After a 20 mile drive yesterday, I noticed that both rear wheels were really quite warm to the touch. I only checked as the trip computer recorded 36mpg, which on a mostly steady 50mph road is rubbish.

 

I had this before when the rear brakes were binding, but both calipers and the discs were actually quite cool, so I'm fairly certain that the brakes aren't dragging.

Both front wheels were pretty cool too.

 

I am assuming that something is dragging given then poor fuel economy, but what apart from the brakes could cause this? Wheel bearings failing?

 

The car has done 182k and is a 2005 1.9 PD DSG estate. 

my guess is that either the rear calipers are sticking or the handbrake cable is...

I had sticking calipers before causing the same issues, and at the time the rear brakes would be red hot.  Both rear calipers have since been been replaced.

 

The discs and calipers both sides were almost stone cold yesterday which is what threw me.

Strange. Why should both bearings fail simultaneously? Sticking brakes would be the easiest solution but that would lead to hot discs - like you said.

They could both have failed over time and never been noticed I guess?

 

I've had poor fuel economy for quite a while now, and have never been able to solve it.

 

I've changed fuel filter, changed the MAF twice, ran a bottle of in the tank injector cleaner, also ran 2 full cans of diesel purge bypassing the fuel filter straight into the fuel pipe on the block . There is zero smoke from the rear when accelerating,

No fault codes on VCDS. Ran turbo boost logs requested vs actual and that came back fine too.

how is the tyre wear on the rears?

if they are out of alignment, they could be excess toe out, caused by...something back there?

bushes or just badly done tracking,

and its causing the tyres to drag a bit?

 

 

Tyre wear is pretty even as far as I can tell.

 

any evidence of the wear cutting across the tyre though, rather than around it? 

its the only reason i can think of that would cause the tyres to over heat but not the brakes.

even if it was the bearings, the heat would go into the brakes..

Did you compare them to the front wheels? It was a very hot day and "quite warm" would be expected.

 

What part of the wheel was quite warm, the tyre or the rim?

 

But for your previous problems would you even be thinking about the rear wheel temperature?

 

Forget wheel bearings and wheel alignment, they are not suddenly going to change on both sides at the same time to increase the fuel consumption on one journey.

 

I agree that a binding brake or brakes could explain it but the wheel(s) would be smoking hot, keep an eye on the fuel consumption on your next long journey and the temperature of all the wheels on every journey.

 

Have a VCDS scan to reveal if its an unrelated sensor or EGR issue.

Yes the front wheels were quite cool in comparison.

 

The rear rim was the warmest. Tyres were warm on the tread, but given the outside temperature, I would expect that.

 

Bad economy isn't something that happened only on one trip, it's something that I've noticed over the last few months, and have been trying to solve. I only noticed how warm the rears were when I bent down to pick something up and my arm went near them.

 

I've never had any EGR related faults with this since I got it 6 years ago.

 

 

I would look at the handbrake compensator where the cable adjusts just behind the handbrake, an Elegance will have a big box so the ashtray and rear bezel will need removing to access it.

 

Pull up the handbrake and if the compensator is not perpendicular to the cable, if its at an angle then you have a self adjust problem on one of the rear calipers, has anyone ever adjusted it because the handbrake was not holding?

 

But again, as we all have said the caliper & disc would be far hotter than the wheel rim.

I don't think the handbrake has ever been adjusted. Certainly while I've had it, I've never done it and no garage has said they have had to.

 

I was expecting the discs to be scorching when I felt them, but they were really not much more above ambient. 

Jack up the back and try spinning the wheels to see or hear if there is anything causing an issue. The wheels should spin freely if all is OK. Also check the handbrake is releasing properly.

 

When one of my wheels got hot, I stripped, cleaned and regreased the caliper pins , brake pads and slides etc. Problem solved.

 

Silly question but have you checked the tyres are not rotational and on the wrong way round? Many tyres have a rotation arrow/marking and having them go the wrong way, is the wrong way to go! You may have accidentally swapped the left wheel for the right, so to speak. Easy to do, though.

If you work out how much extra fuel will have been used during the journey, and its calorific value then you would realise that it would be enough to turn the rear wheels, hubs, suspension and subframe into molten metal, the car would look like something out of an Iron Man movie, it would not be warm tyres getting your attention 

 

Given that the discs & calipers were cool I think you are barking up the wrong tree and should start with a VCDS scan and then move on to measuring blocks etc, a minor boost leak will have a great impact on fuel economy without creating heat.

@softscoop

What tyre pressure are you running the rear wheels at?

What tyres fitted and what size, and load in the car?