Strange Problem

[YarisHybrid2016]

Hi,

At great risk of sounding obsessed with fuel economy, here goes...

1) I know cold temps hammer fuel economy
2) I'm not a "hypermiler"
3) I know weather conditions affect fuel economy
4) Tyre pressures are correct, and everything is in good running order.

Now that's out the way, to my observations:

It started with me noticing a drop in MPG. I don't mean 2 MPG, but more like 15 MPG. I thought it was a blip so thought nothing of it, but after a week of this, I started to think something else was going on (this was late December 2017).

At the same time, I started noticing the car needed more power to hold speed. It got so I couldn't cruise at 70 MPH without going into the power band on level road, and the usual 50 MPH I drive at was also requiring more power. The icing on the cake (sarcasm) was the notably reduced distance I could drive on EV, and the fact that it was driving less in EV overall. This has been getting progressively worse over the last few weeks.

The final "what the hell is going on", was when I started finding roads I would usually need to use some regen braking to hold speed, now required light throttle to cancel regen braking, if not some slight positive power. That is what is prompting this post.

So... last night I filled up and went for a long drive down the motorway (a couple of hours). It's not hilly, but is reasonably gentle in the slopes. As expected by now, the first 30-40 minutes required to hold power into the power band to maintain speed. By this time I was thinking I should maybe head home and take it to the garage (it takes me 40 minutes to get to the motorway so by now everything should already be warm, etc.).

Over a period of some 30 minutes, it started to take less power to hold 70 MPH, so I kept going. By the time I had turned around, and got back to turn off home, the power required to hold 70 was now down 2 tick marks. The journey from the motorway back along A roads at 50 MPH now required less power, and with it, MPG was significantly better.

Tonight, I went and did some shopping, and the car returned a computer reported 68 MPG. This is more like it (just lately the same journey was returning 52 MPG).

I have done this journey countless times. In the last few weeks, the OAT has been +2 or +7 deg. C. I hardly see a change in reported MPG between these temperatures, but needless to say, it has been dire.

Last night was +7 deg. C, with slightly damp roads.

The car is still exhibiting what I would call "increased drag", like a brake is lightly applied, but driving better than it has recently.

I have checked the hubs, and they don't feel hot, and I'm running out of ideas as to what could cause this.

The rear brakes don't feel hot, and the rust doesn't improve after a drive out, suggesting they are releasing correctly.

The only other thing I can think of is possibly a wheel bearing problem, but wouldn't know where to start to diagnose that.

Something isn't right, and posting here in the hope that someone might recognize the symptoms.

[Bomber209]

Hi, in colder weather the air is more dense, so requires more fuel to keep the mixture ratio correct. The EV drive is sorted via electric motors, which don't reach their optimum until they're nicely warmed up. So again if it's colder, more energy required.. That's probably the reason you got a better return on your longer journey, although at 70mph you'd be using the engine more than the Battery to generate drive to the motors.  

[Mike J.]

11 hours ago, YarisHybrid2016 said:

The final "what the hell is going on", was when I started finding roads I would usually need to use some regen braking to hold speed, now required light throttle to cancel regen braking, if not some slight positive power. That is what is prompting this post.

To me this points to binding brakes. jack up each wheel in turn and check for rotation - probably have to stop car in neutral for the front wheels to turn?

[Mick F]

Easiest way to see if any brakes - or anything - is/are binding, is to go down a slope in neutral and coast along to a stop.  If the car comes gracefully to a stop, there's no binding issues.  If - when it stops - there's a feeling of the brakes being applied - even just a little bit - you can tell.  The car should coast and stop gracefully and softly.  If there's a sort of hard stop at the end, you have a binding problem.

Mick.

[YarisHybrid2016]

13 hours ago, Mick F said:

Easiest way to see if any brakes - or anything - is/are binding, is to go down a slope in neutral and coast along to a stop.  If the car comes gracefully to a stop, there's no binding issues.  If - when it stops - there's a feeling of the brakes being applied - even just a little bit - you can tell.  The car should coast and stop gracefully and softly.  If there's a sort of hard stop at the end, you have a binding problem.

Mick.

Thanks - I'll give this a try!

[YarisHybrid2016]

16 hours ago, Bomber209 said:

Hi, in colder weather the air is more dense, so requires more fuel to keep the mixture ratio correct. The EV drive is sorted via electric motors, which don't reach their optimum until they're nicely warmed up. So again if it's colder, more energy required.. That's probably the reason you got a better return on your longer journey, although at 70mph you'd be using the engine more than the battery to generate drive to the motors.  

Right, but what I'm seeing is far beyond that, and doesn't explain the coasting downhill problem, either. Thanks though.

[Mick F]

5 hours ago, YarisHybrid2016 said:

Thanks - I'll give this a try!

Great!  Let us know how you get on.

 

In colder weather, the engine runs more often and for longer to keep you warm.  We set our temp to 21.5degC or sometimes 22degC.  We could make it so it's cooler and wear more clothes, but we prefer comfortable temperatures.  Add that to cold weather engine inefficiency, and the winter means less economy.

If you had an electric car, you would lose range in the winter due to the car heater and air con working harder.

Mick.

Edited February 21, 2018 by Mick F
Added a bit

[YarisHybrid2016]

@Mick F: I did the coast-in-neutral, and it is indeed dragging. More intriguingly, it doesn't drag for the whole rotation, but only partially. Where it is dragging though, it is quite noticeable.

This actually explains a lot:

* When I had my front tyres changed in December, the tyre people said the rears were 6 mm right, 5 mm left, which I found odd.
* The car feels to pull slightly to the left all the time, requiring appreciable steering to the right to correct (it is off-center by around 5 degrees).

From the tyre tread depth issue, it would seem that this has actually been going on for some time (the pulling to the left has been a "feature" for several months - I raised it when it went for service in September, but they just told me it was due to camber of the road, which I knew it wasn't).

I'm getting it booked in to be looked at - I'm expecting them to say the brakes need replacing on the rear...

The brake doesn't seem to be releasing correctly all the time - I found tonight that if I brake, it may release and start rolling again (very slight hill), or it won't. Pressing the brake pedal multiple times affected this. I'm thinking it is a stuck brake caliper.

As for the neutral test - I drove the car to 10 MPH, let it stabilize at 2-3 MPH (!!!) in D, then dropped it to N. The car pulls up with the feeling of the slightest brake pressure. The car wouldn't then roll down the hill most of the time, though a couple of times it started rolling backwards. I then did the same test the other way down the hill, and the car would slowly come to a stop, and not move.

Thank you!!!! I thought something was wrong for some time, and it is good to finally put all the pieces together.

[Mick F]

Good news then.  Not good that you have a problem, but good that you've found it.

Mick.

[YarisHybrid2016]

Hi,

All the brakes were stripped down - general condition very good considering age and relative lack of brake operation. Had the rust cleaned off the pads and everything cleaned up, re-greased the sliders, and checked and adjusted the handbrake (adjusters had seized), and it is as good as new.

Brakepad wear was negligible, and no sign of dragging (heat signs or excessive wear), so it seems just purely corrosion was causing the problem. Car drives great again.

The whole job took my local independent less than 30 minutes to do. Awesome!

Based upon how long the handbrake has been wrong (slack in the bottom part of travel), the brakes want stripping down annually - it started to exhibit slack about a year ago, or at 18 months old.

Why is this not a service item?!