2001 Mk3 2.5l V6, any suggestions what can cause splitsecond power loss when accelerating under load and lowered fuel econemy?

[ancienttyres]

I've had an ongoing problem with my v6 Mondeo. It started to misfire when I would accelerate it hard, usually if I was going up a hill more than anything. I also noticed the fuel economy has gone down quite a bit. The engine was always in fairly good nick and I'd be able to get 400 miles out of a full tank but now I I get around 300 if I drive it like an OAP. This drop in mileage was sudden and seemed to coincide with the misfires happening. Even now though it runs fairly well, it always starts perfectly even in cold weather, never stalls when idling or cuts out on me, and for the most part it drives very smooth, it's just as I say sometimes if I put my foot down it will judder a bit, like a misfire. It doesn't always do it though.

Initial speculation was it might be the ignition system; ie coils, plugs, ht leads. I replaced the coil pack with a new ford one. Misfires seemed to go. But fuel mileage remained lowered. Kept being told my plugs needed doing and I should maybe look at the ht leads too so in the end I just did them all too. MPG didn't recover though, and the other day when driving in the rain I noticed it misfiring again when accelerating up a hill so it hasn't actually solved that either.

Only obvious things other than ignition components that seem like it could be are;

- A sensor. I don't have any DTCs though so if a sensor has started putting out dodgy readings it isn't registering as faulty in the PCM.

- Fuel pump or pressure regulator. My first port of call after the ignition was to check fuel pressure regulator but I can't find it (my haynes manual vaguely said it was under the UIM on the v6 but it isn't on mine, not that I can see anyway).

- The actual injectors

I should note someone told me they were certain it was an egr valve problem, but my v6 is one of the later duratec models that doesn't have an egr valve apparently.

Anyone got any ideas? Would much appreciate the input.

[Mark M]

Have you got access to Forscan or similar? something that can read sensors/values may help you a lot. The only suggestion I have for a cause would be a faulty lambda sensor, they default to a lean condition which means the ecu will chuck fuel in to compensate. That would cause the hesitation on acceleration and poor fuel economy.

[ancienttyres]

Yes, I do have forscan on my laptop, I've only ever used it for checking DTCs admittedly. Lambda sensor sounds promising though, If I have forscan log the o2 sensors output what should I be looking for in the readings?

[Mark M]

I'm not sure of the voltage range on the Mk3, could be from 0-1v, 1-5v, 0-5v or something like that. But when the engine is up to running temperature that voltage should bounce around from one extreme to the other, if it sits at the same voltage for any length of time then it suggests the sensor is faulty.

[ancienttyres]

OK thanks. I'll give it a shot.

[ancienttyres]

Do I have a winner?

I let the car warm up and went for drive. From top to bottom, the first and third sensors did what you said and fluctuated quite a lot. The second sensor really didn't fluctuate much at all. It hovered around the  .7v  mark most of the time only occasionally dipping a bit. The fourth sensor was permanently stuck on 1.27v and didn't change the entire time.

I put them in order, bank 1 sensor 1 and 2, and bank 2 sensor 1 and 2. I don't know which ones are upstream or downstream but I'd guess it's sensor 2 is downstream. So looks like I have two bad downstream sensors?

[Mark M]

I'd say that the 4th one stuck on 1.27v is faulty. The 1st and 3rd that bounce around should be the pre-cat sensors, the other 2 should be after the cats and stay fairly constant but the 4th one sticking that high is the odd one out so I'd replace it as it should be doing what the 2nd one is.

[ancienttyres]

I'm back, as I've discovered that this car doesn't have four 02 sensors, it's got two upstream sensors and only one downstream. That fourth reading on Forscan for "Bank 2 sensor 2" was for a sensor that doesn't actually exist and the 1.27v reading from it is just a red herring. So I'm back at square one it seems. I'm no expert of course so I wouldn't really know from looking at it but I'd say based on what I've been told the sensors are actually working OK?

 

The middle one is the downstream sensor, the other two are the upstreams. This was about four minutes after I'd started the car and began driving it. I recorded the short and long term fuel trims too but, I'm not really sure what they're supposed to be doing, again this was in the same position in the recording as the graph above so nearly 4 minutes since starting the car:

Ignore the flatlining one, I put that PID in by mistake. But the top is Short term fuel trim 1 and the third is STFT2. The bottom two are the Long term fuel trims 1 and 2. Only thing I can see that's possibly out of the ordinary with my limited knowledge of it all is the long term fuel trim 2 is always positive, and never goes negative like the other long term trim does. That looks like my issue right there, PCM thinks bank 2 is lean so it's adding more fuel assuming I'm not just outright misinterpreting. But it doesn't seem to be reflected in the short term trims or the 02 sensor outputs.

[vinylbudgie]

Hi

Did you ever get to the bottom of this? Did swapping an O2 sensor fix things?

My 2003 Mondeo 2.5 v6 (99k miles) is exhibiting very similar symptoms to the ones you describe.  Just like yours it starts and runs fine but can hesitate/judder under load, has very little 'oomph' after about 4000 rpm and gives me only about 240 miles per tank.

I too have changed all of the ignition components (coil, wires, plugs) as well as putting in a new fuel filter.

If there's a simple fix I'd love to try it.

Thanks

 

[nicam49]

I'd say check all the rubber boost pipes for splits. They can be notoriously difficult to find, (the splits that is) due to how the pipe is constructed : a mesh lining tends to hold a split in the rubber outer together, and the split only opens up under boost pressure, so can be tricky to detect without removing them 1st, but I'd start there. 

[vinylbudgie]

Hi Nick

Thanks for the tip. I am not sure my engine has 'boost' pipes as such given its a non turbo model. I will certainly give the vacuum hoses a look over though.