sudden acceleration

[sprite]

1990 Eunos (MX5) manual. The car suddenly accelerates to max revs when driving. On stopping and restarting it can go to max revs, after multiple starts it goes back to normal. The problem is intermittent but can occur every five to ten minutes. Anyone know what to look for or how to solve this problem. (PS there are no floor mats or evidence of anything sticking)

[bruce]

Toyota?
Take it to a mechanic before you kill yourself.

Sticking throttle?- check under bonnet, follow the cable.
Spark plug leads do cause erratic idle, but not acceleration!

[JBT]

Throttle cable inner jamming/unravelling

[Mr Morlock]

I agree with Bruce- get it diagnosed quick- sounds like a recipe for disaster. Let us know how you get on.

[sprite]

The car is with a good mechanic, he drove and experienced the problem but has not diagnosed it yet. In the mean time he is also calling people to see what he can find out but they all have said 'good luck' A possibility is the throttle box but it is expensive and he wants to make sure it is the problem before replacing it.

[93_Clubman]

The car is with a good mechanic, he drove and experienced the problem but has not diagnosed it yet. In the mean time he is also calling people to see what he can find out but they all have said 'good luck' A possibility is the throttle box but it is expensive and he wants to make sure it is the problem before replacing it.

By "throttle box" I'm assuming you mean the 'throttle body' which sits between the intake manifold & the black plastic intake pipe? If so, contact one of our Forum sponsors at the bottom of the page, in either Melb, Syd, or Bris depending on your location for a good well priced 2nd hand throttle body - they're not that expensive for an NA6. If it's not the throttle body, & it is a "box", then may be your mechanic's referring to the engine control unit? Some of these are starting to experience problems in NA8s, but you can also pick them up likewise for a reasonable price 2nd hand from one of our sponsors.

[sprite]

Thanks, you are correct, it was a throttle body. Thanks for the heads up on the suppliers, I will call and get pricing.

[93_Clubman]

Given you can occasionally get an NA intake manifold, perhaps minus throttle body, for $75 or less, you shouldn't have to pay more than $50 for a manual NA6 throttle body, & probably less. I'm sure there'd be a few forumites that would also be happy to sell theirs - may be try a wanted to buy post?

[sprite]

Thanks again. Is there anything else you can think of that might cause unintended sudden acceleration, are there any electrical or electronic inputs to the acceleration system that could send a corrupt signal that would cause this.

[Fatty]

the throttle is a mechanical device, connected to your pedal by a cable. there's really no way it could be an electrical / electronic problem as far i can figure.

there is a spring on the pedal, and also a large spring on the outside of the throttle body that closes the throttle when your foot is off the pedal. if you tug on the throttle cable in the engine bay, you'll notice it is attached to a "wheel" and this has a shaft thru it's centre and the spring sits on the other side of the wheel. when you release the cable you should see the wheel quickly spring back to the closed position.

so i suppose you should just check firstly that the spring is intact and doing it's job.

the shaft continues into the throttle body and the throttle plate is attached to it. i don't know if this is even possible, but maybe the shaft has come loose and therefore the plate is floating around and therefore not closing fully. you would be able to check this out by removing the plastic intake pipe and tug on the throttle cable a few times to see if the shaft is moving freely or not.

whatever the problem is, i wouldn't be driving the car until you are absolutely sure you've fixed the issue!!

[93_Clubman]

Is there anything else you can think of that might cause unintended sudden acceleration, are there any electrical or electronic inputs to the acceleration system that could send a corrupt signal that would cause this.

This issue in a '90 Eunos Roadster can't solely be electronic as the combustion triangle requires heat, fuel & air; & air is administered & admitted mechanically. So, while it's possible there's an additional electronic problem, you certainly have a mechanical issue somewhere between, & including, the throttle pedal to the throttle body, which initially at least your mechanic appears to have diagnosed as throttle body.

[sprite]

An update. The mechanic could not find anything wrong although he experienced the problem twice on road tests. However the problem did not occur in the workshop which made diagnosis difficult. He cleaned, checked and made sure everything was working okay and after this the problem has not resurfaced. I have the car back and it has been running okay so at this point we do not know what happened and hopefully it will continue to run okay. In the mean time thanks for your contributions.

[93_Clubman]

Great to hear it's been ok so far since a mechanic went over it, but I'm sure on something like this, he'd feel more comfortable if he could attribute the problem to a particular component or adjustment.