Hi all! I would value your comments on the following strange engine behaviour:<o:p></o:p>
Today when quickly accelerating to overtake another car (going uphill in an overtaking lane) and accidentally took the engine revs on my Mk3 1.8 RC up to about 6500 rpm in 3rd gear: then suddenly the engine revs started to rapidly pulse up and down between 6250 and 7250 rpm; the car continued at speed and I rapidly caught up a car in front, even though I had taken my foot off the accelerator! I tried dabbing the accelerator and but that didn’t seem to stop it, so I de-clutched and braked, then hit the pedal again. Some time in the middle of all this desperate fumbling, the problem went away. The pulsing lasted only about 20 or 30 sends and then the revs dropped back to a much lower level. I was quite taken aback at this odd behaviour so drove home carefully, listening to the engine for any strange noises. Despite this odd occurrence, the engine sounded OK and drives OK now. <v:shapetype coordsize=“21600,21600” o:spt=“75” o:preferrelative=“t” path=“m@4@5l@4@11@9@11@9@5xe” filled=“f” stroked=“f” id="_x0000_t75"><v:stroke joinstyle=“miter”></v:stroke><v:formulas><v:f eqn=“if lineDrawn pixelLineWidth 0”></v:f><v:f eqn=“sum @0 1 0”></v:f><v:f eqn=“sum 0 0 @1”></v:f><v:f eqn=“prod @2 1 2”></v:f><v:f eqn=“prod @3 21600 pixelWidth”></v:f><v:f eqn=“prod @3 21600 pixelHeight”></v:f><v:f eqn=“sum @0 0 1”></v:f><v:f eqn=“prod @6 1 2”></v:f><v:f eqn=“prod @7 21600 pixelWidth”></v:f><v:f eqn=“sum @8 21600 0”></v:f><v:f eqn=“prod @7 21600 pixelHeight”></v:f><v:f eqn=“sum @10 21600 0”></v:f></v:formulas><v:path o:extrusionok=“f” gradientshapeok=“t” o:connecttype=“rect”></v:path><o:lock v:ext=“edit” aspectratio=“t”></o:lock></v:shapetype><v:shape type="#_x0000_t75" alt=“Confused” id="_x0000_i1025" style=“width: 14.25pt; height: 14.25pt;”><v:imagedata src=“file:///C:DOCUME~1RogerLOCALS~1Tempmsohtml11clip_image001.gif” o:href=“http://www.mx5oc.co.uk/forum/tiny_mce/plugins/emotions/img/original/emotion-7.gif”></v:imagedata></v:shape><o:p></o:p>
Has anyone heard of this happening before? My concerns are that it might happen again, or that there may be some engine damage <v:shape type="#_x0000_t75" alt=“Sad” id="_x0000_i1026" style=“width: 14.25pt; height: 14.25pt;”><v:imagedata src=“file:///C:DOCUME~1RogerLOCALS~1Tempmsohtml11clip_image002.gif” o:href=“http://www.mx5oc.co.uk/forum/tiny_mce/plugins/emotions/img/original/emotion-6.gif”></v:imagedata></v:shape>.<o:p></o:p>
Thanks for the link Goeff: an iInteresting read, but I am not sure if a loose car mat was the cause of the problem. I do have mats that are the fitted (and clipped in place) luxury MX-5 mats. But I do ride with a rubber mat on top, so there is a possibility that that was the cause. However, I recall glancing down at the mat when the throttle seemed to be stuck on full and I thought that it was clear of the pedal - but I cannot be certain as it all happened so quickly and I was in an exposed position on the road with cars all around me.
Having pondered the problem overnight, I am now thinking that the throttle was stuck open at maximum and that the engine was hitting the rev limiter and cutting back, then the throttle kicked in again and revved the engine back to the limiter, and so on. It’s only a theory and I am loath to try and reproduce it. Does that sound plausible? And what is the likley failing/sticking component? Can it be tested? I am not discounting the mat theory yet, but any additional comments would be appreciated.
I really can’t help with this one, I know very little about the mechanics/electrics of the MK3
However, if it were a Mk1 I would have said that hitting the rev limiter does give exactly those effects, I know 'cause I’ve done it many times. Is the throttle on a Mk3 an electrical or mechanical connection?
You can check for a sticky throttle pedal while parked up with the engine off. Try flooring it and see if it gets near the mats, then try pushing the pedal down at an angle in case there is wear at the pivot that jams it.
If it isn’t a mechanical issue with the pedal it could be an electrical issue with the pedal sensor and associated wiring and ECU. Christ knows how you would safely check any of that.
Thanks Captain, I will take a look at the pedal and mats again in the next few days when I have a bit of time on the car.
I am not expecting any wear as the car has only done 4.5k miles but is now out of warranty.
Not having ever seen a diagnostics check done on an Mazda, I can only guess that such a check might reveal something regarding an ECU/electrical fault. But I guess such a check would not be FOC! My Golf recently had a sticking throttle at a 2K miles when it kept loosing power and stalling. VAG diagnostics pinpointed the problem as the motorised throttle unit, and fortunately for me that very expensive assembly was replaced under warranty - an hour to diagnose the fault, an hour to change the assembly and a further hour to run another diagnostics check! Gosh, life used to be much simpler before we had all that electronic gubbins replacing a simple and well proven mechanical linkage - the price we pay for controlling emissions, I guess! Sigh …
I am planning a longer trip at the weekend so am praying that it was a ‘user error’ glitch that will not repeat!
After the interesting suggestions so far, I took a good look at the car mat area and I can’t see how the rubber mat that I have had on top of the fixed Mazda carpet-mat for 4 years could interfere with the accelerator pedal, i.e. in terms of causing it to jam on at full throttle; IMHO, it looks as if it is more likely to get underneath the pedal than on top of it. The Mazda carpets still feel to be very secure.
As suggested, I also tried depressing the pedal with the engine off but could not discern any tendency for it to stick; it seemed to travel smoothly.
So, I am none the wiser!
My knowledge of engine control systems is very limited, but I guess my main concern is that there is a problem with the pedal potentiometer (I assume it is a fly-by wire system, yes/no?), or with (if electrically operated) the motorised throttle control. Does anyone know if the Mazda diagnostics would reveal anything helpful?
Hi, 2 years after your post, I have just had an identical experience! Like you I accelerated in 3rd to overtake, took my foot off the gas to change up but realised the car was still accelerating hard, with the engine revs soon pulsing repeatedly at the top of the range just as you described. Luckily I had a quarter mile to sort things out before the next bend, but it was a bit worrying to say the least. I left the car in neutral and braked until a safe speed was achieved, then managed to hook my toe under the accelerator and normal service was resumed. This after maybe 7 seconds of the very distinctive pulsing at high revs, presumably bouncing off the limiter.
When I got home I checked the operation of the accelerator pedal and it is normal. But the floor mat was further forward than usual and I concluded the pedal did get caught under it. At first I thought the mat wasn’t heavy enough to hold the pedal down on its own, but after some experimentation I found that if the fully depressed accelerator pedal abuts the leading edge of the mat exactly, it WILL stay fully down on its own, even without the weight of the driver’s heels on the mat. And I can reproduce this state quite easily. With a genuine Mazda mat - the sort with one fixing hole. But I then realised there is no retaining clip fitted to my car floor, so that was the cause of my problem. I confess that after I bought the car last year I had looked and failed to find the fixing point, then forgot all about it…
My mats were petty tatty anyway so I’m getting a new set of original Mazda mats with the correct fixing kit.
I thought I’d better post this to warn others to check their mats are fitted securely. Someone else might not be so lucky.
Thanks for the above post, same thing happened to my dads Rover 400 some years ago, it was a heart stopping moment for us all when it happened. Since that day i double check the drivers mat is secured correctly.
