Poor Fuel Economy After Brake Change

I took this video yesterday. If you look carefully, you can find that you can cruise without a big throttle, and the speed will not drop too fast when you don’t step on the throttle.
My tire pressure is 40p and 35psi. It is a 215-45/r17 tire. The resistance may be higher than that of a 195 65 r15 tire.

 

In the past, this car did not travel very lightly. Accelerating requires a lot of throttle, which will basically exceed the power area, and most of the throttle during cruising will exceed half. At that time, the fuel consumption was about 22km/l (mpg=22*2.35).
I did 4-wheel alignment and the effect is obvious, especially when driving at a slow speed is good, with little resistance, and when there is a little regeneration on the downhill, it will also feel like it accelerates quickly and can glide for a long distance. The toe of my front wheel is about 0, and the rear wheel is adjusted too, with spacers, there is a little negative toe, about 2mm, because the rear wheel has a little negative inclination. This adjustment does not need to change low-resistance tires, and there is almost no eccentric wear of the tires, and the tires are very durable.

 

Thanks for this, I've just tried it on my 2010 and no matter how hard I spin, both rear wheels only rotate 1, maybe 1.5 times at most. And there's a faint metal scraping noise that sounds like pad on rotor.

My rear pads where replaced a couple of days ago and I am having terrible economy (~15km/L). My old 05 doing the same driving was getting ~20km/L like clockwork.

The problem is, I only bought the 2010 a few days before getting the pads done (the were due, according to the most recent service check) and I didn't really test out the economy beforehand. I also had 2 new front tyres and an alignment done at the same time.

Any advice for what I ask/tell my mechanic other than "i think you did my brakes wrong"?

TIA

edit: just to add, as another user so eloquently put it, it feels like there's a 2T anchor in my boot when city driving. Highway not so much, but harder to tell.

edit edit: not sure if this means anything, but just took it for a 10 minute drive, came home and the front disks are hot (3 or 4 seconds before can't touch any longer), but the rears are both stone cold by comparison.

 

What's your level of comfort with doing things yourself? Are you comfortable using a jack and jack stands, changing a wheel? Do you have metric socket wrenches?

There are some simple things that can cause rear brake dragging. But, as with so many things in the modern world, if you want some work to be done with actual attention to finding what the cause is and fixing it, doing it yourself is sometimes the most dependable way.

 

Yeah I'm comfortable with all those things, I've got a couple sets of jacks, stands and chocks etc...

 

There's a link in my signature on the rear brake brake issue. It may be misrotated rear caliper pistons, or not. Puzzling that the rear brakes are cold and front hot.

 

Once the wheel is off, you can remove the two slide-pin bolts and slip the caliper off the pads and set it out of the way. (It won't go very far out of the way, because it has both a brake hose and a parking brake cable attached to it, but it can hang out happily behind the caliper bracket there, without putting much stress on the hose).

What do the pads do? There are a couple of V-shaped springs above them, and the pads ought to spread out a little bit, away from the rotor, because of those springs. To slip the caliper back on, you ought to need to use one hand to hold the pads pressed together against the rotor so the caliper fits over them again.

If the pads don't slide in and out that easily, try replacing the "fitting kit"—that's just the springy clips in the mounting bracket where the 'ears' of the pads go. Those clips don't take grease (as exposed as they are, it would only collect road grit). They have a slippery Teflon-like dry coating when they are new. It wears off. A set is cheap, like $16 at the dealer IIRC.

You can also verify that both slide pins do indeed slide. They are greased inside, and the grease can get peanutbuttery after sitting there long enough. It wakes right back up if you just slide the pins in and out a couple times. Many people reflexively pull them out and regrease them whether they need it or not; I only do if they need it, and I think only once in my 14 years playing with Prii have I ever encountered any that needed it. But push/pulling them a few times to wake the grease back up should be a standard practice.

By the way, there is a brake system self-test that always happens, 90 seconds after the car shuts off. You should make a point of learning what that sounds like: go sit in the car with it on for a bit, turn it off, sit there for 90 seconds and listen for the little set of clicks and squish noises happening then.

You do not want that happening while you have the caliper apart (piston will push out, a mess). Some people will always disconnect the 12 volt battery before any brake work, out of an abundance of caution (then you have to reprogram all your radio presets and stuff). I usually don't, but always, always make sure that self-test has already happened, before starting to take calipers off. (And I avoid opening the driver's door, which wakes the brake system up.)

 

Thanks both, I'll report back in a few days hopefully.

 

Ok so I took it back to my mechanic (have been sick and not in the mood for jacking cars up in my freezing cold garage) and he found a stuck caliper that he has resolved, and re-aligned a couple of things back into spec. I'll drive it for a few days to see if that's improved things, if not will personally check for misrotated pistons.

 

If you were to describe what you mean by "it feels like there's a 2T anchor in my boot when city driving... Highway not so much, but harder to tell " is that your 2010's combustion engine revs up much higher and faster instead of using EV mode at city speeds with the same amount of gas pedal input when compared to your 05... causing the low Km/L of gas used?

But then at Highway speeds... Engine behavior seems normal as well as Km/L of gas used?

 

Yes to the first part (higher revs, less/lower EV capability/power than normal.

My highway comment wasn't worded well sorry. It's harder to "sense" it on the highway as there's less go/stop/left/right action happening. It certainly strains the engine more than it should at highway cruising speeds, and the fuel consumption is still horrendous (as of prior to getting the caliper fixed this morning, I haven't re-tested it yet).

 

If your Km/L does not improve... report back. You can read my linked post earlier in this thread as a possible ulterior reason for why you're getting Low MPG as it relates to how the Prius ECU's dictate EV Mode usage in city traffic. I'll explain more if your Km/L doesn't improve.

 

Thank you, I actually stayed up well past my bedtime last weekend reading through your posts. Fascinating and frustrating stuff, hope you get to the bottom of it. I wish Toyota would be more transparent with this stuff.

Anyhoo, whilst not conclusive, I just drove it 7km up the (flat) motorway then back home through (hilly) suburban streets, hitting 22km/l on the end of the motorway leg then 18km/l when back at home. I'd feel more comfortable after a couple hundred more KM's of this but for now is at least head and shoulders better than prior to getting the brake caliper sorted (was getting about 15km/l and 10km/l respectively on the same trip).

 

Before I pop the caliper off, just confirming that I do not need to compress the piston if I'm not actually replacing the pad? Like it doesn't pop out or anything when released?

 

The caliper pistons are quite stable when caliper is pulled off the rotors. The one precaution I take, is to disconnect the 12 volt neg cable (and isolate it): if you don’t, and open the drivers door, there’s a good chance the car will “wake up” and pressurize the hydraulic system, which will likely pop out a front caliper. The rear caliper pistons are safer, adjust by screwing in/out. But yeah, disconnect 12 volt before doing anything, and when everything is reassembled, pump brake pedal several times to take up any looseness, before reconnecting 12 volt. This’ll avoid the car possibly detecting excess pedal travel and throwing a code.

 

Oh man I wrote a huge post then lost it.

Anyway. In brevity - piston rotation looks good to me? The left slide pin felt a little seized up so I missed it in and out a bit to free it up. More concerning though was that I found the rear brake pad resting against the rotor? Like the v-pins weren't holding it back. I've bent out the pins to give them more tension. Does that all sound kosher?

 

You should remove the pins, and clean them off and regrease them.

 

When those V-springs aren't enough to spring the pads apart, your issue normally is the "fitting kit": those springy metal clips you see fit into the caliper bracket notches for the pad "ears" to slide in. The clips come from the factory with a dry, Teflon-like coating (anything non-dry, like grease, would just be a grit magnet), which eventually wears off and/or those clips get rusty.

If you pick up a fitting kit at the dealer, it includes 4 clips, enough to do both sides of the car. (It also includes new V-springs, even though there's probably nothing wrong with the old ones. The problem is the friction with the clips.)

Look closely at the clips you get: they're not all four identical. For each caliper there is one with a longer springy section at the bottom and one where it's shorter. Pay attention to how the old ones are installed: the shorter-springy clip goes at the end the pad naturally moves toward when braking going forward. The longer-springy one goes at the other end. That way, the pad is normally pressed 'forward' in that usual direction, so as not to make a click sound when you brake going forward.

With new clips, and the V-springs in place, you should find the pads a little uncooperative: you'll have to hold them squeezed against the rotor with one hand while you set the caliper back over them with the other.

 

Awesome thanks I will try find one of those kits and make sure they go in the right way.

For now, fuel economy is already looking slightly better (last few days of driving had averaged 16km/l). Cautiously optimistic. Either way, think I'll do the brakes myself next time...

 

Forgot to mention I can also get an easy 3 or 4 free-wheel rotations on each side when it's jacked up now, compared to struggling to get 2 full rotations previously.