Missing at low revs

[Drunkun_Munky]

My MK6 ST 2005 runs fine all through the rev range until you put your foot down a little harder while its a lower revs then it'll miss just once or twice then its fine, I have changed the spark plugs, HT leads and fuel filter but it still does it, I was wondering what my next step should be, I was thinking maybe the TMAP?

 

[Intuit]

On a stock, much older, naturally aspirated and more simple engine design, anytime I experienced a miss under low RPM, high engine load, it was the spark plug wires. If feathered it until higher RPM, one could mash on the pedal all day and not detect an engine miss or loss in performance. Basically the spark would break down when more air was allowed into the cylinder before fire. This of course is a totally different setup.

Next step would be to check for codes and consider the possibility that some modification is creating or contributing to the issue.

 

[Drunkun_Munky]

I am getting the codes read on Wednesday so I will see what pops up, if anything.

 

[M-Sport fan]

Is this the previous generation, naturally aspirated, Fiesta ST?

Is that also a MAF based system, like the current Euro/Brit Mk.7(?) turbo STs??

If so, did you try cleaning the MAF sensor?

 

[Intuit]

For the motorcycle that doesn't have spark plug wires per say, and was behaving similarly, the issue was a coil-on-plug. Fortunately the motorcycle has a superb built-in test that allows you to individually read and actuate sensors and components.

Since I could individually fire any one of the coils with the push of a dash button, I only had to use a spark plug gap tester to compare the distance at which each plug wouldn't fire. Three of them had similar max distances. One plug had a much, much shorter gap at which it wouldn't fire. That was my bad coil. Not much later a second coil failed. So I replaced them all, even though they're expensive, because they're not easy to get to.

I'd look to find if there's some way you can test and compare the gap lengths for all plugs, and see which one if any, is anomalous.

Remember that the gaps will be quite large because they're not firing under atmospheric compression. The gap may even be larger than your gap tester will support. Make sure you don't zap yourself. It'll really hurt.