Ok.. weird question for all you spanner chuckers out there !

Discussion in 'Motorhome Chat' started by Jaws, Jun 2, 2012.

  1. Jaws

    Jaws Funster Life Member

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    I have heard of this before but thought it was a load of cods wallop..

    But now it has happened to me so I can no longer poopoo the issue but REALLY do not understand how the hell it can happen

    I have got 28k on the van so whilst doing the service I decided it was time to change the diesel filter... Ever since then I am getting a good solid measurable 3 - 4 mpg

    tried it twice with as accurate measurements as I can manage.. and got the same result twice ( had to do it twice cos I did not believe the first time )

    Soooo

    Given the filter JUST filters, how on earth can mpg improve when it is changed ?
    I mean, it is before the pump etc and it can hardly change the rate of delivery of the pump nor change the amount an injector is opened

    I simply do not understand this phenomenon

    As a by the by.. When I first had the van it returned 19 to 21 mpg
    I fitted a Torquing of cats black box and that improved it by 2mpg over the next few k miles I then had WOW! do an upload, was not happy and they kindly did a special for me which was magic, and I then averaged 25mpg regularly

    Over the past two years the mpg has slowly dropped.. You know what its like.. Oh its the head wind, oh I had the hammer down a bit hard.. oh there is an R in the month.. you try and think of all sorts of reasons..

    Well changing the filter has shoved the mpg right up to possibly even MORE than the 25mpg previously experienced ( But am reserving judgement until I have done a couple of thousand miles and can confirm )
     
  2. Landy lover

    Landy lover Funster

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    All I can offer is that we always use genuine filters - have stripped the cheap ones down to see what it in then and the difference is amazing - some of them are large cans the same size as the OE yet the filter inside is only a fraction of the size of the OE and often not much more than a coffee filter paper. Some of the look alikes even miss out things like non return valves. On Landies I do know that OE filters and the OE prefered oils do definately give improved MPG. Very much I feel you get what you pay for
     
  3. TheBig1

    TheBig1 Funster Life Member

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    a partially blocked filter restricts the flow rate of fuel so the pump works harder
     
  4. darklord

    darklord Read Only Funster

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    Ditto,..ive been out to trucks that have had a service a few weeks prior, and are down enough on power to be struggling up an incline. On investigation, we changed the filters to genuine after finding the ones fitted were blocked or collapsed inside (measuring flow in, and out). These trucks are serviced regularly on hourage as well as mileage, with a MH, a filter may not cause a problem for a long while.....best to fit genuine.
     
  5. Jaws

    Jaws Funster Life Member

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    It may work harder but it cannot deliver any more ( esp as the filter is upstream of the delivery pump )

    The filter is an insert type.. Not the older cannister one.. And I tell you, the pot was a total ^&*(&*^!"£^ to get to seal properly !! :Angry:
     
  6. TheBig1

    TheBig1 Funster Life Member

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    as its upstream, as in before the pump in the line from the tank, the pump inlet is being starved so making it work harder on a weaker supply. A drop in available power will reduce the mpg
     
  7. Jaws

    Jaws Funster Life Member

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    K.. that is the best explanation I have ever heard.. I shall rock with that ( unless someone comes up with a better theory LOL !!! )

    But.. Less fuel getting through.. so why should the mpg get worse with a partially blocked filter, and not better
    I mean, if less fuel=lower power, then would that not equate to better MPG ( sorry, having difficulty getting my head round this )

    Lets look at a petrol engine. A weak(ish) mixture, while eventually causing damage in the form of burnt exhaust valves and a mouse nibbling the piston crown, will deliver better mpg
     
  8. FULL TIMER

    FULL TIMER Read Only Funster

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    I'am by no means any type of mechanic but I would assume as the engine has less power it has to work harder to acheive the same result therefore increasing fuel consumption
     
  9. DP_JAY

    DP_JAY Funster Life Member

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    Quite simply if the fuel/air ratio isn't correct it won't burn properly & so give less power per rev & so it has to work harder longer.
    This means slightly longer in each gear etc. etc. etc.
     
  10. Jaws

    Jaws Funster Life Member

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    Ok.. so you are saying ( for instance ) a 120bh, 2.3l engine as in one of the more common Fiat MH conversions, will do worse mpg than my 2.8l 160bh ( ?? ) engine ? Well we know that does not work though the premise is good and indeed, in certain circumstances does in fact hold true

    But it would not remain a constant, only work in ( for instance ) stop and start condition or very hilly ones.

    I tend to run the tests over long straight roads with virtually no stops and only one minor hill


    Now this is an interesting one.. and is really close to the crux of the matter

    Although I can strip and rebuild a diesel lump with ease, just the same as a petrol one, I know for sure I could not tweek it for better performance as I simply do not understand how to.
    I know how a diesel works of course, compressive explosion, but thats as far as it goes.

    After spending years squeezing out that last couple of BH from petrol engines, whilst the diesel engine is similar it would seem to me that it is very DISSIMILAR too..

    As mentioned elsewhere, if you wanna risk weakening off a petrol engine you can get more bang for your bucks ( its a common trick used by just about every tuner at some point when pennies are short and you need that tiny bit more ).. But it now seems weakening a diesel mixture will reduce power, so the technologies are pretty disparate to say the least
    And it is obviously that which is stopping my hind brain from grabbing the absolute facts and formulating a reason or theory of my own

    ( Good debate this ... :thumb: !! )
     
  11. jonandshell

    jonandshell Funster

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    We find that fuel quality makes the biggest difference with our Tranny. We have stopped trying to save cash by filling up at supermarkets because we genuinely see a 2mpg difference by buying 'branded' fuel!
    The difference when doing the same in our Toyota IQ is astounding! More like a 5mpg (10%) difference on a strictly urban cycle!:Eek!:

    Have you suddenly stopped using supermarket fuel?
     
  12. Jaws

    Jaws Funster Life Member

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    Nope.. Always use either Sainsburys or ( sorry Shiftzzz LOL !! ) Tescos

    But we have always noted that we get different economy or lack of in cold air .. Warm air always seems to improve matters even though the engine temp remains constant ( as it should as it is controlled by its own internal thermostat )
     
  13. slobadoberbob

    slobadoberbob Read Only Funster

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    we often do not have a choice in the matter

    When it comes to fuel it is not if it is a branded or a supermarket it is can we get in and out with the RV and trailer.

    But going back to oil etc., We are a petrol V10 with a 6.8 ltr engine. What I do is always change the engine oil at or before the recommended change time. Where as the hand book allows different times for dusty conditions, I tend to change the oil every 3k to 5k and I use what the Ford says. But as for oil filters I tend to go with Fram .. used them for all sorts of vehicles and have found them OK ..

    I know petrol engines are different to diesels, but I used to years ago run a small fleet of 6 coaches all diesels .. bedfords and Fords... in those days we run on anything as it was not as sophisticated then. But these days I would only have original equipment fitted.

    Bob
     
  14. Jaws

    Jaws Funster Life Member

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    May I ask what the recommended service interval for the oil is ?
     
  15. vwalan

    vwalan Funster

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    hi jaws ,thats a difficult one .i know i have driven a truck for 500mls .then get out and the next driver drives the same route again .he gets out i get in and so on . these are doing a 1000ml a day 6 days a week. get a sevice check every 6 weeks .but they have done 36000mls. this is in a scania volvo ,merc truck. cornwall to brum ever trip. makes mortor home milage seem insignificent. if the oil isnt changed at that check then it could be 70,000mls . makes you think really doesnt it.
    sometimes they arent even switched off on the change over one gets out the other jumps in .
    all fuel is got from the depot tank. some can do two trips before filling .
     
  16. Jaws

    Jaws Funster Life Member

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    Well that is the point I was going to make Alan..
    Modern oils are simply amazing, and it is actually possible to change it too often ( unless you use some VERY special oil not usually available to the general public )

    The reason is, almost all oils sold in retail outlets nowadays have a detergent component..
    The component is meant to break down and dissipate over about the first 2000 miles of use..**

    So changing the oil too often means that in effect you are running your engine on a semi detergent.. Now that sounds great except as any engine designer will tell you it can cause some fairly odd problems further down the line

    I am a great believer in following manufacturers service recommendations when it comes to oil changes.
    After all, said manufacturers are not going to ruin their reputation by suggesting a service period that will cause the early demise of their engine..


    ** This information taken from a hand out given to me when I was a guest at an oil refinery a couple of years ago
     
  17. vwalan

    vwalan Funster

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    i can say that in the aircooled vw engines i would only use a high detergent straight 30 oil. we found that if not lacquering on the insides of the crankcases caused over heating . also modern oils didnt have emulsifiers in them .if a multigrade was used it caused mayonaise to build up in the breather pots blocking them and causing oil leaks etc due to crankcase pressure. we always used morris,s supreme sae30 after having a good chat with andrew morris .
    using the wrong oil i,m sure killed loads of camper engines . strict 3000ml service, tappets done cold and change oil was the way.
     
  18. TheBig1

    TheBig1 Funster Life Member

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    In colder conditions, diesel is thicker and less viscous, so unless you preheat it in the tank, it affects the amount the pump can suck in over a given time. for want of an analogy, think of mcdonalds (other ways to poison yourself exist) you order a milk shake. try sucking it in when its cold and you dont get much, however wait till it warms and it flows freely. now back to your diesel engine, restrict the fuel at the pump, either by low temp or restriction and your pump works harder. Its not like a petrol engine, squirting out fuel at a set high pressure at a constant rate with constant input. your diesel gets injected into the cylinder and compressed at a set rate. less diesel in the chamber for compression and combustion gives less power. the ecu notices the issue via several sensors and increases the fuel injection timing and therefore you end up burning more fuel to make up the difference

    Think Ive got that right, but its a few years since I got my hands dirty with such things. modern diesel engines are pretty advanced and totally different to petrol, with many many variables and sensor inputs that affect fuel flow and output. Spanners will only get you so far with modern diesels as you know, and the modern breed of technician with a computer is essential for performance tuning
     
  19. Jaws

    Jaws Funster Life Member

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    Great explanation !!! Thank you kind sir !! :thumb:
     
  20. aba

    aba

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    on the older transits etc with mechanical injector pumps it was possible at a belt change to slightly alter the pump timing and a couple of degrees either way could make quite a difference to the performance.

    if you advanced or retarded the pump slightly (cant remember which been a long time) the engine would not be as noisy but would gain extra mpg.
    but if you went slightly too far it would not pull the skin of a rice pudding.
    there was a sweet spot but unlike a car with a distributer that could be tweaked whilst running it was trial and error and carefully marked adjustments.

    modern common rail diesels are electronically controlled and i can see how under the circumstances you have gained a few extra mpg.
    if the pump is struggling to draw fuel from the tank the fuel pressure may be within tolerances but at the lower end therefore when the injector opens it does not get the full pressured amount so will be slightly down on power but you will drive it to the same potential as you expect hence using more fuel.
    it may only cost 3 or 4 BHP but its enough to cause a little more pedal.
     
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