Undriveable due to massive flat spot below 5000rpm



  • N00b question ( and I have looked through the forum trying to find an answer)

    I picked up an MX-3 off eBay a few nights ago. Had been advertised as having simple fault, slight hesitation in low gears/low speed. Hadn't been driven for a couple of months. Then there was an addition to the listing saying it had been driven for a while that day and fault seemed to have disppeared.

    I paid for it, drove it at sensible speeds along New Forest roads, no hesitation on acceleration at all. Got to the roundabout for the motorway, tried to get it sideways, accelerated up the slip road and kept it between 70 and 90 (indicated) for around 25 miles before coming off motorway. Popping and banging, wouldn't rev above idle unless very gentle pressure on throttle, no power until about 4000 revs.

    Filled tank as appeared to be close to empty and set off again on suburban roads, flat spot rendered car undrivable. Called AA who could only drive behind me with flashing lights as I kangarooed, stalled and generally stuttered my way home.

    Following morning 05:45 took it three miles to layby outside mates house so I could borrow one of his vehicles to go to work. Started straight away but no power below 5000 so drove most of way in second.

    Since then it has sat in layby. Had a go at getting fuel filter out yesterday, (AA Suggestion) filter looks a lot cleaner on outside than stuff around it and, if it were blocked how come I can keep it a 5000 revs ok?

    All hoses look in good nick, concertina hose has no cracks. Battery fairly flat after starting attempts so took it home to charge

    Put freshly charged battery on this afternoon, fired on first turn of key, idled at 1500 dropping to 800 or so when warm. Flat spot evident when accelerating in neutral unless taken very gently up the rev range.

    Disconnected TPS but no difference. Removed TPS to try and see what it was doing, Chilton manual is giving some bollo**s about measuring voltages with an ohmeter. Looks as if about 30 ohms resistance when closed and going to infinity about 1/8th throttle. Is this right?

    Any obvious things to recommend before I get the soldering iron out to make a test lamp and some jump wires?

    Thanks in advance



  • I saw that on ebay as I am in southampton and nearly bought that car

    Where in southampton are you I maybe able to help??

    I would check ignition leads for cracks or splits as that caused the same for one of mine in the past, it eventually broke down completely due the electricity leaking out of it

    Take each lead out of the sockets one at a time and look for cracks in the plastic case in the engine end, also check for oil in there if one or 2 are full of oil (common fault) they can cause a short causing the same problem. If they are get a new set of plugs and replace, then when you change them the oil drains into the cylinder and it should fire up immediately - that maybe the cause
    Another one for jumping and flat spots etc is dizzy, change the cap and rotor arm and check for cracks on the plastic and burning on the metal contacts



  • Ditto DJ. I had a miss fire due to the same problem. Had a nasty flat spot. If you find that you have cracks in you HT lead(s). Use some sello tape as a temporary isolator. Worked for me :)

    Also the VAF may require removal and cleaning. As well as the throttle body. Check the inlet pipes for any perishing or cracks. A simple crack can cause alot of head aches.

    Hope this helps.



  • Thanks for suggestions so far guys,

    Car now in Hayling Island, will pull all plugs and replace as a set, got 12 plugs with the car.

    Had a good look at all the inlet and hoses all seems fine, VAF moves easily by hand with no feeling of tightness anywhere in its travel. Will give it anther try tomorrow evening after work



  • Don't wanna sound like how to suck eggs, As DJ suggested take one lead off at a time but with engine running, if no change in running/revs, then it will highlight fault with lead/cylinder/plug etc



  • Sounds like spark trouble a bit to me. I'd borrow DJs disty and see if the coils on it's way out.



  • Thanks again for suggestions.

    One query - if it were problems with sparks wouldnt that cause problems at any speed?

    This idles fine. Over 3k or more it is smooth.

    If very gently accelerated from idle to 3 k it is smooth

    When accelerated like normal it has a horrendous flat spot - possibly more like air/fuel mix?



  • For the sake of 30 mins of fiddling i'd find someone local and borrow their TB inc TPS and IAC, induction pipe and VAF that would rule out the whole lot in one go.
    Find someone willing and start swapping bits out.

    So many things can cause this you just have to rule them out one at a time so start with the easy and free stuff before you go buying stuff that does not need replacing.

    Do the basics remove induction pipe and check for cracks, check all vac lines breather pipes and fuel pipes for kinks or leaks, smell injector area for leaks, ground test the plugs, check the fuel pump is working.

    Is it worse when cold or hot or the same?
    Does it idle ok if left? is it stable or hunting?
    is there any misfire? listen next to the sxhaust for one at idle.



  • @4a56a0479e:

    Is it worse when cold or hot or the same?

    Seems to be the same hot or cold

    @4a56a0479e:

    Does it idle ok if left? is it stable or hunting?

    Idles fine, initially at 1500 when cold, dropping down to 800 warm

    @4a56a0479e:

    is there any misfire? listen next to the sxhaust for one at idle.

    No misfire ate idle, no misfire once above 5000. No misfire if very gently accelerated in neutral, misfires really badly or doesn't pick up at all when accelerated normally in neutral.

    Car is parked in layby on busy road so can't test for acceleration when driving in any safety.



  • @9efc1501d5:

    Is it worse when cold or hot or the same?

    Seems to be the same hot or cold

    @9efc1501d5:

    Does it idle ok if left? is it stable or hunting?

    Idles fine, initially at 1500 when cold, dropping down to 800 warm

    @9efc1501d5:

    is there any misfire? listen next to the sxhaust for one at idle.

    No misfire ate idle, no misfire once above 5000. No misfire if very gently accelerated in neutral, misfires really badly or doesn't pick up at all when accelerated normally in neutral.

    Car is parked in layby on busy road so can't test for acceleration when driving in any safety.



  • hi mate think i have seen your car parked outside the church ?! I am on creek road but i don't have any vsix parts I'm afraid but if you need any tools or a hand gimme a yell



  • @youdirtyfox

    Great, thanks for the offer! And, yes, it is the one outside the church :P



  • Do all the basic checks to rule them out, then….

    Could be the VAF or TPS.

    If it was crank sensor I wouldn't expect it to still work ok at higher revs, same for the fuel pump.

    Is there anyone near enough to pop down and swap some parts over?



  • Might be worth checking for leaks around the variable resonance induction system (vris), those should open at idle but then close after about 1500 then open again at 4000, cant remember the exact values but not far off, its odd to have something effect such a large range of the revs, but the vris does effect a large range of the revs and with v6 engines its very common to have a flatspot thats due in part to the way a v6 works and its need for a variable induction system.

    At idle itd make minimal noticeable difference as theres no load on the engine, but once load is applied and the vris is in the wrong state for the rpm/demand then the fuel to air ratios would be squiffy and bogging your car down ie flatspot.



  • This is now fixed and currently relisted on fleabay



  • Sorry for late replies to you all but have been busy with other things. Car is now fixed and on eBay, 20 watchers but no bids at yet :(

    I checked out the induction system, changed the fuel filter and went to the diag box after making up a test lamp. Unfortunately the box was unmarked and it took me a while to find a pin out for it.

    Plugs were changed and I have been driving it daily for the past week to see if any problems recur. Take the long route along Southwood Road Hayling just to check on the MX-3 parked there.

    I have updated the ebsy ad to point out that if no bids received by Thursdasy it is being withdrawn, I am very impressed with the car now it runs well and will put an MoT onit and keep it.

    thanks to you all dor your helpful suggestions, guided me in the right direction and taught me a bit about modern electronics.

    My other car is a 1954 Daimler Ferret



  • Do we know what the fix was?



  • First off tried to find any air leaks in induction area - no leaks and nothing different when put back together.

    Battery then needed charging - all the while the car was in the layby outside Hayling Church, about 3 miles from home so there were brief visits each time I was passing.

    Tried it with newly charged battery, started ok but hideously flat before about 5k when accelerated hard, would run ok with very gentle acceleration.

    Found it impossible to try and hold multimeter probes on TPS while playing with it on the car so that came bake home to be tested, next the VAF - both tested out ok.

    Ordered a fuel filter from Japarts though filter on car seem clean and shiny on outside, slight improvement but still hesitant.

    Pulled all six plugs, a couple of tubes had oil in them. Replaced plugs and it seemed a lot better. Been driving around the island and brief bursts on the M27 and seems fine.



  • So it was a misfire?



  • Originally it was far more than a misfire.

    I picked the car up and it seemed fine. Got on the M27 at junction 1 and came off at the base of the A3 to go the back way to Tesco to put some fuel in. At that point, pulling away from the traffic lights, it failed to respond to the throttle. It was just driveable by accelerating to 3000 in neutral, putting it in gear and dropping the clutch.

    After filling to see if fresh fuel would help it was just as bad and became worse when trying to get onto Hayling. Eventually stopped to call the AA who were unable to do anything except drive behind as I kangarooed the rest of the way home.

    I think it could be a combination of low fuel/ bits blocking the filter (though it was fairly new), tired plugs and removing, cleaning, replacing components making the electrical contacts more secure.

    No one fault as such but just a combination of things which together made the car sick. As each cause was investigated and eliminated the car got progressively better.


Log in to reply
 

Looks like your connection to UK-MX3.COM was lost, please wait while we try to reconnect.