Ken
I would assume then that when the CC is engaged the computer is "driving" the HL, so it uses the fuel shutoff that summerwind quoted in order to act as a "Jake Brake" that big rig trucks use downhill. If CC is not engaged then fuel is still being injected into the engine. So it seems you have a choice of sorts: use CC downhill to maximize mileage at the cost of slower downhill speed, or disengage it and go faster with a little more fuel consumption. I use both depending on the circumstances running downhill. On the Cajon Pass I don't use CC because running at 80 MPH keeps me with the flow of traffic....:D
 

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Discussion Starter #10
I guess the thing to watch is the rpm of the engine while going downhill. My sense is that the Avalon goes into "coasting" mode: the engine RPM drops off while the downhill speed increases. The HL doesn't seem to behave that way. I'll have to pay close attention to the tach and the gas mileage display in the HL in both modes and see what's going on. The Avy does not have a gas mileage display so I'll just see what the tach does.

I'll report back.

Ken
 

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And the Avalon is not AWD with a constant 50/50 torque split.
You have a transfer case and rear differential and lower gear ratios that the Avalon doesn't have and the Avalon has a six speed tranny versus 5 speed tranny in the HL.

Guess I am still scratching my head why you think the HL and the Avalon should have the same down hill characteristics.:confused:
 

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I guess the thing to watch is the rpm of the engine while going downhill. My sense is that the Avalon goes into "coasting" mode: the engine RPM drops off while the downhill speed increases. The HL doesn't seem to behave that way. I'll have to pay close attention to the tach and the gas mileage display in the HL in both modes and see what's going on. The Avy does not have a gas mileage display so I'll just see what the tach does.

I'll report back.

Ken
My observation of what you're describing is this. When I go downhill with CC off my RPMs remain the same as they would at any speed, so what I'm saying is downhill at 70 MPH and my RPMs are about 2,250. But when CC is on one would expect that as the computer stops the flow of fuel and uses compression to slow the vehicle that the RPMs would increase, but they don't. :confused: My observations have been that at 70 MPH and CC engaged RPMs actually drop to about 1,750. That seems counterintuitive and has always baffled me, but I've always left it at that since ultimately all I care about is that it works properly, if you know what I mean? Having CC that maintains speed downhill is a blessing in areas where speed traps are set up at the bottom of a hill, and many times I've watched people shoot by me and wind up with a ticket. :D
 

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Discussion Starter #13
And the Avalon is not AWD with a constant 50/50 torque split.
You have a transfer case and rear differential and lower gear ratios that the Avalon doesn't have and the Avalon has a six speed tranny versus 5 speed tranny in the HL.

Guess I am still scratching my head why you think the HL and the Avalon should have the same down hill characteristics.:confused:
Summerwind,

Don't get me wrong, I am not trying to say that they should work the same. Since this HL is my first AWD vehicle I am just trying to come to grips with the different way things behave and gain an understanding.

One aspect of driving the AWD I am trying to understand is methods to maximize the fuel economy. When I felt the cruise control "engine braking" the HL on the downhill, I assumed that was bad for gas mileage. But the descriptions shared is that the fuel injectors "cut out" to some degree and this might actually be better for mileage! This was not intuitive to me.

I will check the mileage display next time I am going down a big hill both with and without cruise engaged. Assuming the mileage display does not go off scale this might be indicative.

Ken
 

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My observation of what you're describing is this. When I go downhill with CC off my RPMs remain the same as they would at any speed, so what I'm saying is downhill at 70 MPH and my RPMs are about 2,250. But when CC is on one would expect that as the computer stops the flow of fuel and uses compression to slow the vehicle that the RPMs would increase, but they don't. :confused: My observations have been that at 70 MPH and CC engaged RPMs actually drop to about 1,750. That seems counterintuitive and has always baffled me, but I've always left it at that since ultimately all I care about is that it works properly, if you know what I mean? Having CC that maintains speed downhill is a blessing in areas where speed traps are set up at the bottom of a hill, and many times I've watched people shoot by me and wind up with a ticket. :D
I think you guys are looking at the Tach's impact in the wrong way. Like I said, it's really obvious how it works when driving the Tacoma. If the vehicle is supposed to use engine braking as a feature (rather than coasting and requiring you to always use the brake pedal), then the RPMs should not change when you let off the gas. Engine braking will keep the vehicle in gear which causes the engine to turn at the same rate at any given speed regardless of whether or not you're stepping on the gas pedal (think of it as a manual tranny where you have it in gear and let off the gas). So when the flow of fuel is cutoff, the vehicle isn't coasting but is stuck in gear to use the engine compression to slow the vehicle. Again, when in a specific gear the engine has a specific RPM it MUST turn at for every given speed.

With the Avalon, it's designed to coast and not use engine braking. Therefore, when you let off the gas it essentially puts it in neutral (or something similar but this is a good way to provide an example) which lets the RPMs drop off while the speed remains the same. My suspicion is that engaging CC changes how engine braking is used (since it's all electronic anyway).

See the difference?
 

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One aspect of driving the AWD I am trying to understand is methods to maximize the fuel economy. When I felt the cruise control "engine braking" the HL on the downhill, I assumed that was bad for gas mileage. But the descriptions shared is that the fuel injectors "cut out" to some degree and this might actually be better for mileage! This was not intuitive to me.
Whether it's good or bad is a point of view. You could say you're saving fuel because you're not using any when going downhill. But then the reverse could be said if you're doing a lot of uphill/downhill driving. By cutting off the fuel and using engine braking, you're not building up speed to coast further and get further up the next hill before using more gas. But overall it's a good feature. IMO, it does save fuel and it saves money with regard to making your brake pads last longer because you're not having to use them on every downhill.
 

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The braking effect has nothing to do with AWD or FWD, as other recent Toyota owners of FWD cars have noticed the same and I notice the braking effect with my 2004 FWD Highlander. I believe the torque converter lockup, when engaged, will stay engaged until reaching a lower speed of 20-25 mph, at which point it unlocks and the car will coast better. The service manual of my previous 1989 Camry stated that the torque converter would unlock whenever the gas pedal is released, and I think newer Toyotas are set so the converter unlocks only at lower speeds and not on gas pedal being in the rest position.
 
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