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Sudden fuel usage increase

5K views 14 replies 5 participants last post by  chris vince 
#1 ·
Morning guys over the last month ive noticed my mpg has been slowly going down so last week i reset my computer to get a weekly average and im getting 21mpg on a standard week and i used to get 28-29mpg
Euro 4 automatic 60/40 motorway/ A roads
I do more or less the same route day in day out every week for the last 3 years so my milage and route havent changed so it's the the truck thats some how become less efficient
Anyone got any suggestions what i can check?
The filters religiously get changed every 6 months (last changed in October) and im still useing bp for my fuel
The only other thing ive noticed is when the engine is cold if i give it full boot as in wheels spinning, it sometimes dumps a brick and looses all power untill i shut down and restart but if being run for 5 minutes or more its fine
But its been like that over a year and dosent seem to be getting any worse
Im suspecting the suction control valve for that but havent been able to confirm yet and cant see how that could affect the recent mpg loss
 
#2 ·
Just realised ive posted this to the wrong section if a mod could move it to technical for me please?
Thanks
 
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#4 ·
Back to basics, check rear drums for handbrake locking on, common fault on D22's, don't know if the same applies to 40's?
Similar system and layout though.
 
#5 ·
Never thought about brakes ill check them tomorrow
Wouldn't have thought the maf would be dirty cos i regularly change the filters and its got a egr blank bjt ill give it a good squirt with contact cleaner just in case
 
#11 ·
I know brakes or alignment could be the problem, but my experience with a couple of hundred miles driven is that the maf gets pretty dirty in this weather and gives fairly quick drop offs in fuel consumption.

Noting blanking the egr won't help and even regular filter changes won't stop it getting dirty.
 
#6 ·
It is the hinge point of the lever that tends to seize up.
Best to access it from the 'works' side after removing drum, as from the rear, it is inside a rubber gaiter.
 
#8 ·
Years ago in Cortina days, it was calculated that 1/2" out in the alignment specification was equivalent to dragging the front axle 90' sideways for every mile travelled! (courtesy Haynes).
This on 13" wheels with 4 1/2" wide tyres.
Navaras are much, much bigger.
 
#9 ·
Mmm interesting i did have my tracking done about a month ago because i was burning tires
Might be worth having them recheck it to be sure
think its still inside the 30 warranty so he should do it for free
 
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#10 ·
Tracking is often set incorrectly by technicians; lots of bad data out there. It may also need to be set differently if tyres are not 'standard' circumference or if vehicle has been raised. Or if wheel offset is non-standard.

I'd see if I could find the correct data on tracking myself and check it against what the tech thinks. But this may be red herring for fuel consumption...
 
#13 ·
I used to 'specialise' in wheel alignment, tracking up to 40+ vehicles a day for some 25 years.
Believe it or not factory workshop manuals are not correct all the time, e.g. when the Ford Sierra came out the factory setting was 1/4" toe in, all the local taxis (Sierra's) were wearing tyres out at an alarming rate, I did my first one ignoring the manual and setting it to what I thought correct, excess tyre wear stopped immediately, pretty soon I had almost every Sierra taxi for miles calling in.
G
If you think it may be an alignment problem, take it back and ask them to set it to 1.5mm toe in, no need for a fancy 4 wheel set up, a good accurate old fashioned Dunlop optical gauge is much more suitable.
The manual for a Navara will probably state zero toe in (I honestly don't know), if set to zero, once the truck is in motion the considerable drag and friction from the disc brakes, wheel bearings, driveshafts, front diff and propshaft will act together with, any wear and play in the front suspension the effect will be to pull the front wheels in an arc rearwards, therefore when in motion the front wheels will effectively be running with toe out.
This being the case wear will sooner or later be evident on the front offside tyre.
If 1.5mm toe in, is the at rest setting, when in motion the wheels drop back to run effectively parallel.

The operator SHOULD:
1. Set the touch arms to suit the wheels
2. Remove the gauges and place them together so that the arms are JUST touching each other, making a note of where zero is on the indicator, this is to check the gauge calibration at this arm spacing
3. Replace the gauges on the wheels and take a reading (steering straight ahead)
4. Move the vehicle forward (pushing it, engine NOT running) so that the wheels rotate 180 degrees
5. Replace the gauges on the wheels take a second reading
6. If the two readings are wildly out, one or both of the wheels is buckled (investigations needed)
7. Apply some mathematics, work out the mean reading between the two, this IS the actual current reading
8. Using this measurement, the position of the indicator on the second reading and taking into account the calibration reading, set the alignment to the desired dimension

If he does it properly he should ask you for the position of the steering wheel when driving the truck straight ahead, there is no real need to adjust both sides (unless a very significant adjustment is required, OR the steering wheel is horizontal when driving straight ahead), by adjusting the correct side the steering wheel can be centred up again.
On of the big benefits of the Dunlop gauges over modern 4 wheel systems is that the true-ness of the front wheels can be tested.
After all there is no point aligning the front wheels if one or both wheels are not running true, and this fact is unknown.
 
#15 ·
Honestly, I know very little about D40 suspension, the first thing I would be checking is the ride height, if it has sagged this may have the effect of going to positive camber, due to the arc action of the wishbones.
If this is the case and it has only dropped a small amount, this can be taken out with a camber adjustment, if a big drop possibly new springs required?
 
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