Stumble and juddery gear changes



  • Hi everyone, I have some problems with my mx3 v6, its a 1998, my problems are these…

    When the engine is idling, if I tap and release the throttle the engine revs will drop, then bounce up to about 1500 and then back to idle.

    Also, when I change gears, changing from 1st to 2nd can be wildly jerky, if I let the clutch out really slow the problem isnt so much but you still get a slight jerking. When I change from 4th to 5th the engine will apply excessive braking, its almost like someone has switched off the engine for 1/4 of a second. The engine doesnt die, but it feels like Im trying to bump start it or something.

    I checked the TPS sensor for continuity between 0.15-0.50 and it was fine.

    It makes pulling away sometimes very aggressive as you will have no power when your on the biting point, apply more throttle, but get no response, so dip the clutch, engine will rev to however hard your pressing the throttle (which usually ends up being quite alot since theres no increase in revs) and then you can pull away, sometimes Ive ended up wheel spinning across roundabouts just because of it.

    Its something I really need to find out why/whats wrong as its ruining my enjoyment of driving. I love the car, but coming to a stop, where Im going to have to engage 1st and pull off is pretty offputting right now, and then theres changing into second so I can bounce around the inside of the car like a wasp in a jam jar!

    I admit that I am very new to driving cars, but my girlfriends Astra 1.6 which is the same age as my mx3 is an absolute doddle to drive (infact, I passed my test in it with just 2 minors, both on undue hesitation) Ive owned 7 cars since I was 17, and Ive owned 6 bikes, none of them have ever behaved in such a bizarre manner, that said, this is my first V6, but I doubt V6's should behave this way anyway.

    Oh, and when I turn on an electrical load (ive only tested it with lights and rear defogger) the engine revs will drop, then go back to how it was a prior to turning on the load.

    Thinking maybe bad earths, will get the volt meter out and check for resistance, in the mean time, anyone else had anything like this, if so, how did you resolve it?

    Any help would be greatly appreciated!

    Cheers, J



  • sounds dangerous to drive lol!!, well wouldnt want to pull out of a busy junction like that anyhow.
    No mx is not difficult to drive at all, definately something up.
    Wonder if it is a battery/alternator fault not getting enough juice out and upsetting things??.
    Get it on a jump lead running off another car and see if problems are still there at idle. Hopefully they will dissapear proving you arnt get enough juice out of yours and then start working out whether its alternator or battery.
    Im guessing perhaps battery, as alternator will use battery to keep a constant voltage when loads are applied to the alternator. So battery should be putting out 12-14v, when alternator is put under load it will use the battery to keep the voltage constant. As soon as voltage drops below 10v it will cause a world of problems to anything requiring voltage especially ecu.

    Worth a go, doesnt cost anything, well so far atleaast :D



  • While im querying my car…

    is anyone elses throttle pedal super light? In 3-4 gear itll get upto 5k rpm without being anywhere near the floor. How does one check that the idle stop screw is adjusted correctly? If that makes sense, basically what I mean, is, where should it be, if I tweak it a bit the revs increase (as youd expect) so itd be better to have it so that it doesnt effect idle, which would mean having it so its wound out and doesnt actually touch the throttle arm, but that would surely have a negative effect, what im not sure and dont wish to find out, but I dont really get what its for. On bikes the throttle stop is also the idle setting screw, but on the mx (cars in general? I dunno) you have the IAC, so whats the purpose and what should I set it to?



  • @9849558610=Garfy1981:

    sounds dangerous to drive lol!!, well wouldnt want to pull out of a busy junction like that anyhow.

    Its not as bad as it sounds, Ive done 320miles so far and not hit anything, but its not as second nature as youd hope it to be, Im constantly aware of what Im doing or will have to do in the next few moments. Something is definately a miss, but what I dont know, tbh if someone was closer to my area Id be happy for them to take it for short drive across town, then theyd be able to tell me if its absolutely fine and its just me having trouble making the adjustment from bike to car or not, but as I said, I can hop in my missus's Astra and drive it without a second thought, it doesnt dip from idle, it pulls strongly and you can feel all that your doing and monitor it instinctively, but the mx is lacking any of that, its almost like paying a driving game, except with a car that hates 1st gear. Oddly, put it into second, and after the wild lashing about, you can drive it like its in first gear, Slowed down to a snails pace, then pull away with no problem, it seems to not like having the clutch engaged/disengaged, once its in gear its happy,



  • i would advise checking for air leaks. also if u adjusting the tps and idle make sure you shorrt ten and gnd otherwise your wasting time. if all else fails new vaf maybe??



  • I had a look but couldnt find anything, have read about using butane gas for checking for leaks. Did hear what I though was a leak, only to find out that it was coming from the air box so thats probably just the normal sound of the engine intake. The intake tube looks pretty new, and I couldnt find any damage on it, though it did make me wonder about how the PCV effects the intake as thats after the VAF so thats an unmetered supply of air, therefore could technically be classed as a leak. Trying to think of a way I can route it so that the PCV vents before the VAF not after it.



  • @2c283ab8c2=Jesta:

    While im querying my car…

    is anyone elses throttle pedal super light? In 3-4 gear itll get upto 5k rpm without being anywhere near the floor. How does one check that the idle stop screw is adjusted correctly? If that makes sense, basically what I mean, is, where should it be, if I tweak it a bit the revs increase (as youd expect) so itd be better to have it so that it doesnt effect idle, which would mean having it so its wound out and doesnt actually touch the throttle arm, but that would surely have a negative effect, what im not sure and dont wish to find out, but I dont really get what its for. On bikes the throttle stop is also the idle setting screw, but on the mx (cars in general? I dunno) you have the IAC, so whats the purpose and what should I set it to?

    The idle stop screw is VERY likely your culprit! I had this exact problem this week, and that(as well as a dirty throttle body!!) were the major factors.

    I recommend you properly clean your throttle body(you can find a guide pretty much anywhere), adjust your TPS(throttle position sensor), and then remove the throttle stop screw. Let the butterfly valve close until it bumps the TB, then adjust the screw in small increments until it barely pushes the butterfly valve open. If that doesn't solve your problem, slowly adjust your throttle cable until you can get about 1/4" or 8-10 millimeters of play with the throttle at rest. This should solve your problem; If it doesn't, then I recommend you try and unplug your IAC. If you do unplug the IAC, be careful when you warm your car up on cold mornings. Give it extra time, just to be safe. It won't start up at high RPMs anymore, because the temperature override won't kick in.

    Good luck!



  • Bane92> Genius, problem solved! The stop screw wasnt doing anything, grounded Ten and Gnd, let it warm up a little, wound down the AAC, then wound down the stop screw to where I wanted the revs, took it out of diagnostic mode, the revs dropped a little, so adjusted the AAC 2/3 of a turn and its now perfect, the AAC was wound out 7turns!!. Took it for a drive, absolutely sweet as can be! Had to adjust the TPS after moving the stop screw. Checked at .15mm then .40mm and then .50mm at .40 there was still continuity but at .50 there wasnt, so the TPS is set great aswell.

    Your my hero! Thanks mate, oh, and welcome to the forum, certainly got off on the right foot with me! :D


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