Fiesta Mk4 1.25 Zetec acceleration problems (motorway speed)

[Morie]

Hi all,

Whilst driving at 60-70mph (3200-3500rpm in 5th gear), I regularly experience a lack of power which feels like a sudden headwind or light breaking, whilst maintaining constant pressure on the accelerator pedal. Pressing further on the accelerator pedal does not provide additional power, but if I release the pedal then reapply pressure I can get power back.

I do not think the issue is the transmission, as I had the clutch replaced ~500 miles ago and there is no increase in revs when power loss occurs.

I have also done the following:
Oil filter change and oil renewal
In-line fuel filter change
New air filter
New spark plugs
Compression test (150+ psi on all four cylinders within 3sec of turning the engine over on the starter)
Fuel pressure test at idle (30psi - the value given in the manual)
Replaced throttle position sensor with a used OEM part, left the battery disconnected for a while to reset ECU
Checked the movement of the throttle body by hand - smoothly opens and closes on its spring

The car starts first time, idles at 1000rpm smoothly, and drives (generally) ok through the gears at lower speeds. No oil leaks, no colour in the exhaust fumes, no weird noises, no warning lights.

Any thoughts on what to investigate next would be greatly appreciated

Cheers,

Joe

[Trevor555]

Without seeing/inspecting your car all we can do is guess from our armchairs.

My armchair guess is coil pack?

Slight possibility is a sticky valve?

[Morie]

Hi Trevor555, thanks for the suggestion. Not expecting someone to be able to provide a definitive answer, just ideas on what to look into next. I am pretty new to car mechanics and have been trying to work methodically through the ideas in the Haynes manual but so far without fixing the issue.

[Trevor555]

Morie said:

Hi Trevor555, thanks for the suggestion. Not expecting someone to be able to provide a definitive answer, just ideas on what to look into next. I am pretty new to car mechanics and have been trying to work methodically through the ideas in the Haynes manual but so far without fixing the issue.

Do you have a local, friendly, garage that can plug their computer in and code read it for you?

Even with no engine light on, there may be some stored codes.

Might save you spending more money chasing the fault.

Many garages charge £50 to code read, some as low as £30 if they think they'll get the work afterwards.

Avoid main dealers they're £90 plus normally.

[bangerhoarder]

Absolutely worth trying another coilpack. Changed a few on Zetec-SE engines (and Zetec Rocam).

The aftermarket ones are a waste of money, second hand ones often have more issues than the one you’re replacing, and new ones are very expensive (I wonder how hard it’d be to change to coil on plug?)

[GreenV8S]

Have you scanned for fault codes? The symptoms suggest it's either running lean or misfiring and either of those can leave fault codes.

[Morie]

Thanks for the advice all. I have the coil pack out of the car - resistance on primary windings matches the value given in the manual, but not getting any readings on the secondary windings. A bit of reading suggests this may be the case if the coil pack contains a high voltage diode, so I connected the multimeter on DC volts mode in series: multimeter - battery - coil pack connector pin (primary windings) - HT lead ports (secondary windings) - multimeter

I get no voltage reading when the battery is connected in normal or reverse polarity - so could be a break in the secondary windings?

Anyway, have ordered a new coil pack and a fault code reader for further investigation...

[GreenV8S]

Morie said:

so could be a break in the secondary windings?

Most unlikely given that the car runs.

[JONATHAN_11_80]

Hi, Having driven a few of the 1.25 fiesta's they are willing little cars that need to be revved hard to get them to shift and they have zero torque at lower revs. Have your tried running a fuel cleaner such as forte gas treatment and forte injector cleaner through the fuel system.

[Morie]

JONATHAN_11_80 said:

Hi, Having driven a few of the 1.25 fiesta's they are willing little cars that need to be revved hard to get them to shift and they have zero torque at lower revs. Have your tried running a fuel cleaner such as forte gas treatment and forte injector cleaner through the fuel system.

Thanks for the tip. I have run Wynns injector cleaner through a couple of tanks ago but has not made any difference to the symptoms though

[Morie]

After getting no codes on the code reader, it appears the problem was dirty sensors. Removed the air box and gave both the air intake temperature sensor and MAF sensor a good clean with the proper spray and have had no issues since...and long may that continue

[Limpet]

bangerhoarder said:

Absolutely worth trying another coilpack. Changed a few on Zetec-SE engines (and Zetec Rocam).

The aftermarket ones are a waste of money

I remember this song and dance on my mum's old StreetKa

Car kept going into limp mode and blinking its MIL. Took it to a trusted local indie who diagnosed a coil pack and replaced it (with an aftermarket one).

Car continued going into limp mode and blinking its MIL. Took it back, and the indie stood by their diagnosis, and replaced the coil pack again with another new aftermarket one.

Car continued going into limp mode and blinking its MIL. Out of frustration, took it into the local Ford dealer, who also diagnosed a coil pack and replaced it with a genuine one. Car never missed another beat.