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Thread: Techie question MPG down

  1. #1
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    Nov 2002
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    help Techie question MPG down

    Hi all,

    A bit on the technical side but an answer could benifit us all if my suspisions are correct. Here goes, the full story.

    By the way it's an open floor, ANY ideas, fire them in.

    I dropped my engine to do the clutch and timing belt. No major problems and I changed the Gbox oil as well.
    I also changed the cat complete with the temp sensor and the other sensor, which I am guessing is the Lamda sensor?

    My MPG went from 50+ to 35/36

    I've whipped the cat off and there are fuel, water dropletts on the cat intake end. (The water I can understand although rather surprised so close to the exh manifold but the last run before I did this was a short one) The fuel Its a definate petrol smell.

    I put an AVO across the 4 terminals of the Lamda? and get an open circuit across everything in both directions (polarity) except the black wires (which do not earth to the cat body). This is on both sensors.

    On the "new" (suspect) cat I get a reading of 11.3 Ohms initially, which flicks to 13.2 after about 0.5 seconds. (The flick in the reading could be my AVO but it is s definate reading the same every time).

    On the "old" (good) cat I get 11 Ohms flicking to 12.9 again consistant readings with both figures.

    Has anyone got, or can anyone get the correct figures (has anyone got a new one they could test). If these little beggers are breaking down it could explain the difference in fuel consumption we are getting. I intend to put the old cat back on and see what happens but it's in a bit of a state. (Top seal gone, 1 stud sheared and an un-removable, broken heat sensor). I am in the unfortunate position of having a 93 model so I need the cat for the MOT

    OTHER INFO
    Both cat matrix' are clean and clear.
    Car seems less willing to get up and go but will still rev and get to speed (using a whole lot more fuel)

    Any ideas on anything I may have done wrong with the timing belt most welcome but I get no engine warning light or anything obvious at idle.

    Again it's an open floor ANY ideas, fire them in.
    If it ain't broke, take it apart and see how it works

  2. #2
    Join Date
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    Kent
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    You disconnected the battery and lost the ecu long term memory could be a cause. it will return with running time.
    Steve M
    __________________

    Its all gone Norfolk!!!
    Now in the Lotus position.

  3. #3
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    Nov 2002
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    Hi Tim

    I think you could be on to something there with the idea that the O2 sensor is breaking down or sending inaccurate readings, I too am getting 30 something MPG, im guessing its running very rich given the strong smell of petrol at idle.

    Chapter 6-21in the workshop manual seems to detail a procedure for testing the O2 sensors however my Japanese isn’t the best! Maybe OZ could give it a read?

    I have knocked the centre out of my cat which means a better noise and prob a few extra horses but I regret doing it now because of the dramatic increase in fuel consumption.

    Its defo worth getting to the bottom of this problem alright, petrol price are on the up!

  4. #4
    Join Date
    Aug 2002
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    Ireland
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    92

    MPG

    Mine also does about 30 mpg and always has, I was amazed when I hear you guys talking of 50 mpg. I too get a strong smell of petrol and get some backfiring on over run, its a nice cracking sound but it has to be damaging the exhaust. I am due a service soon and was going to ask them to retard the fuel mix a little. If the O2 sensor contributes to the ratio of fuel to air mix then that could be my problem too. I am getting no fault signals from the ecu so I was hoping nothing was wrong other than the need for a service.

  5. #5
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    Thanks for the quick replies (I'm not the only one taking a day off )

    Well the exhaust is back together and even from just a quick start up and a few throttle blips I feel I've solved the problem.

    I'm going to fill up and we'll see how it goes.

    Any chance over the next week or so you guys could take a resistance reading. Not the easiest plug to get to but if ou're in there anyway it may be useful. I'll blast an e-mail to Oz and see what the Master can come up with.

    Cheers guys
    Tim
    If it ain't broke, take it apart and see how it works

  6. #6
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    Mar 2004
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    northern ireland
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    without going and checking my car how many wires are on the oxy sensor 2,3or 4 i can give you a reading for all of them ..also motor factor sensors at £40ish will do to replace them with

  7. #7
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    Hi Barkley,
    There are 4 wires, 1 green, 1 white and 2 black. There is a web of plastic inside the plug, if you check the terminals across this rather than the pairs on each side you should get an open circuit on one set (green white)and a low resistance on the other (black black). My readings were in Ohms not K or M. ( I also got O/C green black and white black)
    Having given the car a blast to the fuel pump and back I feel it's back to what it was with regards to get up and go but I'll see how it goes on this tank of fuel and post back on here.

    On the pic (if it comes up) check across the bottom pair
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    If it ain't broke, take it apart and see how it works

  8. #8
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    pin 3 and 4 the black black ones should be 10 to 40 ohms according to 4 wire oxy sensor on autodata

  9. #9
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    Is that the working range?
    More to the point does anyone know how the blighter works. 4 wires suggests to me that it's not a simple variable resistor like the temp sensor but I have no idea how it does it's stuff. Time for a google I think.
    If it ain't broke, take it apart and see how it works

  10. #10
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    yes that is the range as for how it works in greater detail cant help ya

  11. #11
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    Well I can definately say changing it has done something. I took it for a longer blast and gunned it. Whilst it still reved through, the engine warning light came on at 7000rpm. I dropped back and switched the ignition off then on again and all was fine until I gunned it through 7000 when the light came on again. It now idles at 1000-1050 whaere as I had set it spot on to 1200 with the other sensor and had no warning light ever.
    I'll reset the idle tomorrow.
    If it ain't broke, take it apart and see how it works

  12. #12
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    Could it be a wheatstone bridge type sensor? These tend to use 4 wires. (i'm not that familiar with car sensors, but in industry this type is often used for gas, temp or flow sensing)

    I think you can attach two other wires to the 'corners' and measure the current drawn. (its been about 12 years since i had anything to do with them!)

    PW
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    'Beatless for 4 years now

  13. #13
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    Originally posted by tinytim
    Whilst it still reved through, the engine warning light came on at 7000rpm. I dropped back and switched the ignition off then on again and all was fine until I gunned it through 7000 when the light came on again. It now idles at 1000-1050 whaere as I had set it spot on to 1200 with the other sensor and had no warning light ever.
    The same thing has been happening to me in the last few weeks. I haven't had a chance to check the codes yet.
    I have a dream

  14. #14
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    Just wondering if your timing was done correctly? a beat I done some work on had that problem and was sorted by doing the timing- just a thought

  15. #15
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    There is no flashing, the light comes on with the ignition then goes out after a few seconds as normal. It drives fine until I take it past the 7000rpm mark. It wasn't doing this with the other O2 sensor on but then I treeked the dizzy to give 1200rpm at idle when I did the timing belt.
    All I did yesterday was change the cat complete with sensors. (The cat that is on now is the one we did in Fishguard). The car runs better (has more willingness) but at 7000 the light comes on and stays on until I switch the ign off. It also idles at 1000-1050 when warm so I'm gussing I just need to put the dizzy back to where it was.
    I may get a strobe and do the whole lot just to be sure.

    Paul,
    Is there any chance of a "common jobs" page. 1st post on each thread is how to do the job and subsequent posts can be tips, where to get substitute bits etc.
    Can I suggest:
    1 Timing
    2 Timing belt
    3 Radio clock setting
    4 Brake pad changes
    I know all the info is on the pages but it can be a pain searching (especially when looking for ECU, cos it's only 3 letters long )

    Anyway, just a thought.
    If it ain't broke, take it apart and see how it works

  16. #16
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    Mmmm, probably a good idea a 'How to..' thread with sticky guides.

    I can start the threads but it would be up to you lot to fill them in!

    PW
    'Beatless for 4 years now

  17. #17
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    That works for me. It'll be a cut and paste for the most part but at least we'llonly have to find the info once
    If it ain't broke, take it apart and see how it works

  18. #18
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    Sorry to join the thread late..... I'll take this up w/ Honda techs soon, see if they have any ideas 4 ya!


    Cheers

    Oz

  19. #19
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    Cheers Oz, I hadn't got round to e-mailing you yet

    Well I started with a diagnostic and got Oxy sensor and No 1 position sensor. No real surprises there, I'm pretty sure I started it with the oxy sensor disconnected and I'd tweaked the dizzy.

    Reset the ecu warmed her up (no warning lights came on) switched off, bridged the diagnostics and checked the timing. It was a good way of, I could only just see the timing mark in the rear of the window
    Reset that at 1200, switched off, removed the bridge and started. She now runs at a steady 1200.

    I whipped the plugs out,,,definately been running rich, rather a lot of carbon.

    I'll let you know on the fuel consumption but it's looking good. I'm still of the opinion it's the oxy sensor failing to flick to a lean mix but from what I've found so far I'll need to get an osciliscope to check it???
    Oz i hope you can come up with a static check, I've been told you can heat the Oxy sensor and it should produce a voltage, can you confirm this ?
    If it ain't broke, take it apart and see how it works

  20. #20
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    Static testing is quite difficult and really not conclusive for the O2 sensor, you can run voltage through it to check heater resistance values with a tester, maybe check for operation (voltage variation) by spraying propane gas, but all the experts say its best to just throw it on the car and do the test. For static testing you'd have to rig a bench probably just for it. Of course the resistance value for each model is different, and no one here knows the Beat's as its a little old... sorry!

    Oz

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