Hi,
My Fiesta has been giving me this problem for over a year now, but it goes away and comes back. I've asked the dealer to look at the problem and they were not sure how to fix it and wanted to start replacing electronics boxes, etc until the problem goes away, all of which I must pay for.

Symptoms of the problem: What happens is that the car loses power intermittently, but especially when accelerating. It's been happening quite a lot this past week. Today when I went out it was quite bad again, on every pull away the car would almost die, and I rev it up then it suddenly bursts to life and shoots forward, just to nose dive again when I change to the next gear, losing power, and then it bursts to life again.

As long as I cruise or accelerate very gently, it behaves more or less like it should. Then after about an hour's driving, the problem disappeared again, and I was able to drive normally, even with fast acceleration.

I have a feeling that the timing goes completely out, my limited understanding of engines is that the computer does what the old vacuum advance used to do in old cars - make the plugs fire faster on acceleration or whenever the engine is laboring hard. This function appears to disappear, or at least the car takes a few seconds to realize that it needs to respond by advancing the timing.

This "symptom" appears sometimes when the car is warm, sometimes when it is cold, some times it stays for days, sometimes it will only show up for a short trip and then be gone again for months.

So.... Any ideas what it could be?

Thanx,
_hartz

While I am noe expert, it sounds more like a fuel related issue to me similar to when a gas filter needs replacing. What year is your Fiesta?

It is a 2004 model 1.6 petrol, manual shift.

I must say that I also think it could be a fuel filter issue, and that at least might be not to expensive to test! I need to take it in for the 120,000 Km service this week, so I'll make sure they change the filter.

i think you should remove the spark plugs and look for signs of a rich mixture in one or all the cylinders. Signs of a rich mixture are black soot on the plug electrode. Leaky injector(s) or poor spray pattern could cause the problems you describe. Injector cleaning may help.If all this fails, then ECM itself may have failed which is very costly.