P38A Mileage Query

V8 4.6 auto 1995 178k
Purely based on the dashboard figures, I used to get approx 20 - 21 mpg
For the past few weeks I get approx 25 - 16 mpg.
Anyone any ideas?
I just changed all plugs and leads and they certainly needed it.
All the plugs looked the right colour so I guess the mixture is ok
I have a nanocom, what should I look for?

I'm guessing anything over 13mpg is an achievement for our antique V8's.

You're creeping into diesel RR territory with that MPG

Minus the reliability, of course

On a run I often get an indicated 22 on the dash, and have even seen it say 25 Ha! Ha!
Not sure its got it right though. I am going to do a brim-to-brim if I can.

My dash may be miles out from actual but is the same dash before so should be comparable.
Dont know what a diesel would do bt it sure wouldnt have the V8 burble

The Dash readout has been known on some to be quite accurate..

I've read quite a few owners who have gotten north of 20mpg with the V8

Depends heavily on your right foot i suppose

The diesels can do from 22-32mpg depends again on the cars health and right foot

Why has mine suddenly dropped to 15 ish tho?
Puzzling

oakey said:

Why has mine suddenly dropped to 15 ish tho?
Puzzling

Click to expand...

More town driving?

If you check on the Nanocom are the O2 sensor readings changing? If not an O2 sensor might be acting up. Or maybe it just needs to get used to the new setup and trim itself?

My diesel has a long term average, checked brim to brim on every fill, of 24 mpg. On a long run it can get close to 30. A recent run on twisty hilly A roads 5 up turned in 27.2mpg.

Reset adaptive values with Nanocom.

From a brim to brim i get 22.1 mpg average out of my V8 which is quite surprising tbh.

My old diesel classic used to get 25-28 so i'm quite happy with my V8

funrover said:

From a brim to brim i get 22.1 mpg average out of my V8 which is quite surprising tbh.

My old diesel classic used to get 25-28 so i'm quite happy with my V8

Click to expand...

Wow, you must have pedals fitted

Thank you all
If O2 sensors were out, then would plugs not show too rich? I will check them hopefully tomorrow ta. And they should be changing all the time?
How do I rest adaptive values? What should I set them to?

120 mile run today and 20.3 indicated and was doing 70 on a lot of it.

Don't reset adaptive values it's pointless and will result in a loss of mpg for a spell! Gems will adapt all by itself. Resetting adaptive values on nanocom is a blunt instrument and resets everything. Having said that is your car on lpg?
I wouldn't worry about it tbh it's an average and reflects long term eg a 150m drive on the motorway may show 23mpg. It would take many short journeys at 13mpg to significantly affect the average. 15-16 mpg is a good representation of shortish journeys and I doubt anything's wrong at that.

No I don't have lpg
I am only getting 16 on a long run albeit not on motorway for long.
But same journey used to be 20 ish.
When it first dropped a few weeks ago it was an average of over 3k at about 20 then slowly plummeted

In fact I'd suggest your fuel trims are very well suited. The only time I'd reset fuel trims is after major engine work. Lpg does mess up gems fuel trims though. Gems is quite a slow adaptor not a fault just made that way which plays havoc with lpg/petrol trims. Thor is much quicker.
It may have taken years to arrive at your fuel trims it may take years before you see 16 mpg again!

oakey said:

Why has mine suddenly dropped to 15 ish tho?
Puzzling

Click to expand...

You inadvertently reset the system so it said "Computing MPG" and since that time you've done NO LONG JOURNEYS only round town ones?

No I have a habit of monitoring mpg.
When it initially dropped I was doing shorter journeys and then reset it when I was doing a long journey to see what happened and it stayed low

oakey said:

No I have a habit of monitoring mpg.
When it initially dropped I was doing shorter journeys and then reset it when I was doing a long journey to see what happened and it stayed low

Click to expand...

Very odd. Whenever I have reset it from appalling MPG (known problems with car and only round-town driving) and restarted/reset the calculations and driven gently on purpose it has always been very positive that I am doing a higher MPG.

It's difficult to recommend anything but check the coolant temp sensor. Check the maf at 1000 and 3000 rpm. Check the actual brim to brim mpg. Check the plugs are the same. Air conditioning on? Alternator/ battery dodgy? Check plug leads for Sparks in the pitch dark. Coil breaking down after plug change?