Apologies - yet another MPG query

[andymc]

I thought about adding to one of the existing threads discussing this topic, but reckoned it might be a bit rude to hijack someone else's query about fuel economy.

So - the 2.0CDT I bought in August. It had a T4 session courtesy of marinabrian - no faults came up other than the need for a new airbag sensor (long since taken care of), and thanks to him it can go as fast as a 2.0CDT should. The car had been starting to show some slight signs of neglect, but had a reasonably lengthy service history and relatively low mileage (around 90k when I got it). I noticed in my first few weeks of ownership that it wasn't quite as economical as either of my other 75s, but thought this would be down to a lack of recent TLC.

In terms of maintenance items that might affect fuel economy, over the past few months the car has had:
- new brake pads fitted at the front
- new discs, pads & calipers at the rear
- new tyres all round (and kept at the right pressure)
- EGR valve and manifold checked/cleaned
- new inline thermostat fitted
- recent full service, with an engine flush and new oil, along with new oil/fuel/air/PCV filters

With the new stat now fitted, according to the OBD the engine temperature after the first three miles in cold weather varies between 83-86 degrees. Occasionally for a few seconds it might go up as high as 88, or as low as 81, but never outside this range. It's not quite reaching the "sweet spot" of 89-90 degrees, but I can't see that making a significant difference.

In terms of fuel economy - on a long (270 mile) motorway run it returned around 42mpg even though I was booting it for sustained periods. That was before the oil & filter service and the fitting of the inline stat, so I thought it would have done better since.

But no - my first to-the-brim refill since then showed that it took nearly 49 litres to cover 440 miles, or just a fraction under 41mpg. Annoyingly, the car will not do better than this, despite the fact that most of my driving involves cruising between 50-60mph. The other CDT we're currently running - which has an extra 40k miles on the clock - is returning a much more reasonable 47mpg. That's with my other half driving it most of the time ("oh sorry, I keep forgetting to change up into fifth ...")

I've brimmed it again today, and put a dose of injector cleaner in (no diesel Redex available when I was looking for it, so I used dipetane). Other than that, the only other things I can think of are

- could the intercooler O-rings need replacing?
- would the existence of a FBH without a remote control make a difference of 6mpg?
- our more economical car is a 2003 build, whereas the thirstier one is on an ‘01 plate. Other than the lack of a FBH, would there be any other relevant differences?

Two other things which may or may not be relevant:

Firstly, I often hear what I can only describe as a sort of "chirring" sound when building up speed from lower revs, e.g. if I've bimbled through the village for half a mile at 30mph in fourth rather than third and accelerate from there up to 55-60mph. This sound fades as the revs climb. I did wonder if that was an engine-mount issue, but it seems not. UBP perhaps? Nothing I can hear at idle, at any rate. Fuel rail, maybe?

Secondly, the aircon got a regas at Halfords (I know, I know) back in late August/early September, but while it initally seemed cooler, it never produced properly cold air. I haven't yet brought it to anyone for this to be fully investigated - my budget has limits and the other items above were of greater importance. Wasn't sure whether the new stat would be relevant to this, but of course the weather's been too cold to use the "Low" setting since then. Could this have any impact on fuel economy?

Is there something I haven’t thought of and need to check?

Sorry for the lengthy post, I'm trying to provide as much info as I can ...

[Coups]

I'm interested in where this goes. I've nothing to aid you but am in pretty much the same boat, although I don't have a fbh. Watching with interest.

Only thing I don't know about with mine is the inter cooler hose/O rings have been changed.

[Mike Noc]

Manual or Auto?

[andymc]

Manual, with a nice light-feeling clutch. It (the thirsty one) is a Club SE, so no electric/heated seats to use up extra juice - unlike the frugal one, which is a Connoisseur! Generally at this time of year I have the cabin air set from 20-22, lowest or second-lowest fan speed.

[bl52krz]

Have you checked your injectors for leak back? Could be dribbling injectors. Best to have a t4 done by one of our experts.

[DMGRS]

I may have missed it in the opening post, but are you measuring economy brim to brim, and not using the fuel gauge and/or computer to work it out?

The computer is renowned for having a mind of it's own - my auto is apparently returning 34MPG at the moment, where it's actually closer to 37MPG when worked out correctly.

[andymc]

Originally Posted by bl52krz

Have you checked your injectors for leak back? Could be dribbling injectors. Best to have a t4 done by one of our experts.

No, I haven't done that - is that something that would have shown up in the T4 check that Brian did when I bought the car? Also, would it show up as leaky fuel eventually making its way to the ground? Haven't seen any trace of that in the five months I've had this car ...

Originally Posted by DMGRS

I may have missed it in the opening post, but are you measuring economy brim to brim, and not using the fuel gauge and/or computer to work it out?

The computer is renowned for having a mind of it's own - my auto is apparently returning 34MPG at the moment, where it's actually closer to 37MPG when worked out correctly.

There's no trip computer on this car (yet!). I'm measuring brim to brim, filling it to the neck and allowing the third click that gets an extra litre or so into the filler neck so as to make it as accurate as possible without getting to the point that I might pose a risk to motorcyclists after my first turn to the left! That said, the fuel gauge does seem to match my calculations, with the needle already on the halfway line by the time I've reached 235-250 miles. This was the case from the time I had the car, but it's the fact that a full service, new tyres & brakes and new thermostat seem to make no difference that concerns me.

By contrast, the 2003 car will be much closer to 300 miles by the time the needle gets to the halfway mark from being brimmed to the neck. It's been pretty consistent over the past 2 years and my driving style hasn't changed. The only difference between the cars that I'm aware of is that the thirsty one has a FBH (and that chirring noise I was talking about).

[andymc]

Having done a search for "Injector Leakback", I came across this:

http://www.the75andztclub.co.uk/foru...eakback&page=2

- with a couple of interesting contributions from marinabrian in response to someone else's query:

Originally Posted by marinabrian

In answer to the OP's question, treat yourself to a bottle of Forte injector treatment, unscrew the fuel filter empty the contents and chuck the entire contents of the bottle of Forte into the filter.

Screw it back into place, turn the ignition to position 2 for 20 seconds, turn off then repeat.

Now start the car and allow the engine to idle for around 45 seconds and switch off.

Leave the car overnight, then go and give it a good hard motorway run.

I bet your injector compensation figures will look a lot more healthy afterwards

Brian

Originally Posted by marinabrian

What you are seeing with the fuelling compensation figures in the live data screen of T4, is the adjustments the ECM is making to the individual injectors to keep the engine running smoothly.

You normally find an injector which is performing poorly having slightly more fuel injected and it's counterpart leaned off slightly.

T4 itself cannot actually detect "leakback" but the figures as presented give the operator a clue as to where a problem may be brewing in the background.

What I'm suggesting costs much less and is far easier than replacing injectors

I have seen many cars improved using this method, so have a bash, you may be pleasantly surprised

Brian

Hopefully the dipetane will have a similarly beneficial effect - I did about 50 miles after filling to the brim today and I've a couple of 20 mile runs to do tomorrow, so I'll see how close I get to the quarter tank by the time I get home!

In the meantime, keep the suggestions coming.

[Ken Lion]

I've kept an exact record of my fuel purchases and mpg in the 2 1/2yrs I've had the car (yes, I know, a bit anal).

I brim it on each fill and the spread-sheet shows the mpg for that trip (sometimes town, sometimes mixed, occasional long runs).

I see a drop of about 5-6mpg over the colder winter months.
The lowest winter mpg has been 37 - lowest summer 43, with an overall average of 46.


I'd shrugged off the winter drop as 'one of those things' but recently the penny dropped (thanks Harry) that my non-working stat means that in very cold spells the fbh starts and keeps on running because the coolant never gets to 77c.

[hamster19]

Originally Posted by Ken Lion

I've kept an exact record of my fuel purchases and mpg in the 2 1/2yrs I've had the car (yes, I know, a bit anal).

I brim it on each fill and the spread-sheet shows the mpg for that trip (sometimes town, sometimes mixed, occasional long runs).

I see a drop of about 5-6mpg over the colder winter months.
The lowest winter mpg has been 37 - lowest summer 43, with an overall average of 46.


I'd shrugged off the winter drop as 'one of those things' but recently the penny dropped (thanks Harry) that my non-working stat means that in very cold spells the fbh starts and keeps on running because the coolant never gets to 77c.

My lowest in the winter has been 47.8mpg, highest in the summer 55.0mpg, all brim to brim in the last 16 months; my average has been 51.4 mpg