MPG

[Worker]

I would like some help please?
I have been the owner of a mark7 manual gti petrol car.  It has been serviced by Volkswagen in July. It has only done 10000 miles.   In October I have observed that I can only manage 30 mpg when I drive steady on a regular route.  I have enjoyed 47 - 52 mpg on this route.  I have no aircon on or heating!  I thought it may revert to the 47 mpg but it hasn’t and if anything it is getting worse!  It is set to e on.
Thank you.

[willni]

Mine's the same at the minute and I've a misfire from a coil pack, not enough to flag a fault but I recorded it via VCDs.

[Worker]

My gut feeling is that it is a brake bind!  Could I do anything to help without going to garage please?

[Adam T7]

Brake bind = unusually hot wheel(s) and car pulling. Are tyre pressures ok?

[NAS23]

Is your petrol particulate filter stuck in regeneration?

About once a year I have significantly reduced sub-30 MPG and other symptoms such as noisier exhaust suddenly appear for this very reason due to frequent short journeys. The first time it lasted for weeks and did raise concerns, but luckily I read an article that detailed the problem and how to easily clear it.

Basically find a quiet dual carriageway maybe at night and get the engine up to temperature in the normal way (oil temp not coolant temp), then put the car into 4th, drop to 40-50mph and accelerate hard up to "around" 70mph but keeping it in 4th gear. Then let the car slow naturally back to 40-50mph without braking or changing gear and then repeat the 4th gear acceleration up to 70mph another 4-5 times.

The engine will be up around 4.5-5K revs and you might use a few extra pence of petrol for the duration but it works immediately and MPG should revert to normal. I found it best to leave the car overnight and then monitor the MPG, otherwise you will be on the tail-end of some brief hard driving and this will reflect in the MPG readings immediately thereafter.

[Exonian]

Welcome Worker 

How old is the car and how long have you had it?

As suggested, tyre pressures would be a good start, and yes, rear brakes could be binding.
Rodents in the airbox? Worth popping the top off to see if there’s any nesting in there, it happens!
Spark plugs and injectors can suffer if the car isn’t used much and old fuel sits in the tank for a long time.

Dealers charge a heap for diagnostics so maybe getting a VCDS cable or OBDeleven set up to have a good scan through fault codes. You can get cheap basic diagnostics devices for £20 too.