Intermittent Engine Misfire Problem

[bales 1]

Hi,

 

I was hoping perhaps someone could shed some light on the problem I have had for a while on my 1.9 205gti. Basically when on the motorway or cruising at a constant speed on part throttle there is quite a pronounced misfire. It doesnt occur during normal driving as generally the revs are lower and not at a constant throttle. It happens much worse when it is hot i.e at the moment, I drove up to the lakes on saturday and it was almost undriveable and wouldnt cruise anywhere between 55mph and 90mph without jerking quite violently.

 

It is a funny one really as when it is cold it doesnt happen to the same degree only very slightly, and if you give full throttle it pulls fine. The other problem which i am guessing is related, is the tachometer jumps up and down between 3 and 5k rpm. This again does it most of the time but gets worse when it is warm weather. The rev counter always jerks and the misfire i think is always there when this happens but only very mildly in the majority of driving.

 

This makes me think it is probably ignition or the tachometric relay as i have messed with the airflow meter and i dont think it is fuel related. Does anyone know any methods of testing things to try and find the problem such as the ignition system or the tachometric relay.

 

Any help would be very much appreciated, thanks.

 

Alex

[pug_ham 244 3 Cars]

When did your car last have a set of HT leads, distributor cap & a rotor arm?

 

I did an engine swap last weekend & one of the HT leads was arcing to one of the water pipes intermittently & when the car got hot it was almost undrivable. New leads fixed it instantly. once I managed to get the right set.

 

Graham.

[bales 1]

Actually I have just found the problem, I am bit confused to it having such an effect, but it was the vacuum advance pipe from the dizzy to the throttle body.

 

I wouldnt have thought it would make that much difference really, I have taken it off for now and it runs fine so it must have been blocked or something. Think I am going to leave it off though as I am not hugely convinced to how much effect it has on the mpg anyway. Unless one of you guys knows a specific reason that will cause problems if i dont have a vacuum advance.

 

I have had all new HT leads and dizzy bits and also redid the ignition amplifier module as the aluminium heat sink was covered in grime and was surrounded by wires, I took that off cleaned it up and put on a nice layer of thermal paste. I thought that was the problem originally bit that tends to cause a miss under full throttle as opposed to gentle throttle openings like on mine.

 

The only remaining problem is the tachometer jumping up and down when it gets to about 4k, anyone have any suggestions for this, tachometric relay?