Strange and frightening car problem - any ideas?

[Velorum]

Mrs V returned home from work about an hour ago and was very shaken.

She was travelling home earlier on the M4 in her 2007 MK3 MX5, gently accelerating in 5th gear from 60 to 70.

However the engine kept increasing in revs when she eased off the accelerator and she then took her foot off the accelerator completely. Revs and speed kept increasing and the car was very quickly at 85 and still accelerating.

She started to press the brake pedal and it felt hard it wouldn't move and the car was still accelerating.

She then pressed the clutch in and hit the brake pedal as hard as she could - moving it down into 4th to try and get it to slow down. It then started to slow down but with very high revs. There was a smell of burning rubber.

After making a terrific noise (over - revving I suspect) she describes the hard brake pedal as suddenly freeing up and working as normal.

She continued the remaining 5 miles to home at a low speed.

I have just taken the car out along the local country lanes taking it through all of the gears and accelerating hard then the motorway at up to 85. Everything seems fine.

I have a Mazda service centre just round the corner and I am going to book it in there for a thorough check - in the meantime it will remain on the drive.

Meanwhile has anyone got any ideas re this?

I cant think what would affect throttle control and braking function at the same time in this way.

[JasonM]

Nightmare! Was the floor mat stuck over/ under the pedals ?

[stefmcd]

V scary. As you say, weird combo of affecting throttle and brake. My hunch is electrical issue but I'd go on to pistonheads and describe it to see if you get answers there.

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[Papa Hotel]

Accelerator stuck? If not the pedal itself then a bit of the throttle system maybe just hung up a bit. What exactly I don't know, but I'd say worth getting it looked at, you don't want that happening again.

The burning rubber smell won't have been rubber, it'll be clutch if she had it depressed at the same time as the throttle was going crazy.

[Velorum]

The rubber matt issue was the first thing I asked about - all was OK with this though

[notnowkato]

My wife has an auto gen 3 mx5 and the overmat now moves around quite a bit as the securing pegs have gone missing, might be worth checking.

[Robbo12]

Off the top of my head i would say its the ECU, this is linked to abs system so thats why the brake pedal felt hard.

My thinking is that when you took it out everything was fine, so sounds electrical, turning it off and then starting again would reset ECU. I would get online and find a pic of ECU and check all plugs around this area, they could be loose !

You could also disconnect the battery completely and try starting ( completely drain the electricity ) as this will hard reset ECU.

Ps make sure you have any codes for sound system Sat nav as they wont work without

[Velorum]

Thanks for that

My main fear is that it is an intermittent fault and the garage cant find anything wrong - I am reluctant for her to drive it again in that case

[zelig]

I had an old Renault 5 that did something similar...

In the days before drive by wire - the throttle cable linkage got caught on a rubber water hose that ran just above it. One of the hose clips failed, so it sagged & at higher throttle the mechanism got stuck under the hose. Easy fix - but was a bit disconcerting at the time.

Luckily, Renault had a built in safety feature ... a pathetically puny 1.1 litre engine.
...so there wasn’t enough power to accelerate too quickly...



z

[meridian]

That has potential to be a split induction pipe somewhere- thereby increasing the intake air volume and upping the rpm without throttle and at the same time depriving the servo of manifold depression. Not a given but sure a possible.

[Markw]

I had something similar in an E30. Took a while to find exact cause. Had a new air flow meter fitted which helped. New temp sensor too.

[Beanie]

You could also disconnect the battery completely and try starting ( completely drain the electricity ) as this will hard reset ECU.

Ps make sure you have any codes for sound system Sat nav as they wont work without[/QUOTE]

In the Land Rover world, a hard reset is achieved by disconnecting both battery terminals and joining them together (I have used jump leads). This will discharge the electrical system. Make sure nothing else goes near the battery or terminals when you do this. There may be a different procedure in Mazda land of course, forums generally will have this type of information.
I can understand how this event would upset your wife, it would shake anyone up.

[Huertecilla]

Off the top of my head i would say its the ECU, this is linked to abs system so thats why the brake pedal felt hard.

The power assist is operating with inlet ´vacuum´.
With the throttle for some reason stuck open, there is no low pressure behind them.
After braking the first time, the vauum is used and then there is none, changing the brake system in a non assited brake. That would feel hard wioth hardly any braking.
It is a secondary effect unrelated to a cause.

I would first look at the throttle cable and then throttle body.

I can imagine it is unsettling. Personally I would not worry too much about it as that gen. MX5 it is not crammed full of electronics and it will not be a ´Gremlin´ thing.

Again; check the whole throttle system. Take it apart, adjust/lubricate/replace as you see fit.

Meanwhile; swap cars. She drives your MX, you drive hers. The confidence thing is serious.

Btw. compliment, sérious compliment to Mrs.V.
Quite a cool head and capable of coming up with different driving solutions.
Chapeau indeed!!

[Foxy100]

Don't bother with Mazda, take it straight to Skuzzle Motorsport just outside Winchester. Nick, who runs the place, is a genius when it comes to ECUs and he knows everything there is to know about Mk3s. Give him a tinkle on Monday on 01962 776 167.

[Robbo12]

If there was no check engine lights its something stupid as below on the induction side or ECU . You are right about taking it to garage as unless there are any stored codes then they wont have a clue

I would pop the bonnet and start checking all connectors any corrosion ect then see if you can see any splits in the rubber vac lines , although I would have thought you would be getting a lumpy idle if it was one of these, but it could be as the pressure increases under throttle there is a small hole causing the problem.

ECU is normally inside the car or outside of the engine bay not sure on a MX 5, but check plugs ,take them apart if poss any crap corrosion ect , clean them up with some contact cleaner.

Just a quick story that makes me think its this. My step sons boss had a year old Mercedes Vito this had the same problem in the outside lane of the M4 coming upto stationary traffic, Bloody frightening ! Mercedes diagnosed faulty ECU ,needless to say he sold the van just after getting it fixed as he no longer trusted it.

[Mr Curta]

Almost certainly possessed; probably just got bored with the bedside lamp trick.

[learningtofly]

My money’s on the mat.

[RABbit]

My money’s on the mat.

Yep, Occam’s razor.

[petethegeek]

My money’s on the mat.

My first MX-5 was a UK SE Jasper Conran which included an upgraded interior. It was eighteen months old when I bought it and on checking I found that they had been subject to a mandatory recall for the driver's floor mat. So yes, certainly worth some close scrutiny.

[Huertecilla]

My first MX-5 was a UK SE Jasper Conran which included an upgraded interior. It was eighteen months old when I bought it and on checking I found that they had been subject to a mandatory recall for the driver's floor mat. So yes, certainly worth some close scrutiny.

Never heard of that version (nor of the designer), thanks.




I have taken the froor mats out of my MR. They always seem like a suit over a suit to me. In a 4x4 used in the mud a set of rubber ones with raised edges makes sense but carpet on carpet? There´s even a wear resistant rubber pad under the pedal box!

[Velorum]

I also posted about this on the MX5 OC forum and received lots of suggestions relating to ECU's. split induction pipes etc but the most common suggestion is about the floor mats. A few have described exactly the same problem occurring.

I fitted heavy duty rubber mats from MX5 parts to mine and Mrs V's cars some time ago.

I did have similar experience in my MK2.5 after it had been valeted - however the mat was over the pedals as the cleaner had just rammed it in haphazardly. This would not be the case with Mrs V's MK3 - any problem would need to under the upper surface of the pedals

This morning I am going to fiddle about with the mat and see if I can replicate yesterdays events.

If I can then its simply a case of taking the mat out!

I have not completely ruled out demonic possession or witchcraft on the part of my previous wife of course.

[farquare]

Gamma rays, seriously.

2nd half of this podcast is terrifying:

https://www.podtrac.com/pts/redirect...bitflipfix.mp3

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[Liner33]

I cant think what would affect throttle control and braking function at the same time in this way.

No single fault would, as with others 99.9% sure this will be floor mat related

[Franky Four Fingers]

If the throttle was sticking for whatever reason in an attempt to slow the vehicle down i.e. Multiple brake pedal presses she would have exhausted all of the vacuum from the brake servo......the pedal would go hard. I suspect when she knocked it in to lower gear and started to slow the car down she was probably off of the brake which would have again allowed vacuum back in to the servo and normal operation would have returned. MX5s aren't my forte but I believe them to be cable operated throttle? In which case I'd being looking at the mechanism p, disconnecting it and making sure it's not sticky

[Liner33]

If the throttle was sticking for whatever reason in an attempt to slow the vehicle down i.e. Multiple brake pedal presses she would have exhausted all of the vacuum from the brake servo......the pedal would go hard. I suspect when she knocked it in to lower gear and started to slow the car down she was probably off of the brake which would have again allowed vacuum back in to the servo and normal operation would have returned. MX5s aren't my forte but I believe them to be cable operated throttle? In which case I'd being looking at the mechanism p, disconnecting it and making sure it's not sticky

MK3 MX5 is drive by wire

[Huertecilla]

If the throttle was sticking for whatever reason in an attempt to slow the vehicle down i.e. Multiple brake pedal presses she would have exhausted all of the vacuum from the brake servo......the pedal would go hard.

See above....

[Huertecilla]

Why????? have they become just about ´mandatory´?

Ok, in a 4x4 used in the mud, it makes sense to have a rubber matt with high sides collecting the mud, keeping it off the carpet.
But in a road going vehicle, why have carpet on carpet.
I mean the factory carpet already has wear resistant rubber inserts under the pedal box.

Not even all thát long ago, base model cars did not have any carpeting, only rubber matts. Carpeting was a luxury which only about mid seventies evolved with a sound deading function to become essential and now we have a flying carpet on top of that carpet.

What´s more, they cause a lót of potentially véry hazardous problems, like causing the throttle pedal to stick. There is a lót about that on mx-engineinwrongplace fora p.e.
I myself have not experienced it but clutch pedal issues yes.
What´s not fitted can´t cause problems, so mine are out but remains the why do we, me included, per default see it as an essential?!

[Velorum]

Well Ive fiddled about with the mat a while:

Simply pushing it forward as far as it will go doesn't seem to interfere with the pedals in any way that I can see

If the end is folded back on itself and then rammed all the way forward it does interfere with them but try as I might I couldn't get the accelerator to stick open

Putting the mat on top of all of some of the pedals has the biggest effect and will make the accelerator stick open especially of the brake or clutch are depressed - however I cant see how Mrs V would have not been aware of this - and she did say that she checked the mat after she had slowed down.

She was in an anxious state though and perhaps with all the frantic pedal pushing and gear changing it had flopped back into place?

Ive removed it from the car anyway

[Huertecilla]

Ive removed it from the car anyway

Common sense prevails!

Did you try while seated?
Point is that when it is pushed forward against the pedal arm, the weight on the feet keeps it there. As the return spring is rather light, that easily looses out.

Fingers crossed it was the issue.

´Thanks´ for raising the issue btw.
I´ve been thinking of taking the carpeting out altogether and drop in pieces of those silicon anti slip kitchen cupboard matts.
There is only 5 kg. of it in the MR and no heavy insulating rubberfoam so it did not seem worthwhile but it is not a lot of work either and 5 kilos are 5 kilos, not counting accumulated sand/dirt, for free!
Also, a lót easier to vaccuum/clean.
Next week.

[Robbo12]

I'am glad your other half is ok , it must have been quite frightening !

I worked in a garage as a apprentice and the mechanics I knew ALL told me that the most reliable car is a Petrol Japanese !

I 'am wondering if the two problems are linked , under fuelling , oil starvation (obviously) for it to go bang. No doubt you will get the full investigation report shortly.