Vehicle is a 2006 Ford Courier, started having hesitation/loss of power after fill up at a gas station that I'd never been to before.

I depend on it for transport so I'm forced to still drive it, which does work, but it refuses to go above ~3000 rpm (even with codes reset, and it doesn't sound like it's hitting a rev limiter or limp mode)

I've replaced the fuel pump and fuel filter, no effect. I did obviously miswire the fuel sender, because the gauge no longer works, but does the PCM use that for anything or is it for display only?

Recurring codes: P0171, P0174

Data log of idle with rev up to 3000: data log of idle with rev up to 3000

The data in the graphs is offset a bit because of the slow update rate of my scan tool, but the drop to 0v on both O2 sensors surprises me, any ideas on this?

I've also tried spraying carb cleaner all over the intake/air lines in case of vacuum leak but no surging or noticeable difference that I could hear.

My suspicions at this point are:

  • Fuel issue after the pump
  • Blocked catalytic converter (exhaust has a strange sound to it sometimes)

Very lost at this point, but I'm trying to stay scientific instead of throwing parts at it. I don't have enough experience to be able to interpret the data from the graphs, but they are clearly indicating an extremely lean condition.

Edit: Data from 30s Idle, 30s 3k RPM, 30s Idle

30s Idle, 30s 3k RPM, 30s Idle

Edit 2: Log while driving ~50km/h

Driving Log

  • Very good written question! We might need the help of Diagnose Dan (youtube) for this one. Just a quick old analog question, have you tried driving with you fuel cap open? To make sure that there's no vacuum due to a faulty ventilation or fuel cap?
    – Markus
    Commented Apr 15, 2019 at 14:36
  • If you started having issues after the fill-up at an unknown fueling station, I'd suggest you got some poor gas. Probably some water in the gas. As for the graphs, did you look at the airflow? That seems really screwy to me. G/S should increase with engine speed ... it needs more air as engine speed increases ... this graph doesn't show that. Then there is the big bobble there between 91-94, which doesn't make sense, either. This doesn't seem to correlate with the O2 sensors, but who knows. Commented Apr 15, 2019 at 16:29
  • Could you do another trace? But this time add in throttle position and do the following (starting with a warm engine before the trace) 1. Idle for ~30 secs accelerate up to where you hit the 3k rpm "wall" and hold it for another 30 secs and then 3. let it return to idle for another 30secs. and (approximately) mark on the trace the different phases? (I've got a theory - I'm not just yanking your chain) Commented Apr 15, 2019 at 16:53
  • @Markus running with fuel cap removed makes no difference unfortunately, may have reduced the maximum rpm very slightly. Commented Apr 16, 2019 at 0:01
  • Ok, sorry to hear that, would have been a quick fix. So I'm no export at this, but from your first image it looks like the st. fuel trim for some reason goes down, which should make the engine run lean, which the nb. O2 sensors seems to confirm... do you have a wb O2 as well? Maybe that is faulty and the computer regulates after that and not the nb? In the second image, doesn't it look like the engine is running lean all the time? Shouldn't the voltage be 0.5 for lambda=1? And what changed in half time in the run?
    – Markus
    Commented Apr 16, 2019 at 7:56

2 Answers 2


From the data we have so far (thanks for supplying the second set of traces OP!) I'm leaning towards the Mass Air Flow (MAF) sensor being the culprit.

Looking at the initial traces the air flow figures look way, way too low - unless you're trying this at the top of Mt Everest - and the "spike" from t92 to t94 kind of defies physics, the revs are dropping at this point in time which should lead to a reduction in the airflow. This makes me think the MAF can't be trusted.

If the MAF is essentially under reading most of the time then the other readings start to make sense - if the engine is actually getting more air then it "thinks" then it'll be insufficiently fueling, which means it'll be running lean (hence the flatlined O2 readings during the first run) and it will try and reduce the fueling to compensate, hence the precipitous drops in fuel trim on both banks.

Testing this shouldn't be too hard to do - clear the codes (I'm guessing you'll have the lean codes after running the second trace) and unplug the MAF, it'll probably throw a code for the lack of the MAF and might go into limp mode but it should default to using the MAP sensor and a slightly rich fueling. The O2 sensor traces shouldn't flatline like they have so far, the fuel trims shouldn't have that steep drop and generally be a bit more stable and it won't throw the lean codes.

This was my theory yesterday and I hoped the second trace would confirm this, unfortunately it didn't quite do that, the time lag before the O2 readings flatline after going WOT don't quite fit which is concerning but not enough to make me throw the theory out. Would be good to see airflow readings for that run to see if that clarifies things a bit.

If it's not the MAF my next suggestion would be that there was injector issues, possibly dirt or contamination from the suspect fuel - if that's causing a restricted fuel flow then it would make sense that it was running lean when the revs rise. It fits with the inciting incident being the fill up - but not with the weird airflow readings in the first trace, if the MAF angle doesn't play out I'd be tempted to run some injector cleaner through and repeat the test to see if there's any difference.

Finally I appreciate that you need to be getting from A-B so are having to drive it but I'd be wary of driving it more than you absolutely have to (and gently at that) if it's running as lean as it appears then you're going to be seeing higher than normal exhaust gas temps (EGT) and you run the risk of damaging things like the catalytic converter and the lambda probes.

  • I'm leaning more toward fuel supply/injector issues at this point, I've logged another vehicle that has no issues and the MAF rate is the same low level (at idle) Commented Apr 17, 2019 at 14:07
  • I have updated my question with another log from driving that has better MAF data vs throttle position and RPM. I am unsure what the spikes visible in the data are, but I'm going to blame them on the quality of my scan tool rather than read too much into them (or a bad ground affecting both the TPS and MAF? seems a bit far fetched) Commented Apr 17, 2019 at 14:18
  • @AaronPatrickson Yeah from Trace #3 the MAF looks a lot healthier and the log of the known good baseline suggests its a red herring - g/s numbers still look a little low to my eyes but some scan tools aren't great at calculating that and tbf I'm extrapolating from other vehicles anyway. I'd expect a bad ground to produce more scattershot symptoms but it's not impossible I guess, the most likely area now is fuel supply as you say - given you've replaced the fuel filter and pump that points at injectors. Commented Apr 17, 2019 at 14:54
  • @AaronPatrickson Some injector cleaner is probably the next logical step then. Commented Apr 17, 2019 at 14:54
  • Apologies, I completely spaced out and forgot to mention it has had several doses of injector cleaner through it, to no effect. The first few were weeks ago and didn't cause any change, yesterday I put another 2 bottles through and they caused it to run extremely poorly, which also put me on the fuel supply issue path. My thinking is that if it has a restricted supply, it is already starving for fuel, and by adding the injector cleaner I have reduced the proportion of combustible fluid, making the situation worse. Any thoughts on that concept? I don't know what exactly is in the cleaners. Commented Apr 17, 2019 at 15:13

Sorry for the late update, and thanks for all the help. We finally got the issue fixed: injectors. The old ones had basically no flow at all, except for one, which had way too much flow. I think this weird combination was what contributed to it being so hard to diagnose from the logs. In hindsight the massive boost to fuel was a big sign, but I only interpreted that as too much air rather than not enough fuel... thanks again for your help!

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