Idle drops when sitting in traffic

[B210Bandit]

hi,

Car has been fully serviced (carb is fine according to the mechanic) but still have this problem. Sit in traffic and the idle will drop fairly low, causing the engine to vibrate a bit. Does this for 10 seconds then back to normal idle. Doesn't seem to be related to the electrics being used. Am guessing this is something to do with the electronically controlled carb? Doesn't stall ever, but is a bit annoying. Had it out on the motorway and when back in town did not seem to do it i.e. when the engine is nice and warm. Any ideas?

 

[porkpie700]

If you have the 1.2 engine with full electronic ignition and computer set-up then this is normal. It's just the computer turning various things on and off (such as the throttle open control system and exhaust gas recirculation system). The change in revs is just the result of changes to the mixture and to the vaccum in the inlet manifold. Like for example if you keep pumping the brakes with the car idling the engine will die out to a point because the brake servo is altering the vaccum level inside the inlet manifold. Don't worry about it, if you want you can just increase the idle speed a little so when it's in this phase it doesn't shake so much.
I should also point out that removing the emmision control systems does not cure this problem as i have two super s's, one stock, one with the emmision control systems removed, and they both have this very same characteristic.
It wouldn't hurt to check your vaccum pipes for leaks though as this can amplify little jitters and also cause them.

Hope this helps you out.

 

[B210Bandit]

Thank you. That is very helpful. I did try unplugging the box and it made it worse. I will check the vacuum hoses but they actually look pretty new. I'll have a fiddle with the idle if I can work out how (not the most mechanically minded as you might guess!)

 

[porkpie700]

Thank you. That is very helpful. I did try unplugging the box and it made it worse. I will check the vacuum hoses but they actually look pretty new. I'll have a fiddle with the idle if I can work out how (not the most mechanically minded as you might guess!)

It's small cross scerw at the back of the carb, it's faily obvious if you look at the throttle linkage because it rests agains the back of the screw. You don't even need to remove the airbox.
With the vaccum line it's not normally the acctual line itself that breaks, it's the little rubber sleves (link pieces) at the ends that connect them to the nipples. They tend to harden and crack which you don't notice sometimes unless you have a real good close look at them. I replaced them on my car with chopped sections of spare fuel line and then cable-tied them just to make sure (Y)

 

[B210Bandit]

Thanks. that's great advice and I will check them closely to see the state of the rubber sleeves.

 

[B210Bandit]

Thanks a bunch, porkpie. I checked all of the vacuum tubes and some were rather loose, so put new screw clips on them. Hey presto! Instead of the engine idle dropping, it picks up when accessories are on as the electronic unit does its stuff as it is supposed to do. (Y)

 

[porkpie700]

Thanks a bunch, porkpie. I checked all of the vacuum tubes and some were rather loose, so put new screw clips on them. Hey presto! Instead of the engine idle dropping, it picks up when accessories are on as the electronic unit does its stuff as it is supposed to do. (Y)

Excellant news, now you get on with enjoying the drive (Y)

 

[B210Bandit]

Want to replace the vacuum pipes on my 1.2 as I think something is still not right. What diameter should the pipe be? Cheers.

 

[porkpie700]

Whatever fits and is strong enough not to collapse under the vaccum. Standard 0.8mm fuel line will do it, if you still have the origional hard platic pipes fitted then you will only need to change the rubber ends that connects them to the vac nipples. Chop up small sections of the fuel line pipe and use them as the new ends. i.e. 2inch bit of fuel pipe, plug the vac nipple into one side and the vac pipe into the other. It is best to also cable-tie the ends to be certain of a good seal. I have used this very same method over 4 years ago on my car and they are all still working perfectly.