Fiesta 1.25l Duratec - P0172 Engine running rich issue

[FiestaZetec1]

Hi,

Having an issue with my 2008 mk6 (mk5.5) Fiesta 1.25l Duratec engine and wondering if anyone can help. Mileage is 66k.

The car appears to be misfiring intermittently and hesistant in low revs. The car struggles to build power unless flat out on the throttle and high revs. However if I turn the car off and on again and continue to drive, it runs fine until the next idle or sometimes after few minutes again where you feel the car is struggling.

The car had an EML showing P0171 and P0172 error codes with a check engine light but they have been cleared. Recently got P0172 showing again since doing the following fixes from my mechanic:

- Engine temp sensor
- New spark plugs
- New coil pack and leads

It's had a new O2 sensor fitted around Jan 2025 so there is a chance this could be faulty again. However reading the live data shows the following:
O2 sensor is cycling between 0.1 and 0.9v so appears normal

MAP sensor appears normal. Runs at 30-40kPa and with throttle goes up to 90-100kPa.

Short trim fuel goes to -25% when misfiring otherwise its around -5 to 0%

Long trim fuel is sitting around 1 and -5%

I’ve attached an image from Forscan DTC.
My suspicions now are to do with Fuel system, either injectors, rail or fuel pressure regulator?

Wondering if anyone has any idea what it could be and if anyone has had the same issue before. Look forward to hearing all your thoughts
Thanks

 

[Handy Andy]

I’ve attached an image from Forscan DTC.
My suspicions now are to do with Fuel system, either injectors, rail or fuel pressure regulator?

When it runs rich, there may be several simple things combining into a larger problem that seems to lead in one direction, but really is a simple series of fixes that - until now, weren't on "The List Of Things" needing service.

Well, let's go take a look.

• Throttle - how is it doing? Has it been cleaned? Taken off and inspected the bore? Looking for caked residue and possibly found debris from the "coke" forming in the Manifold - possibly obstructing the air-flow.
• Fuel, Changed Brands - or have you started seeing "Now with 15% Alcohol" or "New Formulation" along with the price going up? (They may have had to switch suppliers without your knowledge - changing the quality of fuel)
• Wiring within and the Fuel Rail condition - any rust or kinks?
• PCV valve - is the Hose to the Intake from the engine-side - collapsing in idle (high) vacuum conditions - making it seem hard to keep idle up because of how it affect the air mixture "budget" it has.
  • This includes the Foul Air purge line, the hose in the upper Valve cover that goes to the Air-Box - there is a foam filter that needs to be inspected and even cleaned - your vehicle is a 2009 - so the MAF sensor can also need some attention as well as the PCV intake or "Foul air purge" line from the air-box.

Since you own a Ztec - the earlier models had a slightly different setup than those of the Sigma motor the Fiesta became. So it's where I can only offer - their are other results similar in symptoms - sounds like there may be more but FORSCAN and DTC is not telling you anything else at the moment.

Hmm...The One On the Right - looks Like @scotman has been here...?
​

This can also mean issues like Oil changes and plugged filters - onto more serious conditions of blockage either at the Exhaust or carbon build up from Injectors placed elsewhere - not cleaning the Intake valves and sludge is built up. Or from a LEAKY intake gasket making the engine mis-fire which then makes the system think it's too rich because of the mis-firing is making the exhaust seem rich.

Wait...Elsewhere? Well, yes, (in a way it would be under the Duratec) some Sigma motors have had GDi while other used Ti-VCT (Focus models) as well as TDCI (Diesel) Either way, the timing of the Camshaft usually affects the hesitation - so you may have some internal oil-lubrication issues that have not yet become severe enough to make the DTC list it out as possible causes. Oil Pressure as well as the oil port feeding the solenoid - can get plugged with debris if the Oil Filter got plugged by dirt or water sludge - affecting the medias ability to filter. It's slowed the rate of flow - but not by volume. So it ran thru the bypass valve in the Oil-pump to keep the motor spinning. This means the oil got there, but the dirt in it may have left behind residue that prevents the oil pushing past into the solenoid as a flow-rate as per-usual, it's plugged or lower - so it sees timing issues differently.

This is why I Said the above...


You have a 2009 with 66K - that's making us think; pretty low miles and a lot of start and stops and sitting along the way - unless you idled it for long periods of time which then the MRTBF versus the MTBF with one being the Mean RUN Time Between Failures versus actual miles driven - the results can be the same - a fail in the longevity of vehicle lubrication due to excessive wear as well as lack of use and short term drives. So we see a new motor - but low miles - only now the age of the motor can also play a role here too. It takes too much time to boost the oil pressure up and advance the cams so the thing stutters due to lack of advance - it's simply waiting for the oil to push the cams forward to affect the mixture and make the spark plug fire in the right mixture, not an enriched one caused by timing not advancing the cams to let more air get in it. So it gets more fuel, but there less air in the chamber for this; giving you this same result.

Suggestions?

Change the oil to 5W-30 or even 10W-30 for hot weather - but also think about changing it more often due to fuel dilution and the newer blends the EU is imposing on the Gas stations - Alcohol puts a twist on the fuel and how it works with oil when it forms by-products. So a review of what you use from now on may be in order - both in fuel and oil, along with filter, service interval - and brands used for this.