Unexplained huge drop in MPG

Disclaimer - I don't think this is a DPF/EGR issue.

I have a Insignia estate, elite nav, ecoflex 140bhp version. It's been great, I really like the car and has been very reliable and good in many aspects. It's a 2013 facelift version and has done a lot of miles - 118k before I bought it 2 years ago and now up to about 135k. It's well serviced and oil pump has been recently replaced due to a leak.

I'm most interetest in the car running really efficiently and getting good MPG and low emissions for the type of driving I do, mostly out cruising on fast roads. Very little urban driving.

With cruise control set at 70mph over several hours of driving, I can get excellent MPG levels, 70+, sometimes even 75mpg averaged.

However, on occations, without any explanation or noticable issue happening, the MPG will suddenly drop. Right down to around 30mpg (while doing 70mph on the flat). There's a slight feeling that the engine is struggling a bit, but it's not massively noticeable.

When I notice it, I find if I pull over and turn the car off and leave it for 10 or 15mins, then the problem corrects itself and I can carry on as normal. A short switch off doesn't seem to help. When you stop you can smell heat off the engine and the fans are going. Also not normal for when it's running fine. There's no heat of the brakes and no warning lights etc.

If you just ignore it the problem doesn't seem to go away by itself and of course, I'm concerned it will be doing damage to the engine.

I've checked the back pressure off the DPF and seems to be fine. Also removed the EGR and given it a deep clean.

Any pointers would be great. I thought maybe it's got a variable fin turbo and they're getting stuck. Or maybe the MAF is faulty and giving a bad reading. But all strange that it recovers when switched off for some time.

Thanks very much, Jake

 

A bit late I know but having seen your post I can confirm that mine does exactly, and I mean exactly, the same.
The way I fix mine is by doing a several mile thrash, say 5 or 10 miles in 3rd gear at well over 2000 rpm.
It seems as if the innards of the engine need a good blast, including the dpf, and if you watch your instant mpg readout on your fuel computer you will see the figures go back up to the "normal" 60 - 70 mpg after this blast.
That is until the next time, at which point you do it again.
I have no idea why this happens especially when you see the mpg suddenly plummet as you describe.
Do you get any warning message on your dashboard. On mine there is no message.
Buy some Forte DPF cleaner and run it through as instructed. I did and it seemed to help.