ECU/Electrical problem?

[Olethros]

So my '98 TU3JP-engined (that's 1.4l petrol, in case you didn't know) Multispace 'lingo is a bit under the weather at the moment. Which is kind of strange, as it has braved the <-20ºC Norwegian winters with no significant problems thus far. Anyway, to put it in chronological order:

• Date of purchase to the Friday before last: Engine running more or less like clockwork.
  
• last Monday: Stuttering at idle and accelleration. Noticeably less power. Tachometer not working.
  
• Saturday: Tachometer suddenly working again. Idles at 3000 rpm.
  
• Today: Tachometer not working. Idles at ~3000 rpm, but settles down to about 1000-1500 rpm after a few minutes.

I've checked the connections between the crancshaft position sensor and ECU, and it seems just fine.

The workshop claimed, with absolute certainty, that the stuttering and power loss is caused by worn-out spark plugs and said they could change them for £150. I replied pull the other one, it has bells on it. Spark plugs do not, to my admittedly limited knowledge, spontaneously wear out overnight with the car parked in the driveway.

So is there an easy way to diagnose ECU/electrical problems that could be related to these issues? And have anyone else here had similar experiences with elderly petrol-fueled Mk 1s?

[Andre]

This really sounds to me like your idle control valve. I had similar symptoms in an old Pug 206 i used to own; irregular idling, cutting out, and idling at any rpm it liked (those times when the rpms weren't all over the place).

[Olethros]

Both the idle control valve and the spark plugs could, in theory, be contributing factors, but the erratic behaviour of the tachometer and the fact that the fuel pump does not disengage a couple of seconds after the ignition is turned on and the engine is not started unfortunately point to a fault in the ECU itself. I'm having it looked at at a dealership this Wednesday. Hopefully I won't have to start looking for a replacement ECU, but if worst comes to worst it shouldn't be too expensive to get a second-hand one from a breaker's yard.

Just to be on the safe side I replaced the spark plugs with brand new ones this afternoon, and just as I expected it did absolutely sod-all. The old ones don't look too worn, either. There seem to be some slight carbon fouling on them, but nothing that points to any serious faults.