Irritating misfire and backfiring


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New member
Aug 28, 2016
Afternoon. I'm scratching my head trying to find the source of this misfire. Coil pack has been replaced, HT leads show less than 6k ohms and spark plugs look fine. I replaced the pre-cat lambda sensor today hoping that would sort it as the car runs fine with the temperature sensor unplugged. Am I right in thinking that in models with two lambda sensors, it's only the pre-cat one that deals with fueling?

The misfire started a few weeks ago; after more than an hour of driving at motorway speeds, when going down a hill between 70 and 80 mph, the car would violently judder when I applied gas after the hill. If I lifted off, dropped back to 60 or so, then tried again, the problem went away. It's now spread to Lower speeds and it's particularly bad when accelerating in a high gear after having travelled with no accelerator (down a hill or something). The car will judder and continuously backfire but be ok if the revs get high or I lift off and wait a bit.

Like I said before, when the coolant temperature sensor is unplugged, the car behaves perfectly. I cleaned the mass airflow sensor, but don't want to replace it unless it's definitely the culprit. If I unplug it completely (the mafs) and try to drive, the problem gets worse, but the car is still just about drivable. There are no obvious vacuum leaks. Any ideas?

I'm also assuming that the cat is toast at this point.
Sounds like a blocked fuel filter to me. When was it last changed & has the car been driven low on fuel on a regular basis ie often when the yellow light on the dash shows?
If you get to the bottom of this problem quickly your Cat will need no more than a run for an hour on a motorway to get rid of the build up.

The cat will clog the mesh inside it if you are burning loads of oil . At that stage you will have to remove to clean it, however at the moment it's not the cause of your problem. Replace the fuel filter.
I changed the fuel and air filters as soon as I noticed the first sight misfire, it's since got a lot worse. It's able to cruise at relatively high speeds most of the time and accelerate ok, once the revs are higher than about 2k. It also backfires continuously when it's misfiring so I'm convinced the fuel supply is ok and it's ignition or a sensor. The real problem appears when accelerating in a high gear after I've lifted off for a time. Will recheck the spark plugs and make sure they've not been gummed up.
I'm not normally a fan of throwing parts at a problem, but if your confident in the fuel system then I would honestly replace the entire ignition circuit from the coil pack to the spark plugs, and take the time under the hood to really triple check for vacuum leaks, clean the IACV, inspect the PCV lines, etc. This Era Ford ignition systems can be really finicky and they always seem to respond best when everything is replaced together, not old and new parts mixed and matched.

If you can take the time, it might not hurt to go as far as removing the injectors to clean and bench-test them. If you don't want to do that, at least give them a good flush with your favorite fuel injector cleaner. A good, aggressive seafoam treatment can work wonders.
Thanks, will certainly double check the spark plugs, clean the iacv and check for leaks. I've got a scanner on order so will see what that says.
Ok, I've plugged in the obd, the maf is working fine, but the second O2 sensor is showing 0v, I was under the impression that it was only the first that controls fueling, but will replace and see if that helps
I doubt its the second 02 sensor as from what I've read on here in the past , that is a fine trimmer to help adjust the emissions after the cat, many fitting a tubular manifold leave it disconnected without issues.
Reading back through what has been done so far , there is no mention of that you have checked the TPS (Throttle Position Sensor).
Clean the contacts with contact spray
As I understand its function could do an ohms test on it as it's effectively a variable resistor , perhaps its got a dead spot as the throttle butterfly opens.
BTW, your leads sound fine with the readings you have.
I've had a quick look at the tps, it says 15% open at idle, seems to increase fairly smoothly on acceleration though.
The variable cam pulley can sometimes go faulty and stick, however I don't think it's that because the OBD would have thrown up an error code.

1)Do you have the earth braided wire fitted from the back of the head to the bulkhead and if so is it in good shape?
2) Is the fuel pump noisy when the engines running ?
3) If you can get up live data for fuel pressure, your looking for approx 4 bar?
Ok, interesting outcome. It got worse and worse until I eventually got a code saying cylinder 2misfire. I pulled the plugs and cylinder 2's was a bit oily but not awful (not sure how sensitive modern plugs are to oil fouling). I swapped it with the cylinder 1 plug expecting the misfiring cylinder to either change or stay the same, but it's stopped. No idea why. Will obviously change all plugs now.

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