My 56 Fiesta Ghia (55000 miles) has a problem where I am constantly getting hesitant acceleration and flat spots when driving. I will be driving at a constant speed but the power will suddenly drop\flatten, but then picks up again a few seconds later.

I have just heard back from Ford garage (who were crap) that they can't find a problem (but can replicate the issue) and have replaced the spark plugs.......for £85.

basically:

- occurs especially when lightly pressing accelerator, but also when pressing harder

- no engine lights

- getting progressively worse

- doesn't overheat

- no excessive fuel consumption

- no mis-fire or stalling

- no unusual engine noises

- tick over seems fine

any ideas?

Moved to Fiesta section.

Could be something to do with the throttle pedal. I know the electronic movement switches can start to corrode and it interrupts the signal getting to the ECU on some cars as they get older.

thanks i wouldnt know where to start looking for corrosion on that..

basically I don't know what to do, ford have been uninterested, they put the computer on it and found nothing wrong, checked hoses, changed spark plugs...my local garage don't know...where to turn?

Do you have an induction kit? Is this happening more at low revs? If soo it could be drawing in too much air and causing the engine too bog down..... Could be a cracked inlet manifold or vacuum pipe blown off etc

Here are some possible reasons for problem(s) -

Worn potentiometer in throttle pedal (as above) when you get the problem - what happens when you "floor" the throttle? (when safe to do so)

Plugs, leads and/or coil packs - you would hear the engine "missing" so its unlikely - £85 for a set of plugs??? - the garage were clueless so just stuck these in - probably did not even need them - i hope they gapped them properly

Engine is drawing air - could be through breather pipes/ PCV valve, or the like

Dirty or blocked fuel filter/ pipes,/ low fuel head / some type of fuel surge or starvation water dirt in the fuel/ poor/ stale fuel - see if a fresh full tank makes a difference

Blocked air filter/ engine getting choked/ air hose/ inlet getting squeezed etc / air hose coming off and drawing warm air

Exhaust getting blocked baffles or cat moving around blocking gas flow

Faulty sensors/ (eg crank or cam position sensor) - should show up on codes

intermittent electrical fault / like a loose connection to the throttle pedal potentiometer

thanks for the detailed replies! £85 was for 'diagnostics' and plugs. Ford were indeed clueless.

one last but of maybe relevant info I do seem to get a decrease in the problem if I use the premium petrol available everywhere nowadays. what may this point to?

thanks for the detailed replies! £85 was for 'diagnostics' and plugs. Ford were indeed clueless.

one last but of maybe relevant info I do seem to get a decrease in the problem if I use the premium petrol available everywhere nowadays. what may this point to?

Well the premium fuel has a higher octane raiting so if the engine is runnig lean (eg - drawing air somewhere) it may help but is not nessisarily the problem

This seems like one of those faults that is/are tough to diagnose and sometimes you need to go through everything "with a fine tooth comb" eliminating everything you know is ok

I was brought to my attention recently that plugs are often pre- gapped to 1.3mm and these cars run better with 1mm plug- gaps, you should ask them what the plug gaps are set to

There is a valve in the engine called the PCV valve, sometimes it gets "gummed up" worth checking this and that all the breather hoses are ok

You could replace the throttle pedal / potentiometer but i always feel its best to find out what the problem is rather than "throwing" new parts on/ potentially wasting money un-nessisarily - some items are "service" items, though (like the air filter, oil/filter, fuel filter, and er, spark plugs etc) i think its worth changing these things for fresh ones if they have not been done recently

Just curious.. where is the PCV valve located on the 1.25 Zetec 2009 model? is it the tiny part on the side of the air box with the filter inside?

Edit:! I do have generally the same issue as above thus! I am looking at simple checks before its back to Ford :)