Rough when cold

[MBS3]

Help required for the mechanically(and electrically)un-gifted:
My S3 has just started playing up when cold. On initial start up it will not idle - this lasts for approx five mins then it will idle just about ok. It then runs very rough(appearing to gag)until the temperature gauge gets to just under horizontal, it then runs fine. Accelerating appears to be ok during the dodgy warm up period it is only when running in traffic at constant speed with little throttle that the problem arises. I do have the 'Bible' and was wondering if it could be the thermotime switch/sensor, at what sort of temperature does this cease to be required to be operating?
One other recent 'feature' is the rev counter stays at 1000 rpm when the engine is turned off dropping down to nearly zero when the electics are turned on again.

All help very simply explained appreciated

Martyn

[Dave_H]

quote:Help required for the mechanically(and electrically)un-gifted:
My S3 has just started playing up when cold. On initial start up it will not idle - this lasts for approx five mins then it will idle just about ok. It then runs very rough(appearing to gag)until the temperature gauge gets to just under horizontal, it then runs fine. Accelerating appears to be ok during the dodgy warm up period it is only when running in traffic at constant speed with little throttle that the problem arises. I do have the 'Bible' and was wondering if it could be the thermotime switch/sensor, at what sort of temperature does this cease to be required to be operating?
One other recent 'feature' is the rev counter stays at 1000 rpm when the engine is turned off dropping down to nearly zero when the electics are turned on again.

All help very simply explained appreciated

Martyn

Hi Martyn,

I can answer your rev counter problems, your rev counter stays at 1000rpm when turned off due to gravity, "They all do that Sir" I think is the officiai answer

They all do that, it's not a problem.

Cheers,

Dave.

[GreenV8S]

quote:your rev counter stays at 1000rpm when turned off due to gravity,

If it's the same as mine, the rev counter has a small stepper motor inside that moves the needle, when you switch off it freezes. If you blip the throttle before you switch off you will see it stop at other readings. This can be very confusing when you're waiting in a line of TVRs listening to a burbling V8, rev counter shows normal tickover but when you bring the clutch up nothing happens, D'oh!

[samdring]

Your other problem on rough running when cold shows same symptoms as mine when throttle pot 'going'. Now on 4th pot in 4 months. Normally when mine's going (as it is now - again!) the cold rough running is accompanied by fluctuating tick over when warm i.e. sometimes 600 rpm sometime 1200 rpm

cheers

sam

[MBS3]

Cheers for the replies;
It sounds like the stepper motor may be the cause of the rev counter 'feature' as it has only done this for the last two weeks.
I have read that Chimp throttle pots last better than the normal ford ones used in the S's are these only suitable for the V8s or are they OK on the V6s as well. I had a new pot about 2 months ago, so I suppose it has lasted quite well really!!! But if it is the pot why does it run perfectly when warm??

Martyn

[Roy C]

"Rough Running when cold" - do you mean that the engine is trying to cold idle at hot/slow idle revs?

If so, it probably means that your air bypass valve (also, confusingly, a stepper motor) - the silver tube with a plug in it, mounted on the front of the plenum - is probably sticking. It works like a choke to let more air (and fuel) into the system to provide a fast cold idle.

They usually just need cleaning (the piston sticks in the cylinder due to a build up of dirt in the airflow).